PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...
PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...
PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...
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SPECTROSCOPIC INVESTIGATIONS OF THE<br />
ELECTRONIC STRUCTURE OF Pr 1−x Ca x MnO 3<br />
SYSTEM<br />
A THESIS SUBMITTED TO THE<br />
HOMI BHABHA NATIONAL INSTITUTE<br />
FOR THE DEGREE OF<br />
DOCTOR OF PHILOSOPHY IN SCIENCE<br />
(PHYSICS)<br />
MAY, 2009<br />
By<br />
MANAS KUMAR DALAI<br />
INSTITUTE OF PHYSICS,<br />
BHUBANESWAR 751 005,<br />
INDIA
CERTIFICATE<br />
This is to certify that the thesis entitled Spectroscopic investigations of the<br />
electronic structure of Pr 1−x Ca x MnO 3 system, which is being submitted by<br />
<strong>Manas</strong> <strong>Kumar</strong> <strong>Dala</strong>i in partial fulfillment of the degree of Doctor of Philosophy<br />
in Science (Physics) of <strong>Homi</strong> <strong>Bhabha</strong> <strong>National</strong> Institute is a record of his own<br />
research work carried out by him. He has carried out his investigations for the last<br />
six years on the subject matter of the thesis under my supervision at Institute of<br />
Physics, Bhubaneswar, India. To the best of my knowledge, the matter embodied<br />
in the thesis has not been submitted for the award of any other degree.<br />
Signature of the Candidate<br />
<strong>Manas</strong> <strong>Kumar</strong> <strong>Dala</strong>i<br />
Institute of Physics<br />
Bhubaneswar<br />
Date :<br />
Signature of the supervisor<br />
Dr. B. R. Sekhar<br />
Associate Professor<br />
Institute of Physics<br />
Bhubaneswar<br />
ii
iii<br />
To My Parents
Acknowledgements<br />
I owe a debt of gratitude to many people who made this thesis possible due to their<br />
support, suggestions, and encouragements.<br />
First of all, it is a great pleasure for me to express my sincere gratitude and indebtedness<br />
to my thesis supervisor Prof. B. R. Sekhar for his invaluable guidance<br />
and constant encouragement to complete this work. His excellent training in handling<br />
the intricacies of various experiments and making me acquainted with the data<br />
analysis have been an inspiration for me to pursue my career as an experimental<br />
condensed matter physicist. I wish to thank him for giving me the complete freedom<br />
both academically and socially during the period of my work.<br />
I would like to thank my group members Prabir and Rupali for their untiring<br />
support during the experiments and for many interesting discussions. I would also<br />
like to thank our technical staff Mr. Chittaranjan Swain for his help during the<br />
experiments. My sincere thanks go to Prof. C. Martin (Laboratoire CRISMAT,<br />
France) for providing good quality samples for this thesis work.<br />
I would like to thank Dr. S. R. Barman, Dr. S. Banik, Dr. A. K. Shukla and Dr.<br />
R. Dhaka (UGC-DAE CSR, Indore) for their help and discussions for inverse photoemission<br />
measurements. I would also like to thank Prof. Stefan Schuppler, Dr. Peter<br />
Nagel and Dr. Michael Merz for their help and cooperation during the high resolution<br />
photoemission and x-ray absorption measurements at WERA beamline of ANKA<br />
synchrotron, Karlsruhe, Germany. I would like to thank Dr. Federica Bondino, Dr.<br />
Marco Zangrando, Dr. Elena Magnano and Dr. Bryan Doyle of ELETTRA synchrotron,<br />
Trieste for their help and technical discussions for XAS experiments.<br />
It is a great pleasure for me to thank Prof. N. L. Saini (Departimento di fisica,<br />
Universita di Roma, Rome, Italy) for his fruitfull discussion at different stages of<br />
my Ph. D. career. I also thank Prof. Anil <strong>Kumar</strong> and Prof K. K. Nanda of IISc<br />
Bangalore for discussion and suggestions during their short visit to IOP.<br />
I would like to thank all my teachers, who had taught me at various stages of<br />
my educational career, especially my M. Sc teachers Profs. N. Barik, L. P. Singh, L.<br />
iv
Maharana, P. Khare, D. K. Basa, K. Maharana, S. Mohapatra and D. Behera and<br />
B. Sc teacher Mr. Dipak Singh for their encouragement and excellent teaching. It<br />
is my great pleasure to thank Profs. A. Khare, A. M. Jayannavar, S. G. Mishra, D.<br />
P. Mohapatra, K. Kundu, P. V. Satyam, S. Varma, S. Mukherji, J. Maharana, G. V.<br />
Raviprasad, A. M. Srivastava, S. M. Bhattacharjee, A. <strong>Kumar</strong>, P. Agrawal, T. Som,<br />
G. Tripathy, S. K. Patra for their teaching, suggestions and encouragements. I thank<br />
our director Dr. Y. P. Viyogi and ex-director Dr. R. K. Choudhury for their support<br />
and encouragements in all respect.<br />
I would like to thank Mr. Santosh Choudhury, Mr. Anup Behera, Mrs. Ramarani<br />
Das, Mr. Arun Dash, Mr. Khirod Patra, and Dr. S. N. Sarangi for providing technical<br />
help when needed. I would also like to thank the library staff Rama, Rabaneswar,<br />
Duryodhan and Kissan for their cooperation at every stage. I thank Dipankar, Srijesh<br />
and Dhiren for their help and cooperation at every computer related problems. I wish<br />
to thank the hostel mess staff for their wonderful services.<br />
I am thankful to our registrar Mr. C. B. Mishra for his imense help and cooperation<br />
in all administrative problems. I also thank Mr. Sk. Kefaytulla and Rajesh<br />
Mohapatra for their administrative help.<br />
I take this oppurtunity to thank my M.Sc friends Rajesh, Tapas, Swayambhu, Hitu<br />
and Manmohan and village friend Suda for their constant support and encouragement<br />
in all stage of my life. A special thank to Ananta for his constructive criticism, support<br />
and company for both academic and social activities in my Ph.D life.<br />
It is my pleasure to thank Zashmir bhai, Satya bhai, Jhasa bhai, Satchi bhai,<br />
Bedanga bhai, Pal bhai, Kamal bhai, Pradeep bhai, Bishnu bhai, Birendra bhai,<br />
Sudhira bhauja and Raj babu for their encouragement and cooperation during my<br />
stay at IOP. I would like to thank Millind, Sinu, Boby, Chandra, Hara, Amulya, Ajay,<br />
Trilochan, Smruti, Chitrasen, Jatis, Ashutosh, Kuntala, Sadhana, Mamata, Ranjita,<br />
Binata, Jaya, Poulomi, Saumia, Pramita, Probodh, Jay, Dipak, Sumalay, Bharat,<br />
Anupam, Umanand, Dilip, Suchi, Srikumar, Sankha, Nabyendu, Somnath, Sandeep,<br />
Sourabh, Subrat, Sanjay, Raghavendra, Vanaraj, and Tanmoy for making my stay at<br />
the Institute a memorable one. I also thank Dr. V. R. R. Medicherla for his valuable<br />
discussions.<br />
I would like to give my special thanks to Dr. Arun <strong>Kumar</strong> Pati and Dr. Biswajit<br />
Pradhan for making my stay in the beautiful campus of IOP an exciting, memorable<br />
and pleasing one.<br />
v
I am extremely grateful to Mrs. Ranjana Sekhar for making my stay homely<br />
during the course of my work.<br />
Gratefully I would like to thank my younger sister Tuin and younger brother Tulia<br />
for their unlimited love, affection, encouragements, assistance and support at every<br />
stage of my life. I do not find proper words to express my sense of indebtedness to my<br />
wife <strong>Manas</strong>mita, whose unlimited love, affection, encouragements, and support have<br />
become a special inspiration to me during my Ph. D life. I also thank my brother<br />
in-law Munu for his love, affection and support.<br />
Finally, I wish to express my deep sense of gratitude to my parents and late grand<br />
parents for their immense patience, love, affection, encouragements and support. I am<br />
indebted to them for giving me the freedom to choose my career and for constantly<br />
providing me the moral and emotional support at every stage of my life. It is only<br />
because of their kind blessings that I could complete this work.<br />
Date: 22.05.2009<br />
(<strong>Manas</strong> <strong>Kumar</strong> <strong>Dala</strong>i)<br />
vi
List of Publications/Preprints<br />
[1] Valence band electronic structure of Pr 1−x Sr x MnO 3 from photoemission studies,<br />
P. Pal, M. K. <strong>Dala</strong>i, B. R. Sekhar, S. N. Jha, S. V. N. Bhaskara Rao, N. C.<br />
Das, C. Martin, and F. Studer; J. Phys. : Condens. Matter 17, 2993<br />
(2005).<br />
*[2] Electronic structure of Pr 0.67 Ca 0.33 MnO 3 near the Fermi level studied by ultraviolet<br />
photoelectron and x-ray absorption spectroscopy, M. K. <strong>Dala</strong>i, P. Pal,<br />
B. R. Sekhar, N. L. Saini, R. K. Singhal, K. B. Garg, B. Doyle, S. Nannarone,<br />
C. Martin, and F. Studer; Phys. Rev. B 74, 165119 (2006).<br />
[3] Near Fermi level electronic structure of Pr 1−x Sr x MnO 3 : Photoemission study, P.<br />
Pal, M. K. <strong>Dala</strong>i, R. Kundu, M. Chakraborty, B. R. Sekhar, and C. Martin;<br />
Phys. Rev. B 76, 195120 (2007).<br />
[4] Pseudogap behavior of phase-separated Sm 1−x Ca x MnO 3 : A comparative photoemission<br />
study with double exchange, P. Pal, M. K. <strong>Dala</strong>i, R. Kundu, B. R.<br />
Sekhar, and C. Martin; Phys. Rev. B 77, 184405 (2008).<br />
*[5] Electronic structure of Pr 1−x Ca x MnO 3 from photoemission and inverse photoemission<br />
spectroscopy, M. K. <strong>Dala</strong>i, P. Pal, R. Kundu, B. R. Sekhar, S. Banik,<br />
A. K. Shukla, S. R. Barman, and C. Martin; Submitted to Physica B.<br />
*[6] Spectroscopic investigations of the electron-doped Ca 0.86 Pr 0.14 MnO 3 , M. K.<br />
<strong>Dala</strong>i, P. Pal, R. Kundu, B. R. Sekhar, M. Mertz, P. Nagel, S. Schuppler,<br />
and C. Martin; Submitted to Phys. Rev. B.<br />
*[7] Core level electronic structure of Pr 1−x Ca x MnO 3 using x-ray photoelextron spectroscopy,<br />
M. K. <strong>Dala</strong>i, P. Pal, R. Kundu, B. R. Sekhar, and C. Martin; To<br />
be submitted.<br />
vii
[8] Electron spectroscopic investigations of the paramagnetic insulator to antiferromagnetic<br />
insulator transition in Pr 0.5 Sr 0.5 MnO 3 , P. Pal, M. K. <strong>Dala</strong>i, R.<br />
Kundu, B. R. Sekhar, S. Schuppler, M. Merz, P. Nagel, M. Channabasappa and<br />
A. Sundaresan; Submitted to Phys. Rev. B.<br />
(*) indicates papers on which this thesis is based.<br />
viii
Synopsis<br />
The perovskite type manganites having general formula R 1−x A x MnO 3 (where R is a<br />
trivalent rare earth element like La, Pr, Nd. and A is a divalent alkaline earth element<br />
like Sr, Ca, Ba.) have been one of the interesting compounds among the transition<br />
metal oxide systems due to their important physical property such as colossal magnetoresistance<br />
(CMR) [1, 2, 3, 4]. CMR is defined as the colossal decrease of resistance<br />
by the application of magnetic field. These materials are insulators at high temperatures<br />
and poor metals at low temperatures. Accompanying this insulator-metal<br />
transition is a magnetic transition from a high temperature paramagnetic phase to a<br />
low temperature ferromagnetic phase. These transitions occurs around the same transition<br />
temperature T C . Applying a magnetic field around T C will greatly reduce the<br />
resistivity, i.e negative magnetoresistance effect. These materials were first described<br />
in 1950 by Jonker and van Santen [5] in perovskite type of manganites. One year later<br />
(1951), Zener [6, 7] explained this unusual correlation between magnetism and transport<br />
properties by introducing a novel concept called ”Double exchange” mechanism<br />
(DE). The pioneering work of Zener was followed by Anderson and Hasegawa [8] in<br />
1955 and de Gennes [9] in 1960. Double exchange theory argues that the electron<br />
hoping is related to the relative spin orientations of the neighbouring sites. Although<br />
it qualitatively explains the connection between the insulator-metal and the magnetic<br />
transitions, it was noticed latter by A. J. Millis et. al. [10] that it fails to explain<br />
the quantitative change in resistivity and the very low T C . Thus it is revealed that<br />
the double-exchange mechanism is not enough to explain the CMR effect. Several<br />
mechanisms [11, 12, 13, 14], based on electron-phonon interactions (the polaronic<br />
effect), electronic phase separation, charge and/or orbital ordering etc. have been<br />
proposed to account for the discrepancy. Although a detailed picture is still unclear,<br />
there seems to be a growing consensus that one or more of these additional effects<br />
probably exists, with the various phenomena both competing and cooperating with<br />
each other. This leads to a very rich and exotic behaviour, as observed in these<br />
ix
manganites. This mixed valence manganites have been studied for more than five<br />
decades but are still considered modern materials because of their wide potential for<br />
technological application.<br />
Experimental electronic structure studies [15, 16, 17, 18] on these materials have<br />
contributed substantially to the understanding of their unusual behavior which could<br />
not be fully accomodated within the framework of the DE model. The temperature<br />
and doping dependent metal-insulator transitions in these materials are found to be<br />
closely related to the unique electronic structure derived from the Mn 3d and O 2p<br />
hybridized orbitals. Most of their electrical and magnetic properties originate from<br />
the Jahn-Teller distortion in the MnO 6 octahedra around the Mn 3+ ions. The strong<br />
interplay between spin and orbital ordering originating from the single occupancy of<br />
the doubly degenerate e g orbitals of Mn 3+ (t 3 2ge 1 g) ions make the phase diagram of these<br />
compounds rich with physics. Many of these compounds, for example L 1−x Ca x MnO 3<br />
(L = La, Pr) show strong charge and/or orbital ordered (CO/OO) insulating nature<br />
at low temperatures, especially when x equals commensurate value.<br />
Electron spectroscopy is a very powerful experimental tool for probing the electronic<br />
structure of these materials. For this thesis work, the electronic structure of<br />
Pr 1−x Ca x MnO 3 series have been investigated using ultra-violet photoelectron spectroscopy<br />
(UPS), X-ray photoelectron spectroscopy (XPS), Inverse photoemission spectroscopy<br />
(IPES) and X-ray absorption spectroscopy (XAS). Pr 1−x Ca x MnO 3 is one of<br />
the interesting among these materials due to their insulating phase at all temperatures<br />
and great variety of ordered phases, that are very sensitive to the cation/anion<br />
doping. For 0.3 ≤ x ≤ 0.8, a charge ordering of Mn 3+ and Mn 4+ was found and an<br />
antiferromagnetic (AF) ordering can be observed with neel temperature ranging from<br />
100 to 170 K. For x ≤ 0.25, a ferromagnetic insulating state (FMI) is observed and<br />
with no CO, whatever the temperature. In this region the metallic state is never realized<br />
even upon the application of magnetic field. But the charge ordered insulating<br />
state can be melted into ferromagnetic metallic state (FMM) upon application of a<br />
magnetic field [4]. Interestingly the x = 0.33 doping shows the coexistence of FM and<br />
AFM phases. Around x = 0.9 a metallic cluster glass domain [19] has been observed.<br />
Using valence band photoemission and O K edge x-ray absorption, we studied<br />
the temperature dependent finer changes in the near E F electronic structure of<br />
Pr 0.67 Ca 0.33 MnO 3 , which is regarded as a prototype for the electronic phase separation<br />
models in CMR systems. With decrease in temperature the O 2p contributions<br />
x
to the t 2g and e g spin-up states in the valence band were found to increase until T c .<br />
Below T c , the density of states with e g spin-up symmetry increased while those with<br />
t 2g symmetry decreased, possibly due to the change in the orbital degrees of freedom<br />
associated with the Mn 3dO 2p hybridization in the pseudo-CE-type charge or orbital<br />
ordering. These changes in the density of states could well be connected to the electronic<br />
phase separation reported earlier. By combining the UPS and XAS spectra,<br />
we derived a quantitative estimate of the charge transfer energy E CT (2.6±0.1 eV),<br />
which is large compared to the earlier reported values in other CMR systems. Such<br />
a large charge transfer energy was found to support the phase separation model.<br />
For a complete understanding of the changes in the occupied and unoccupied<br />
electronic states we performed a detailed study of the Pr 1−x Ca x MnO 3 system across<br />
their ferromagnetic - antiferromagnetic phase boundary using UPS and IPES. We<br />
used three compositions x = 0.2, 0.33 and 0.4. Our photoemission studies showed<br />
that the pseudogap formation in these compositions occur over an energy scale of<br />
0.48±0.02 eV. Here again, we have estimated the charge transfer energy in these<br />
compositions to be of the order of ∼ 2.8±0.2 eV. We have also undertaken a detailed<br />
study of the core level electronic structure of Pr 1−x Ca x MnO 3 ((x = 0.2, 0.33, 0.4 and<br />
0.84) using X-ray photoelectron spectroscopy. These studies have been performed at<br />
the Mn 2p, Ca 2p and Pr 4d levels. We have made systematic measurments of the<br />
changes in the binding energy positions of these levels as a function of the charge<br />
carrier concentration. We also have analyzed the line shapes of these core levels.<br />
To a large extent the traditional models employing the charge-spin coupling, have<br />
been able to explain the CMR in many of the hole doped compositions. But, much<br />
less is known about the electron-doped versions of these materials in which the charge,<br />
orbital and spin ordering add more complexity to the ferromagnetic (FM) double exchange<br />
and the superexchange interactions. These Mn(IV) rich compositions exhibit<br />
marked differences in the electronic and magnetic properties from their Mn(III) rich<br />
counterparts. It has been shown that the substitution of Ca by a trivalent cation<br />
in the G-type antiferromagnet, CaMnO 3 leads to the formation of a FM component<br />
with a maximum for optimal electron doping. Depending on the nature of the trivalent<br />
cation, the x opt was found to lie between 0.135 and 0.16. This FM component is<br />
correlated to the distortion of the MnO 6 octahedra and thereby the e g band width.<br />
With low filling of the narrow e g band, these systems have strong electron-electron<br />
interactions and consequent charge localizations. Using high resolution photoelectron<br />
xi
spectroscopy and O K edge x-ray absorption spectroscopy we have studied the temperature<br />
dependence of the near Fermi level electronic structure in the electron doped<br />
CMR, Ca 0.86 Pr 0.14 MnO 3 . We found that the temperature dependent changes in the<br />
electronic structure is consistent with the conductivity behavior of the electron doped<br />
systems in general. As the temperature is lowered from 293 K a feature due to the e g<br />
states starts appearing near the E F . This increase in the DOS is a signature of the<br />
semi-metallic behavior of the sample in the range 110 - 300 K. Surprisingly, though<br />
the resistivity measurement on this sample showed an insulating behavior below its<br />
transition temperature T c (110 K), the intensity of this feature further increases below<br />
the T c . Further, we have found an increase in the e g band width (W) also at<br />
low temperatures. It was also interesting to note that the increase in the near E F<br />
DOS in this compound is apparently different in nature from that associated with the<br />
pseudogap formation in the other systems like Pr 1−x Sr x MnO 3 . We have discussed our<br />
results from the point of view of phase sepation models considering the temperature<br />
dependent structural changes and consequent localization of the electrons in a narrow<br />
e g band.<br />
xii
Bibliography<br />
[1] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, and K. Samwer; Phys. Rev.<br />
Lett. 71, 2331 (1993).<br />
[2] Y. Tokura, A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido and<br />
N. Furukawa; J. Phys. Soc. Jpn. 63, 3931 (1994).<br />
[3] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, and Y. Tokura;<br />
Phys. Rev.B 51, 14103 (1995).<br />
[4] Y. Tomioka, A. Asamitsu, Y. Moritomo, and Y. Tokura, J. Phys. Soc. Jpn 64,<br />
3626 (1995).<br />
[5] G. H. Jonker and J. H. Van Santen Physica 16, 337 (1950).<br />
[6] C. Zener, Phys. Rev. 81, 440 (1951).<br />
[7] C. Zener, Phys. Rev. 82, 403 (1951).<br />
[8] P. W. Anderson and H. Hasegawa, Phys. Rev. 100, 675 (1955).<br />
[9] P. G. de Gennes, Phys. Rev. 118, 141 (1960).<br />
[10] A. J. Millis, B. I. Shraiman, and P. B. Lttlewood, Phys. Rev. Lett. 74, 5144<br />
(1995).<br />
[11] A. J. Millis, Nature 392, 147 (1998).<br />
[12] A. Moreo, S. Yunoki, and E. Dagatto, Science 283 2034 (1999).<br />
[13] A. Moreo, M. Mayr, A. Feiguin, S. Yunoki, and E. Dagotto, Phys. Rev. Lett. 84<br />
5568 (2000).<br />
xiii
[14] H. Kuwahara, Y. Tomioka, A. Asamitsu, Y. Moritomo, Y. Tokura; Science 270,<br />
961 (1995).<br />
[15] T. Saitoh, A. E. Bocquet, T. Mizokawa, H. Namatame, A. Fujimori, M. Abbate,<br />
Y. Takeda, and M. Takano; Phys. Rev. B 51, 13942 (1995).<br />
[16] D. D. Sarma, N. Shanthi, S. R. Krishnakumar, T. Saitoh, T. Mizokawa, A.<br />
Sekiyama, K. Kobayashi, A. Fujimori, E. Weschke, R. Meier, G. Kaindl, Y.<br />
Takeda, and M. Takano; Phys. Rev. B 53, 6873 (1996).<br />
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U. Idzerda; Phys. Rev. Lett. 76, 4215 (1996).<br />
[18] A. Chainani, H. Kumigashira, T. Takahashi, Y. Tomioka, H. Kuwahara, and Y.<br />
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xiv
Contents<br />
CERTIFICATE<br />
Acknowledgements<br />
List of Publications/Preprints<br />
Synopsis<br />
ii<br />
iv<br />
vii<br />
ix<br />
1 Introduction 1<br />
1.1 An overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2<br />
1.2 Colossal magnetoresistance . . . . . . . . . . . . . . . . . . . . . 3<br />
1.3 Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4<br />
1.4 The basic electronic structure . . . . . . . . . . . . . . . . . . . . 5<br />
1.5 Double exchange mechanism . . . . . . . . . . . . . . . . . . . . 7<br />
1.6 Jahn-Teller effect in CMR manganites . . . . . . . . . . . . . . 8<br />
1.7 Phase separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10<br />
1.8 Spin, Charge and Orbital ordering in manganites . . . . . . . 11<br />
1.9 Pseudogap behavior in manganites . . . . . . . . . . . . . . . . 13<br />
1.10 Basic properties, phase diagram and CMR effect in Pr 1−x Ca x MnO 3<br />
system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16<br />
2 Experimental Techniques 23<br />
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24<br />
2.2 Photoemission Spectroscopy . . . . . . . . . . . . . . . . . . . . . 24<br />
2.2.1 Basic principle . . . . . . . . . . . . . . . . . . . . . . . . . . 24<br />
2.2.2 Theoretical interpretation of photoemission . . . . . . . . . . . 25<br />
2.2.3 The electron escape depth . . . . . . . . . . . . . . . . . . . . 29<br />
2.2.4 Experimental set-up . . . . . . . . . . . . . . . . . . . . . . . 30<br />
xv
2.3 Inverse Photoemission Spectroscopy . . . . . . . . . . . . . . . . 38<br />
2.3.1 Historical background . . . . . . . . . . . . . . . . . . . . . . 38<br />
2.3.2 Basic principle . . . . . . . . . . . . . . . . . . . . . . . . . . 38<br />
2.3.3 Basic interpretation of inverse photoemission . . . . . . . . . . 39<br />
2.3.4 Experimental Set-up . . . . . . . . . . . . . . . . . . . . . . . 41<br />
2.4 X-ray Absorption Spectroscopy . . . . . . . . . . . . . . . . . . . 45<br />
2.4.1 Basic principle . . . . . . . . . . . . . . . . . . . . . . . . . . 45<br />
2.4.2 Detection techniques . . . . . . . . . . . . . . . . . . . . . . . 47<br />
2.5 Synchrotron Radiation . . . . . . . . . . . . . . . . . . . . . . . . 49<br />
3 Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 59<br />
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60<br />
3.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61<br />
3.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . 62<br />
3.3.1 The UPS study of Pr 0.67 Ca 0.33 MnO 3 . . . . . . . . . . . . . . 62<br />
3.3.2 The XAS study of Pr 0.67 Ca 0.33 MnO 3 . . . . . . . . . . . . . . 67<br />
3.3.3 The Charge Transfer Energy (E CT ) . . . . . . . . . . . . . . . 69<br />
3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72<br />
4 Electronic Structure of Pr 1−x Ca x MnO 3 77<br />
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78<br />
4.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79<br />
4.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . 80<br />
4.3.1 The UPS study of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33 and 0.4 . . . 80<br />
4.3.2 The IPES study of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33, 0.4 and 1 . 84<br />
4.3.3 The core levels of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33, 0.4, and 0.84 87<br />
4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91<br />
5 Electronic Structure of Ca 0.86 Pr 0.14 MnO 3 96<br />
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97<br />
5.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98<br />
5.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . 98<br />
5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103<br />
6 Summary and Conclusions 108<br />
xvi
List of Figures<br />
1.1 Temperature dependence of the resistivity of La 1−x Sr x MnO 3 (x =<br />
0.175) under various magnetic fields [2]. . . . . . . . . . . . . . . . . . 4<br />
1.2 Schematic view of the cubic perovskite structure. . . . . . . . . . . . 5<br />
1.3 Phase diagram of temperature versus tolerance factor for the system<br />
R 0.7 A 0.3 MnO 3 , where R is a trivalent rare earth ion and A is a divalent<br />
alkaline earth ion [12]. . . . . . . . . . . . . . . . . . . . . . . . . . . 6<br />
1.4 A schematic of the electronic structure of Mn 3+ (a) and Mn 4+ (b). . . 7<br />
1.5 Schematic features of the double exchange mechanism. . . . . . . . . 8<br />
1.6 Schematic view of the Jahn-Teller effect for the d 4 ion. . . . . . . . . 9<br />
1.7 Schematic representation of a macroscopic phase separated state (a),<br />
as well as possible charge inhomogeneous states stabilized by the longrange<br />
Coulomb interaction (spherical droplets in (b), stripes in (c))<br />
[20]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10<br />
1.8 Different types of spin arrangements. . . . . . . . . . . . . . . . . . . 11<br />
1.9 A Schematic picture of antiferromagnetic CE-type structure with the<br />
charge and e g orbital ordering. . . . . . . . . . . . . . . . . . . . . . . 12<br />
1.10 A schematic picture of the antiferromagnetic pseudo-CE-type structure<br />
in the case of Pr 1−x Ca x MnO 3 (x = 0.4) [38]. . . . . . . . . . . . . . . 13<br />
xvii
1.11 (a) DOS of the one-orbital model on a 10×10 cluster at J H = ∞ and<br />
temperature T = 1/130 (hopping t = 1). The four lines from the top<br />
correspond to densities 0.90, 0.92, 0.94, and 0.97. The inset has results<br />
at 〈n〉 = 0.86, a marginally stable density at T = 0. (b) DOS of the<br />
two-orbital model on a 20-site chain, working at 〈n〉 = 0.7, J H = 8, and<br />
λ = 1.5. Starting from the top at ω − µ = 0, the three lines represent<br />
temperatures 1/5, 1/10 and 1/20 respectively. Both (a) and (b) are<br />
taken from Ref. [40]. (c) DOS using a 20-site chain of the one-orbital<br />
model at T = 1/175, J H = 8, 〈n〉 = 0.87, and at a chemical potential<br />
such that the system is phase separated in the absence of disorder. W<br />
regulates the strength of the disorder, as explained in Moreo et. al.<br />
[41] from where this figure was taken. . . . . . . . . . . . . . . . . . . 14<br />
1.12 Schematic explanation of pseudogap formation at low electron density<br />
[40]. In (a) A typical Monte Carlo configuration of t 2g -spin is sketched.<br />
In (b) the corresponding e g density is shown, with electrons mostly<br />
located in the FM regions. In (c), the effective potential felt by the<br />
electrons is presented. A populated cluster band is formed (thick line).<br />
In (d) the resulting DOS is shown. . . . . . . . . . . . . . . . . . . . 15<br />
1.13 Phase diagram of Pr 1−x Ca x MnO 3 . Figure taken from ref. [44] . . . . 17<br />
1.14 (a) Temperature dependence of the resistivity of Pr 1−x Ca x MnO 3 with<br />
x = 0.3 at various magnetic fields. The inset is the phase diagram<br />
in the temperature magnetic field plane. The hatched area indicates<br />
the hysteretic region [45]. (b) The temperature dependence of the<br />
resistivity of Pr 1−x Ca x MnO 3 (x = 0.5, 0.4, 0.35 and 0.3) at magnetic<br />
fields of 0, 6 and 12 T [44]. . . . . . . . . . . . . . . . . . . . . . . . . 18<br />
2.1 Principle of a modern photoemission experiment. The photon source is<br />
either UV light or X-ray, which hit the sample surface under an angle<br />
ψ with respect to the surface normal. The kinetic energy E kin of the<br />
photoelectrons can be analysed by use of electrostatic analysers. . . . 24<br />
xviii
2.2 Schematic view of the energetics of the photoemission process [3], which<br />
gives the relation between the energy levels in a solid and the electron<br />
energy distribution produced by photons of energy hν. The electron<br />
energy distribution which can be measured by the analyser as a function<br />
of kinetic energy (E kin ) is more conveniently expressed in terms of<br />
the binding energy E B when one refers to the density of states inside<br />
the solid. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25<br />
2.3 ”Universal curve” for electron inelastic mean free path λ. . . . . . . . 30<br />
2.4 Photoemission spectrometer at Institute of Physics, Bhubaneswar. . . 31<br />
2.5 Schematic diagram of the VUV source. . . . . . . . . . . . . . . . . . 32<br />
2.6 Dimensional outline of the VUV source HIS 13. The viewport can be<br />
used for inspection of the discharge and for adjustment of the light<br />
spot position on the sample. The dimensions are in mm. . . . . . . . 32<br />
2.7 Schematic of the X-ray source in operation . . . . . . . . . . . . . . . 33<br />
2.8 Schematic diagram of the EA 125 hemispherical analyser. . . . . . . . 34<br />
2.9 The principle of operation of the concentric hemispherical analyser,<br />
schematic diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35<br />
2.10 Schematic diagram of inverse photoemission spectroscopy (IPES). . . 39<br />
2.11 (a) IPES spectrometer at UGC-DAE consortium for scientific research,<br />
Indore, (b) External view of photon detector attached to the IPES<br />
chamber, and (c) Inside view of the chamber showing a rotatable (left)<br />
and fixed (right) electron gun and the sample holder . . . . . . . . . . 42<br />
2.12 A schematic diagram of the electron gun [30]. . . . . . . . . . . . . . 43<br />
2.13 Schematic cross section of the photon detector showing the window<br />
(1), O-ring (2), cylindrical cap (3), detector tube (4), teflon spacer (5),<br />
anode (6), double sided DN40 CF flanges (7, 10), gas inlet and outlet<br />
(8, 9), barrel connector (11), teflon sleeve (12), DN40 CF flange (13)<br />
and SHV connector (14). . . . . . . . . . . . . . . . . . . . . . . . . . 44<br />
2.14 Schematic diagram of x-ray absorption spectroscopy . . . . . . . . . . 46<br />
2.15 The x-ray loses its intensity via interactions with material. The absorption<br />
coefficient decreases smoothly with higher energy, except for<br />
special photon energies. When the photon energy reaches a critical<br />
value for a core electron transition, the absorption coefficient increases<br />
abruptly. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47<br />
xix
2.16 Schematic of the Si-photodiode. . . . . . . . . . . . . . . . . . . . . . 49<br />
2.17 Schematic of the synchrotron radiation. . . . . . . . . . . . . . . . . . 50<br />
2.18 Schematic representation of an insertion device. . . . . . . . . . . . . 51<br />
2.19 Optical layout of the WERA soft x-ray beam line, ANKA. . . . . . . 52<br />
2.20 Optical layout of the BEAR beam line. . . . . . . . . . . . . . . . . . 53<br />
2.21 Optical layout of the BACH beam line. . . . . . . . . . . . . . . . . . 53<br />
2.22 The variable included angle monochromator : the combined movement<br />
of PM1 (plane mirror) and one of the four spherical gratings SG1(-4)<br />
leads to the monochromatization of the light keeping both the entrance<br />
and exit slit fixed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54<br />
3.1 Valence band photoemission spectra of Pr 0.67 Ca 0.33 MnO 3 taken using<br />
He I photon energy (21.2 eV) at 77 (solid line), 110 (dotted), 150<br />
(dashed), 220 (dot-dashed), and 300 K (double-dot dashed). All the<br />
spectra have been normalized and shifted along y-axis by a constant<br />
for clarity. The subbands around -1.2 (e g↑ ), -3.5 (t 2g↑ ), and -5.6 eV (e g↑<br />
+ t 2g↑ ) are marked as A, B, and C respectively. . . . . . . . . . . . . 63<br />
3.2 High-resolution photoemission spectra of the near-E F region of the<br />
valence band of Pr 0.67 Ca 0.33 MnO 3 . In (a) the spectrum taken below T c<br />
(red) is compared with the normal state (300 K) spectrum (blue). The<br />
difference spectrum (green) obtained by subtracting the spectra at 300<br />
K from 77 K and multiplied by 5 is also shown in the panel. Similarly,<br />
in (b), (c), and (d) the near-E F spectra taken at 110, 150, and 220 K<br />
are compared with that taken at 300 K. The feature in the difference<br />
spectra corresponds to the tail feature A (e g↑ ) in Fig. 3.1. . . . . . . . 64<br />
3.3 Temperature dependence of the area of the three valence band features<br />
obtained from fitting the spectra with Lorentzian line shapes using a<br />
χ 2 iterative program. We have used an integral background, which was<br />
kept the same for all the spectra. The energy positions and FWHMs<br />
were determined by finding the best fit common to all the spectra by the<br />
iterative program. The final fit for all spectra at different temperatures<br />
were obtained with the same energy positions and FWHMs. (a), (b),<br />
and (c) correspond to the features A, B, and C positioned at -1.19,<br />
-3.49, and -5.63 eV with FWHMs 2.56, 1.93, and 1.37 eV respectively. 65<br />
xx
3.4 O K edge x-ray absorption spectra of Pr 0.67 Ca 0.33 MnO 3 taken at 300<br />
(solid line), 150 (dashed line), and 95 K (dotted). The pre-edge feature<br />
centered around 529.5 eV (marked by a box) is where most interest lies.<br />
Since this feature consists of two peaks (a main line and a shoulder on<br />
the low-energy side), this part of the spectrum was fitted with two<br />
components of Lorentzian line shapes. Results of the curve fit are<br />
given in Table 3.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67<br />
3.5 (a) Combined spectra from valence band photoemission (occupied energy<br />
states) and pre-peak in O K edge x-ray absorption spectra (unoccupied<br />
energy states) of Pr 0.67 Ca 0.33 MnO 3 taken at room temperature<br />
(above T c ). The three features corresponding to the subbands in the<br />
valence region are marked A, B, and C.The two peaks of the fitted O<br />
K edge pre-peak are marked A ′ and B ′ . We used integral background<br />
for both sides of E F . Details of the fitting are mentioned in the text.<br />
(b) Combined spectra of Pr 0.67 Ca 0.33 MnO 3 below T c . The valence band<br />
spectrum was taken at 77 K and O K edge XAS was taken at 95 K. . 70<br />
3.6 Schematic diagram of the near-E F energy levels of Pr 0.67 Ca 0.33 MnO 3<br />
in the occupied and unoccupied parts. The diagram is not drawn to<br />
scale. Some of the e g↑ states are occupied (at the Mn 3+ sites) and some<br />
are unoccupied (at the Mn 4+ sites). Hence the last occupied and first<br />
unoccupied states are marked with dotted lines. . . . . . . . . . . . . 71<br />
4.1 Phase diagram of the Pr 1−x Ca x MnO 3 system. In this study we have<br />
used the x = 0.2, x = 0.33 and x = 0.4 compositions. (FMI = ferromagnetic<br />
insulating, CO-AFMI = charge ordered antiferromagnetic<br />
insulating, CG = cluster glass, T N = Neel Temperature, T C = Critical<br />
Temperature, T CO = charge ordering temperature). . . . . . . . . . . 79<br />
4.2 The angle integrated valence band photoemission spectra of the Pr 1−x Ca x MnO 3<br />
(x = 0.0, 0.2, x = 0.33, x = 0.4 and x = 1.0) samples taken at room<br />
temperature and 77K using He I photons (21.2 eV). . . . . . . . . . . 81<br />
xxi
4.3 The near E F region of the valence band spectra (panel(a): 300 K,<br />
panel(b): 77 K) of the three samples plotted against that of CaMnO 3 .<br />
The difference spectra (lower panel) is obtained by subtracting the<br />
spectra of CaMnO 3 from those of the three Pr 1−x Ca x MnO 3 compositions<br />
and multiplied by 5. The difference spectra corresponds to the<br />
density of e g states. The red and blue peaks are A’ and A” respectively.<br />
The green line shows the spectral background used in the fitting. . . . 83<br />
4.4 The IPES spectra of the Pr 1−x Ca x MnO 3 samples (x = 0.2, 0.33 and<br />
0.4) and the CaMnO 3 sample taken at room temperature. The spectra<br />
represents the unoccupied electron states of the samples. . . . . . . . 85<br />
4.5 Panel(a) shows the IPES spectra of the near E F unoccupied states.<br />
Panel(b) shows the difference spectra obtained by subtracting the spectra<br />
corresponding to the x = 0.2 from those of other compositions. . . 86<br />
4.6 The Mn 2p core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken<br />
at room temperature (a) and 77 K (b) . . . . . . . . . . . . . . . . . . 88<br />
4.7 The Ca 2p core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken<br />
at room temperature (a) and 77 K (b). . . . . . . . . . . . . . . . . . 89<br />
4.8 The Pr 4d core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken<br />
at room temperature (a) and 77 K (b). . . . . . . . . . . . . . . . . . 90<br />
5.1 The angle integrated valence band photoemission spectrum of Ca 0.86 Pr 0.14 MnO 3<br />
taken using 124 eV photons. The spectra taken at different temperatures<br />
(30 K, 70 K, 110 K, 220 K and 293 K) are normalized for their<br />
intensities and shifted along Y-axis by a constant for clarity of presentation.<br />
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99<br />
5.2 The high resolution spectra of the near E F region of the valence band<br />
of Ca 0.86 Pr 0.14 MnO 3 for 30 K, 70 K, 110 K, 220 K and 300 K. This<br />
feature refers to the e g band. . . . . . . . . . . . . . . . . . . . . . . . 100<br />
5.3 The O K-edge x-ray absorption spectra of the Ca 0.86 Pr 0.14 MnO 3 taken<br />
at 30 K, 80 K, 150 K, 200 K and 300 K. The pre-edge feature appears<br />
between 527 - 533 eV is mostly due to the strong hybridization between<br />
Mn 3d and O 2p orbitals, where most interest lies. Origin of other<br />
features are given in the text. . . . . . . . . . . . . . . . . . . . . . . 101<br />
5.4 The pre-edge peak of the O K -edge XAS spectra of Ca 0.86 Pr 0.14 MnO 3<br />
for temperatures 30 K, 80 K, 150 K, 200 K and 300 K. . . . . . . . . 102<br />
xxii
Chapter 1<br />
Introduction<br />
1
Introduction 2<br />
1.1 An overview<br />
The perovskite type manganites having general formula R 1−x A x MnO 3 (where R is a<br />
trivalent rare earth element like La, Pr, Nd. and A is a divalent alkaline earth element<br />
like Sr, Ca, Ba.) have been one of the interesting compounds among the transition<br />
metal oxide systems due to their intriguing physical property such as colossal magnetoresistance<br />
(CMR) [1, 2, 3]. CMR is defined as the colossal decrease of resistance by<br />
the application of magnetic field. These materials are insulators at high temperatures<br />
and poor metals at low temperatures. Accompanying this insulator-metal transition<br />
is a magnetic transition from a high temperature paramagnetic phase to a low temperature<br />
ferromagnetic phase. These transitions occurs around the same transition<br />
temperature T C . Applying a magnetic field around T C will greatly reduce the resistivity,<br />
i.e negative magnetoresistance effect. These materials were first described in<br />
1950 by Jonker and van Santen [4] in perovskite type manganites.<br />
One year later (1951), Zener [5, 6] explained this unusual correlation between<br />
magnetism and transport properties by introducing a novel concept, so-called ”Double<br />
exchange” mechanism (DE). The pioneering work of Zener was followed by Anderson<br />
and Hasegawa [7] in 1955 and de Gennes [8] in 1960. Double exchange theory<br />
argues that the electron hopping is related to the relative spin orientations of the<br />
neighbouring sites. Although it qualitatively explains the connection between the<br />
insulator metal and the magnetic transitions, it was noticed latter by A. J. Millis et.<br />
al. [9] that it fails to explain the quantitative change in resistivity and the very low<br />
T C . Thus it is believed that double-exchange mechanism is not enough to explain<br />
the CMR effect. Several mechanisms, such as electron-phonon interactions (the polaronic<br />
effect), electronic phase separation, charge and/or orbital ordering etc. have<br />
been proposed to account for the discrepancy. Although the detailed picture is still<br />
unclear, there seems to be a growing consensus that one or more of these additional<br />
effects probably exists, with the various phenomena both competing and cooperating<br />
with each other. This can lead to very rich and exotic behaviour, as observed in the<br />
manganites. This mixed valence manganites have been studied for more than five<br />
decades but are still considered modern materials because of their wide potential for<br />
technological applications. Potential applications of the CMR effect in these manganites<br />
include magnetic sensors, magnetoresistive read heads, magnetoresistive random<br />
access memory (MRAM) etc.
Introduction 3<br />
The temperature and doping dependent metal-insulator transitions in these materials<br />
are found to be closely related to the unique electronic structure derived from<br />
the Mn 3d and O 2p hybridized orbitals of MnO 6 octahedra. The 3d orbitals of Mn<br />
in the MnO 6 octahedra, which are split by the crystal field into t 2g and e g states,<br />
are further split by the Jahn-Teller distortion. With charge carrier doping, many of<br />
them show well defined charge ordering (CO) and/or orbital ordering (OO) at low<br />
temperatures, especially when x equals a commensurate value (e.g x = 0.5). The CO<br />
state where the Mn 3+ (t 3 2ge 1 g) and Mn 4+ (t 3 2g) ions arranged like in a checkerboard was<br />
found to bear a strong influence on the one electron band width (W) of the e g band<br />
and the transfer interaction of the e g holes (electrons).<br />
The work presented in this thesis consists of the investigations of the electronic<br />
structure of Pr 1−x Ca x MnO 3 using different spectroscopic techniques as photoemission<br />
spectroscopy, inverse photoemision spectroscopy and x-ray absorption spectroscopy<br />
etc. The thesis is structured as follows : Following this overview, the Chapter 1 gives<br />
a general idea about the physical properties (structural, electronic and magnetic) and<br />
the mechanisms associated with the CMR manganites. Chapter 2 describes the principles<br />
and operations of the experimental techniques used for this thesis work. Chapter<br />
3 presents the electronic structure of Pr 0.67 Ca 0.33 MnO 3 at different temperatures<br />
using ultra-violet photoelectron and x-ray absorption spectroscopy. The electronic<br />
structure of the Pr 1−x Ca x MnO 3 series at different temperatures using ultra-violet<br />
photoemission and inverse photoemission spectroscopy are discussed in Chapter 4.<br />
In Chapter 5, the electronic structure of Ca 0.86 Pr 0.14 MnO 3 at different temperatures<br />
using high resolution photoemission and x-ray absorption spectroscopy are discussed.<br />
Chapter 6 gives the summary and conclusions of this thesis.<br />
1.2 Colossal magnetoresistance<br />
Colossal magnetoresistance is a property of some materials, which is the gigantic<br />
decrease of resistance by the application of magnetic field. The magnetoresistance<br />
can be defined as,<br />
MR =<br />
R(H) − R(0)<br />
, (1.1)<br />
R(0)<br />
where R(H) and R(0) are the resistances with and without magnetic field H respectively.<br />
Expressing the results as a percentage (i.e multiplying by an additional factor<br />
100), it has been shown by Jin et. al. [10] that the MR value as large as -100,000
Introduction 4<br />
Figure 1.1: Temperature dependence of the resistivity of La 1−x Sr x MnO 3 (x = 0.175)<br />
under various magnetic fields [2].<br />
% near 77 K. Which corresponds to thousand-fold magnetoresistance. The temperature<br />
dependent resistivity of La 1−x Sr x MnO 3 (x = 0.175) single crystal under several<br />
magnetic fields are shown in the Fig. 1.1.<br />
1.3 Crystal structure<br />
The CMR samples used in this thesis work are of perovskite type (ABO 3 ) structure.<br />
The general formula for perovskite type manganites is R 1−x A x MnO 3 , where R is the<br />
trivalent rare earth element such as La, Pr, Nd, and Sm etc. and A is the divalent<br />
alkaline earth ion such as Sr, Ca, and Ba etc. An ideal cubic perovskite structure is<br />
shown in the Fig. 1.2, which contains a MnO 6 octahedron, with one Mn ion at the<br />
center and oxygen ions at each of the six corners. The trivalent or divalent ions (R<br />
or A) are situated at the corners of the cube. The stability of the cubic perovskite<br />
structure depends strongly on the size of the cations (R, A or Mn). If there is a<br />
size mismatch occurs between the cations, then the cubic perovskite structure gets
Introduction 5<br />
Figure 1.2: Schematic view of the cubic perovskite structure.<br />
distorted. This size mismatch is governed by a factor called tolerance factor (t), and<br />
is defined by Goldschmidt [11] as,<br />
t = (r A + r O )<br />
√<br />
2(rMn + r O ) , (1.2)<br />
where r A , r Mn and r O are the ionic radii of the A-site(R or A), Mn- and O- atoms<br />
respectively. For cubic perovskite structure t = 1, where the Mn-O-Mn bond angles<br />
are relatively straight. Due to the ionic size mismatch the Mn-O-Mn bond angles<br />
deviate from 180 o and lead to distorted perovskite structure. When t decreases (0.96 <<br />
t < 1), the cubic structure transforms to rhombohedral and then to the orthorhombic<br />
structure (t < 0.96). The properties of manganites strongly depend on the tolerance<br />
factor. In 1995 Hwang et. al. [12] has reported the structure-property relationship<br />
as a function of tolerance factor for 30 % dopant concentration with a variety of<br />
rare earth and alkaline earth ions, shown in the Fig. 1.3. It shows the presence<br />
of three regimes: a paramagnetic insulator at high temperature, a low-temperature<br />
ferromagnetic metal at large tolerance factor and a low-temperature ferromagnetic<br />
insulator at small tolerance factor.<br />
1.4 The basic electronic structure<br />
In manganites the electronically active orbitals are the Mn 3d orbitals. In the cubic<br />
lattice, the five-fold degenerate 3d-orbitals of an isolated Mn atom or ion are split into
Introduction 6<br />
Figure 1.3: Phase diagram of temperature versus tolerance factor for the system<br />
R 0.7 A 0.3 MnO 3 , where R is a trivalent rare earth ion and A is a divalent alkaline earth<br />
ion [12].<br />
a triple degenerate (lower energy levels) d xy , d yz , and d zx orbitals, called t 2g states<br />
and a double degenerate (higher energy levels) d x 2 −y 2 and d 3z 2 −r 2 orbitals, called e g<br />
states [13]. Energy separation between the t 2g states and e g states is 10Dq due to the<br />
crystal field splitting of MnO 6 octahedra. The schematic of the electronic structure<br />
from ionic point of view are shown in the Fig. 1.4. Since the e g orbitals point towards<br />
the six O atoms, they are expected to have a larger hybridization with the O 2p<br />
states (p x , p y , and p z ) and hence are more delocalized than the t 2g states. So the e g<br />
state electrons play an important role of conduction electrons in this type of crystals.<br />
In Mn 3+ ions three electrons occupy the t 2g states and one electron occupies the e g<br />
states. The strong Hund’s coupling (J H ) in these systems favors the alignment of<br />
the e g electron spins with the core-like t 2g spins. When the manganites are doped<br />
with holes through chemical substitution, the Mn 4+ ions are formed. In Mn 4+ ions,<br />
there are only three electrons occupying the t 2g orbitals. When the excess electron is<br />
present in case of Mn 3+ ion, a further distortion known as the Jahn-teller distortion<br />
may lower the crystal symmetry and break the degeneracy of the e g states. This can<br />
be energetically favorable as the excess electron will fill the lower energy orbital while<br />
the upper energy orbital remains empty.
Introduction 7<br />
Figure 1.4: A schematic of the electronic structure of Mn 3+ (a) and Mn 4+ (b).<br />
1.5 Double exchange mechanism<br />
The starting point for understanding the properties of manganites is the so-called<br />
”double” exchange (DE) mechanism and was first proposed by C. Zener [5, 6] in<br />
1951, which says the simultaneous hopping of e g elctrons from Mn 3+ sites to O 2−<br />
sites and then from O 2− to the empty e g orbitals of Mn 4+ sites. This transfer of e g<br />
electron from Mn 3+ to Mn 4+ by DE is the basic mechanism of electrical conduction<br />
in manganites. In 1955, the DE theory was expanded by P. W. Anderson, and H.<br />
Hasegawa [7], where the hopping probability (t ij ) of the e g electron from Mn 3+ to<br />
neighbouring Mn 4+ is,<br />
t ij = t cos( θ ij<br />
), (1.3)<br />
2<br />
where θ ij is the angle between the Mn spins in the case of strong Hund’s coupling.<br />
The concept of double exchange mechanism is illustrated in Fig. 1.5. For parallel spin<br />
configuration (θ ij = 0) the hopping probability is maximum and for antiparallel spin<br />
(θ ij = 180 o ), the hopping probability is zero, that means the hopping cancels. Thus it<br />
predicts maximum conductivity at ferromagnetic state where as insulating behaviour<br />
at antiferromagnetic state. This provides the qualitative explanation for the close<br />
connection between the insulator-metal transition and the magnetic transition. It<br />
also explains why the external magnetic field will increase the conductivity since θ ij<br />
will be reduced as the external magnetic field polarizes the core (t 2g ) spins of the<br />
Mn ions. Using mean-field approximations, P. -G. degennes [8] in 1960 suggested<br />
that the interpolation between the antiferromagnetic state of the undoped limit and<br />
the ferromagnetic state at finite hole density, where the DE mechanism works, occurs
Introduction 8<br />
Figure 1.5: Schematic features of the double exchange mechanism.<br />
through a ”canted” state, similar as the state produced by a magnetic field acting over<br />
an antiferromagnetic state. Although DE mechanism gives an intuitive explanation<br />
for the coupling between the spin and charge degrees of freedom, it fails to predict<br />
the correct magnitude of the change in resistivity. For paramagnetic case (T > T C ),<br />
θ ij = 90 o , and so the hopping probability will be reduced to cos ( 90o ) = 0.7 times<br />
2<br />
that in the ferromagnetic case. While experimentally it is seen that the conductivity<br />
decreases across the ferro - para transition may be many orders of magnitude. Which<br />
says that some other mechanisms beyond DE are necessary to explain the conductivity<br />
change across the transition.<br />
1.6 Jahn-Teller effect in CMR manganites<br />
A. J. Millis et. al. [9] in 1995 pointed out that, the DE model alone is not enough to<br />
explain the CMR effect in manganites, where he argued that DE produces wrong T C<br />
by a large factor and the resistivity that grows with reducing temperature (insulating<br />
behaviour) even below T C . By taking these reasons Millis et. al. [9] concluded that<br />
the model based only on a large J H is not adequate for the manganites and claimed<br />
that the necessity of Jahn-Teller phonons to explain the change in curvature of the<br />
resistivity close to T C . The e g electrons on a given MnO 6 octahedron are strongly<br />
coupled to the distortions of the O 6 octahedron. In the pseudocubic materials such<br />
as La 1−x Ca x MnO 3 the strong coupling is due in part to the Jahn-Teller effect, the
Introduction 9<br />
Figure 1.6: Schematic view of the Jahn-Teller effect for the d 4 ion.<br />
essence of which is that a local distortion in which some Mn-O bonds get shorter and<br />
other gets longer breaks the local cubic symmetry and therefore splits the degeneracy<br />
of the e g level on that site (Fig. 1.6). If only one electron is present then it will in<br />
the absence of hybridization reside in the lower level and gain energy. For example,<br />
a distortion in which the Mn-O bond get longer in the ±z direction and get shorter<br />
in the ±x,y directions raise the energy of the d x 2 −y2 orbital and lowers the energy of<br />
the d 3z 2 −r 2 orbital (Q 3 mode). Millis et. al. [14, 15, 17, 18] argued that the physics of<br />
manganites is dominated by the interplay between a strong electron-phonon coupling<br />
and the large Hund’s coupling effect that optimizes the electronic kinetic energy by the<br />
generation of a FM-phase. The large value of electron-phonon coupling in manganites<br />
is clear in the regime below x = 0.2, where a static JT distortion plays a key role in the<br />
physics of the material and a dynamical JT effect may persists at higher hole densities<br />
[16], without leading to long-range order but producing important fluctuations that<br />
localize electrons by splitting the degenerate e g levels at a given MnO 6 octahedron.<br />
Millis et. al [14] argued that the ratio λ eff = E JT<br />
t eff<br />
(coupling strength) dominates<br />
the physics of the problem. Here E JT is the static trapping energy at a given octahedron,<br />
and t eff is an effective hopping that is temperature dependent following the<br />
standard DE discussion. In this context it was conjectured that when the temperature<br />
is larger than T C the effective coupling λ eff could be above the critical value that<br />
leads to the insulating behavior due to electron localization, while it becomes smaller
Introduction 10<br />
Figure 1.7: Schematic representation of a macroscopic phase separated state (a), as<br />
well as possible charge inhomogeneous states stabilized by the long-range Coulomb<br />
interaction (spherical droplets in (b), stripes in (c)) [20].<br />
than the critical value below T C , thus inducing metallic behavior. The calculations<br />
were carried out using classical phonons and t 2g spins. The results of Millis et. al.<br />
[14] for T C and the resistivity at a fixed density n = 1 when plotted as a function of<br />
λ eff had formal similarities with experimental results (which are produced as a function<br />
of density). In particular, if λ eff is tuned to be very close to the metal-insulator<br />
transition, the resistivity naturally strongly depends on even small external magnetic<br />
fields.<br />
1.7 Phase separation<br />
Another possibly complementary route to understand the CMR effect in manganites<br />
is the phase separation. It has been recently predicted from computational studies<br />
of realistic models that an electronic phase separation can occur in manganites in a<br />
certain range of doping [19]. In particular at low doping level and at low temperature,<br />
a phase separation between hole-poor antiferromagnetic (AFM) regions and hole-rich<br />
ferromagnetic (FM) regions is energetically more favourable than the homogeneous<br />
canted AFM phase. The energy of charge carrier is minimal for FM ordering. With<br />
a density insufficient for establishing the FM ordering in the entire sample, the carriers<br />
concentrate into droplets or stripes which become FM inside insulating AFM<br />
matrix [20](Fig. 1.7). The fluctuations length scales range from nm to µm. This<br />
point towards the conduction in metallic phase and the CMR effect is argued as a
Introduction 11<br />
Figure 1.8: Different types of spin arrangements.<br />
result of percolation [21, 22, 23]. On increasing the doping level the FM regions become<br />
connected to each other and a low electrical resistance is achieved above the<br />
percolation threshold. There are many reports about the coexistence of FM clusters<br />
in the CO AFM insulating phase of La 1−x Ca x MnO 3 using different experimental<br />
techniques [23, 24, 25, 26, 27]. And very recently the neutron diffraction and the<br />
inelastic neutron scattering studies of Pr 1−x Ca x MnO 3 have shown the coexistence of<br />
ferromagnetic fluctuations in antiferromagnetic domains [28, 29, 30, 31].<br />
1.8 Spin, Charge and Orbital ordering in manganites<br />
The unusual properties of manganites challenge the current topic of research in the<br />
field of strongly correlated electrons system due to their strong spin, charge and<br />
orbital degrees of freedom. The strong interplay between spin, charge and orbital<br />
ordering originating from the single occupancy of the doubly degenerate e g orbitals<br />
of Mn 3+ (t 3 2g e1 g ) ions make the phase diagram of these compounds rich with physics.<br />
With charge carrier doping, some of them for example, La 1−x Ca x Mon 3 system show<br />
strong charge and/or orbital ordered (CO/OO) insulating nature at low temperatures,<br />
especially when x equals a commensurate value. The CO state where the Mn 3+ (t 3 2ge 1 g)<br />
and Mn 4+ (t 3 2g ) are arranged like a checkerboard influences strongly the one electron<br />
bandwidth of the e g band [32] and the transfer interaction of the e g holes (electrons).<br />
There are various types of ordering occurs across the phase diagram of manganites,<br />
which are described below.<br />
The lattice distortion in manganites is due to the J-T interaction acting on the<br />
Mn 3+ ion, which lifts the degenerate e g orbitals in the cubic electronic field. The J-T
Introduction 12<br />
Figure 1.9: A Schematic picture of antiferromagnetic CE-type structure with the<br />
charge and e g orbital ordering.<br />
distorted Mn 3+ O 6 in LaMnO 3 form an orthorhombic structure (space group P bnm )<br />
where the e g charge occupies alternating 3d 3x 2 −r 2 and 3d 3y 2 −r2 orbitals. The magnetic<br />
structure of LaMnO 3 was found to be of A-type antiferromagnetic, where all Mn 3+<br />
moments lying in c-plane are aligned ferromagnetically and the the FM layer stacks<br />
antiferromagnetically along the c-axis by the weaker superexchange interaction along<br />
this axis. The A-type AFM ordering exists below 140 K [33]. Different types of spin<br />
arrangements are shown in the Fig. 1.8. Upon substituting divalent cations such<br />
as Ca, Sr, Pb etc. to the parent compound like LaMnO 3 , the magnetic structure<br />
changes from A-type AFM to the FM with the rotation of the spin direction from<br />
the b-axis in the c-plane towards the c-axis associated with the metallic conduction,<br />
which was interpreted successfully in terms of DE mechanism at an early stage of the<br />
investigation. Further increase of doping, the charge ordered (CO) state near x = 0.5<br />
is observed. For example in La 1−x Ca x MnO 3 (x = 0.5), the ordering becomes C-E type<br />
(Fig. 1.9), where the Mn 3+ and Mn 4+ line up forming alternate stripes in the C plane<br />
and stack also along the c-axis. This phenomenon in manganites was first observed<br />
by Wollan and Koehler [34] in the neutron powder diffraction of La 0.5 Ca 0.5 MnO 3 .<br />
Later these charge-ordering phases are observed in the low band width compound<br />
Pr 1−x Ca x MnO 3 with a broad doping region 0.3 < x < 0.75 [28, 35, 36, 37, 38].<br />
The CO state for Pr 0.6 Ca 0.4 MnO 3 is called pseudo-CE type ordering [38] is shown
Introduction 13<br />
Figure 1.10: A schematic picture of the antiferromagnetic pseudo-CE-type structure<br />
in the case of Pr 1−x Ca x MnO 3 (x = 0.4) [38].<br />
in Fig. 1.10, where the arrangement of spin is canted along the c-axis while the<br />
CE-type ordering is kept within the orthorhombic ab-plane. For the ground state<br />
of R 1−x A x MnO 3 , i.e AMnO 3 (x = 1), the magnetic structure is called G-type AFM<br />
[34, 39] ordering in which the spins on all Mn 4+ sites are antiparallel to their nearest<br />
neighbours.<br />
1.9 Pseudogap behavior in manganites<br />
The conducting properties of the materials depend on the finer changes associated<br />
with the density of states (DOS) near the Fermi level. If there are no allowed states<br />
at the Fermi level a gap forms by making the system insulating and finite density of<br />
states makes the system metallic. A Pseudogap exists in the mixed-phase regimes of<br />
manganites, which suggests that the prominent depletion of spectral weight (density<br />
of states) at the chemical potential, but not exactly zero (few states associated with<br />
it). This feature is similar to that extensively discussed in high T c superconductor.<br />
The calculations in the pseudogap context in manganites was carried out by Moreo<br />
et. al. [40, 41] using both one- and two-orbital models (Fig. 1.11). The density of<br />
states of the one-orbital model on 2D cluster varying the electronic density slightly<br />
below 〈n〉 = 1.0 is shown in the Fig. 1.11(a). At zero temperature, this density<br />
regime is unstable due to phase separation, but at the temperature of simulation<br />
those densities still correspond to stable states, but with a dynamical mixture of
Introduction 14<br />
Figure 1.11: (a) DOS of the one-orbital model on a 10×10 cluster at J H = ∞ and<br />
temperature T = 1/130 (hopping t = 1). The four lines from the top correspond to<br />
densities 0.90, 0.92, 0.94, and 0.97. The inset has results at 〈n〉 = 0.86, a marginally<br />
stable density at T = 0. (b) DOS of the two-orbital model on a 20-site chain, working<br />
at 〈n〉 = 0.7, J H = 8, and λ = 1.5. Starting from the top at ω − µ = 0, the three<br />
lines represent temperatures 1/5, 1/10 and 1/20 respectively. Both (a) and (b) are<br />
taken from Ref. [40]. (c) DOS using a 20-site chain of the one-orbital model at<br />
T = 1/175, J H = 8, 〈n〉 = 0.87, and at a chemical potential such that the system is<br />
phase separated in the absence of disorder. W regulates the strength of the disorder,<br />
as explained in Moreo et. al. [41] from where this figure was taken.
Introduction 15<br />
Figure 1.12: Schematic explanation of pseudogap formation at low electron density<br />
[40]. In (a) A typical Monte Carlo configuration of t 2g -spin is sketched. In (b) the<br />
corresponding e g density is shown, with electrons mostly located in the FM regions.<br />
In (c), the effective potential felt by the electrons is presented. A populated cluster<br />
band is formed (thick line). In (d) the resulting DOS is shown.<br />
AFM and FM features. A clear minimum in the density of states at the chemical<br />
potential can be observed. Very similar results also appear in 1D simulations [40].<br />
Fig. 1.11(b) shows the results for two-orbitals and a large electron-phonon coupling,<br />
at a fixed density and changing temperature. Clearly a pseudogap develop in the<br />
system as a precursor of the phase separation that is reached as the temperature is<br />
further reduced. Similar results have been obtained in other parts of parameter space,<br />
as long as the system is near unstable phase separated regimes. A pseudogap appears<br />
also in cases where disorder is added to the system. Fig. 1.11(c) shows the results<br />
for the case where a random on-site energy is added to the one orbital model.<br />
Moreo et. al. [40] explained this phenomenon tentatively for the case of ”without<br />
disorder”. A schematic of explanation is shown in the Fig. 1.12. In Fig. 1.12(a) a<br />
typical mixed phase FM-AFM state is sketched. In the FM regions, the e g electrons<br />
improve their kinetic energy, and thus they prefer to be located in those regions as<br />
shown in Fig. 1.12(b). The FM domains act as effective attractive potentials for<br />
electrons, as shown in Fig. 1.12(c). When other electrons are added FM clusters<br />
are created and new occupied levels appear below the chemical potential, creating<br />
a pseudogap [Fig. 1.12(d)]. These results are compatible with the photoemission
Introduction 16<br />
experiments by Dessau et. al. [42] for bilayer manganites (La 2−2x Sr 1+2x Mn 2 O 7 , x =<br />
0.4). In this experiment it was observed that the low-temperature ferromagnetic<br />
state was very different from a prototypical metal. Its resistivity is unusually high,<br />
the width of ARPES features are anomalously broad, and they don’t sharpen as they<br />
approach the Fermi momentum. In addition, the centroid of the near E F peak never<br />
approach close than approximately 0.65 eV to the Fermi level. This implies that<br />
even in the expected metallic regime, the density of states at the Fermi level is very<br />
small. Dessau et. al. [42] refers to these results as the formation of a pseudogap. This<br />
effect is present both in ferromagnetic and paramagnetic regimes. Scanning tunneling<br />
microscopy data of single crystals and thin films of hole doped manganites by Biswas<br />
et. al. [43] showed a rapid variation in the density of states for the temperatures<br />
near the Curie temperature, such that below T c a finite density of states is observed<br />
at the Fermi level while above T c a hard gap opens up. This result suggests that<br />
the presence of a gap or pseudogap is not just a feature of bilayers, but it appears in<br />
other manganites as well.<br />
1.10 Basic properties, phase diagram and CMR<br />
effect in Pr 1−x Ca x MnO 3 system<br />
The Pr 1−x Ca x MnO 3 is a low band width manganite. The band width W is smaller<br />
than in other manganites like La 1−x Sr x MnO 3 and La 1−x Ca x MnO 3 system. In the lowbandwidth<br />
compounds, a charge-ordered (CO) state is stabilized in the vicinity of x =<br />
0.5, while manganite with large W (La 1−x Sr x MnO 3 as example) present a metallic<br />
phase at this concentration. The Pr 1−x Ca x MnO 3 presents a stable CO state in a<br />
broad density region between x = 0.3 and 0.75, as showed by Jirak et. al [38]. Part of<br />
the phase diagram is shown in the Fig. 1.13 [44]. A ferromagnetic insulating (FMI)<br />
state exists in the range from x = 0.15 to 0.30. For x≥0.30, an antiferromagnetic CO<br />
state is stabilized. Neutron diffraction studies by Jirak et. al. [38] showed that at<br />
all densities between 0.30 and 0.75, the arrangement of charge/spin/orbital order of<br />
this state is similar to the CE-state (Fig. 1.9). However, certainly the hole density is<br />
changing with x, and as consequence the CE-state can not be perfect at all densities<br />
but electrons have to be added or removed from the structure. Jirak et. al. [38]<br />
discussed a ”pseudo”-CE-type structure for x = 0.4 that has the proper density.<br />
The effect of magnetic fields on the CO-state of Pr 1−x Ca x MnO 3 is remarkable. The
Introduction 17<br />
Figure 1.13: Phase diagram of Pr 1−x Ca x MnO 3 . Figure taken from ref. [44]<br />
resistivity vs. temperature graph of Pr 1−x Ca x MnO 3 (x = 0.3) under various magnetic<br />
fields [45] is shown in the Fig. 1.14(a). At low temperatures, changes in resistivity<br />
by several orders of magnitude can be observed. The stabilization of metallic state is<br />
realized upon the application of magnetic field. This state is ferromagnetic according<br />
to magnetization measurements, and thus it is curious to observe that a state not<br />
present at zero field in the phase diagram, is nevertheless stabilized at finite fields.<br />
The magnetic field effects on the other compositions of Pr 1−x CaxMnO 3 (x = 0.35,<br />
0.4 and 0.5) are also shown in the Fig. 1.14(b). The shape of the curves in Fig. 1.14<br />
present a large magnetoresistance effect, and a possible origin based on percolation<br />
between the CO- and FM-phase. Tomioka et. al. [46] showed that, as x grows away<br />
from x = 0.3, larger fields are needed to destabilize the charge-ordered state at low<br />
temperatures (e.g., 27 T at x = 0.50 compared with 4 T at x = 0.30).<br />
This thesis is based on the study of electronic structure of Pr 1−x Ca x MnO 3 series<br />
using different spectroscopic techniques.
Introduction 18<br />
Figure 1.14: (a) Temperature dependence of the resistivity of Pr 1−x Ca x MnO 3 with<br />
x = 0.3 at various magnetic fields. The inset is the phase diagram in the temperature<br />
magnetic field plane. The hatched area indicates the hysteretic region [45]. (b) The<br />
temperature dependence of the resistivity of Pr 1−x Ca x MnO 3 (x = 0.5, 0.4, 0.35 and<br />
0.3) at magnetic fields of 0, 6 and 12 T [44].
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Chapter 2<br />
Experimental Techniques<br />
23
Experimental Techniques 24<br />
2.1 Introduction<br />
Spectroscopic methods are some of the most powerful techniques for studying the<br />
electronic structure of solids. This chapter is intended to describe the details of the<br />
different spectroscopic techniques which have been used in this thesis work.<br />
2.2 Photoemission Spectroscopy<br />
2.2.1 Basic principle<br />
The principle of photoemission spectroscopy is based on the principle of photoelectric<br />
effect. The photoelectric effect was first discovered by the German Physicist Heinrich<br />
Hertz [1] in 1887 and latter explained by Albert Einstein [2] in 1905 as a manifestation<br />
of the quantum nature of light.<br />
Figure 2.1: Principle of a modern photoemission experiment. The photon source is<br />
either UV light or X-ray, which hit the sample surface under an angle ψ with respect<br />
to the surface normal. The kinetic energy E kin of the photoelectrons can be analysed<br />
by use of electrostatic analysers.<br />
When light is incident on a sample, electron can absorb the photon and escape<br />
from the sample with a maximum kinetic energy as shown in the Fig. 2.1. If a photon<br />
having energy hν is absorbed by the electron of binding energy E B , then the kinetic<br />
energy E kin of the electron is given by,<br />
E kin = hν − E B − φ, (2.1)<br />
where φ is the work function of the material. The resulting photoelectron spectra<br />
is a plot of the number of emitted electrons versus their kinetic energy, which<br />
gives the information about the electronic structure of the material. Fig. 2.2 shows
Experimental Techniques 25<br />
Figure 2.2: Schematic view of the energetics of the photoemission process [3], which<br />
gives the relation between the energy levels in a solid and the electron energy distribution<br />
produced by photons of energy hν. The electron energy distribution which<br />
can be measured by the analyser as a function of kinetic energy (E kin ) is more conveniently<br />
expressed in terms of the binding energy E B when one refers to the density<br />
of states inside the solid.<br />
schematically how the energy level diagram and the energy distribution of photoemitted<br />
electrons relate to each other.<br />
2.2.2 Theoretical interpretation of photoemission<br />
There have been many important theoretical studies to describe and analyse the spectral<br />
features in photoemission process [3, 4, 5, 6]. The most general and widely applied<br />
theoretical description of the photoemission spectrum is based on using Fermi’s golden<br />
rule as a result of perturbation theory in first order. The photo current produced in<br />
the photoemission spectroscopy experiment is due to the excitation of electrons from<br />
the initial states i with wave function ψ i to the final states f with wave function ψ f<br />
by the photon field having vector potential A. According to the Fermi’s Golden Rule,
Experimental Techniques 26<br />
the transition probability can be written as,<br />
w ∝ 2π¯h |〈ψ f|r|ψ i 〉| 2 δ(E f − E i − hν) = 2π¯h m fiδ(E f − E i − hν) (2.2)<br />
where m fi is the square of the transition matrix element. To discuss about the<br />
transition matrix element, one has to take certain assumptions of the wave functions<br />
contained in it. In the simplest approximation one can take a one electron view for<br />
the initial and final state wave functions. The final state is in addition with a free<br />
electron having kinetic energy E kin . The initial state wave function is then written as<br />
a product of the orbital φ k from which the electron is excited and the wave function<br />
of the remaining electrons ψi,R(N k −1), assuming that system under consideration has<br />
N electrons, where the index k indicates that the electron k has been left out and R<br />
refers to the remaining. Now one can write,<br />
ψ i (N) = Cφ i,k ψ k i,R(N − 1) (2.3)<br />
where C is the operator that antisymmetrizes the wave function properly.<br />
Similarly the final state can be written as a product of the wave function of the<br />
photoemitted electron φ f,Ekin and that of the remaining N −1 electrons ψf,R k (N −1),<br />
ψ f (N) = Cφ f,Ekin ψf,R k (N − 1) (2.4)<br />
So the transition matrix element can be written as,<br />
〈ψ f |r|ψ i 〉 = 〈φ f,Ekin |r|φ i,k 〉〈ψf,R(N k − 1)|ψi,R(N k − 1)〉 (2.5)<br />
Thus the matrix element is the product of one electon matrix element and the<br />
(N − 1) electron overlap integral. Let us assume that the remaining orbitals (called<br />
the passive orbitals) are the same in the final state as they were in the initial state,<br />
meaning ψf,R(N k − 1) = ψi,R(N k − 1), which renders the overlap integral unity and<br />
the transition matrix element is just the one-electron matrix element. Under this<br />
assumption [7] the PES experiment measures the negative Hartree-Fock orbital energy<br />
of the orbital k meaning,<br />
which is sometimes called Koopman’s binding energy.<br />
E B,k ≃ − ǫ k , (2.6)
Experimental Techniques 27<br />
Let us now assume that the final state with N − 1 electrons has s excited states<br />
with the wave function ψf,s k and energy E s (N − 1).<br />
The transition matrix element can be calculated by summing over all the possible<br />
excited final states yielding,<br />
〈ψ f |r|ψ i 〉 = 〈φ f,Ekin |r|φ i,k 〉 ∑ s<br />
c s (2.7)<br />
with<br />
c s = 〈ψf,s k (N − 1)|ψk i,R (N − 1)〉 (2.8)<br />
Here |c s | 2 is the probability that removal of electron from orbital φ k of the N<br />
electron ground state leaves the system in the excited state s of the N − 1 electron<br />
system. For strongly correlated systems many of the c s ’s are nonzero. In terms of PE<br />
spectrum this means that for s = k one has the so-called main line and for the other<br />
non-zero c s additional so-called satellite lines occur. If the correlations are weak, then<br />
ψ k f,s(N − 1)≃ψ k i,R(N − 1) (2.9)<br />
meaning that c s 2 ≃ 1 for s = k and c s 2 ≃ 0 for s ≠ k, i.e one has only one peak.<br />
Now the photocurrent I detected in a PE experiment as,<br />
I∝ ∑ |〈φ f,Ekin |r|φ i,k 〉| 2∑<br />
f,i,k<br />
s<br />
|c s | 2 δ(E f,kin + E s (N − 1) − E 0 (N) − hν, (2.10)<br />
where E 0 (N) is the ground state energy of the N-electron system. The photo<br />
current thus consists of the lines created by photoionizing the various orbitals k where<br />
each line can be accompanied by satellites according to the number of excited states<br />
s created in the photoexcitation of that particular orbital k.<br />
The above discussion is very convenient for atoms and molecules [4]. However,<br />
it is not always easy to apply for solids. For solids, a slightly different formalism<br />
has been developed [3, 8, 9] and hence the above equation can be rewritten in the<br />
following form<br />
I∝ ∑ f,i,k|〈φ f,Ekin |r|φ i,k 〉| 2 A(k, E) (2.11)<br />
where A(k, E) is the so-called spectral function for the wave number k and energy<br />
E. The spectral function can be related to the single particle Green’s function by
Experimental Techniques 28<br />
A(k, E) = π −1 Im{G(k, E)} (2.12)<br />
Here A(k, E) describes the probability of removing (below the E F ) or adding<br />
(above the E F ) an electron with the energy E and the wave vector k from (to) the<br />
N-electron system (Interacting).<br />
For a non interacting system with the one electron energy Ek,<br />
0<br />
G 0 (K, E) =<br />
where ǫ is very small<br />
and<br />
1<br />
(2.13)<br />
E − Ek 0 − iǫ,<br />
A 0 (k, E) = δ(E − E 0 k) (2.14)<br />
which means the spectral function is a δ-function at E = Ek 0 . This result is<br />
identical to that of equation (2.6) as in a noninteracting system the binding energy<br />
is by definition the Koopman’s binding energy.<br />
In an interacting electron system the electron energy gets renormalized by the<br />
so-called self energy Σ(k, E) = Re{Σ(k, E)} + iIm{Σ(k, E)}, yielding<br />
and<br />
G(K, E) =<br />
1<br />
E − E 0 k − Σ(k, E) (2.15)<br />
A(k, E) = 1 π<br />
Im{Σ(k, E)}<br />
[E − E 0 k − Re{Σ(k, E)}]2 + [Im{Σ(k, E)}] 2 (2.16)<br />
If we assume that the spectrum of the interacting system is not too different<br />
from that of the noninteracting system (Im{Σ(k, E)}), then the poles in the Green’s<br />
function of the interacting one occur at<br />
with the condition<br />
E 1 k = Re{E1 k } + iIm{E1 k } (2.17)<br />
E 1 k − E0 k − Σ(k, E1 k ) = 0 (2.18)<br />
This allows to separate the Greens function into two parts, a pole part and a<br />
non-singular term G inc , i.e,
Experimental Techniques 29<br />
and<br />
G(k, E) =<br />
Z k<br />
E − (Re{E 1 k } + iIm{E1 k }) + G inc (2.19)<br />
A(k, E) = 1 π<br />
Z k Im{E 1 k }<br />
(E − Re{E 1 k })2 + (Im{E 1 k })2 + A inc (2.20)<br />
where Z k is a number smaller than one. The first term in (2.19) is similar to<br />
the result for the noninteracting system, and one can therefore say that it describes<br />
a slightly modified electron (with renormalized mass) which is generally called a<br />
quasiparticle. The first term in (2.19) is called the coherent part of the Green’s<br />
function, it gives rise to the coherent part of the spectrum. And the second term in<br />
(2.19) is called the incoherent part of the Green’s function, which gives rise to the<br />
incoherent part of the spectrum. In a very approximate way one can identify the<br />
coherent part of the spectrum with the main line and the incoherent one with the<br />
satellites in equation (2.20) [3].<br />
2.2.3 The electron escape depth<br />
In photoemission spectroscopy, when electrons are impinged on, or emitted from a<br />
sample surface, the fundamental phenomenon occurs is inelastic scattering of electrons.<br />
The escape depth of electron from the sample surface is depend on the inelastic<br />
mean free path (IMFP) λ, which is the average distance between the scattering<br />
events. λ is small for low energy electrons in solids and hence gives rise to surface<br />
sensitivity. So ultra - high vacuum (UHV) condition is necessary to study the solid<br />
surfaces [3, 10, 11]. Because a monolayer is adsorbed on the surface of the sample<br />
in about 2.5 Langmuirs (1L = 10 −6 torr.s). At a pressure of 10 −9 torr it therefore<br />
takes about 1000 seconds, or less than an hour, until a coverage of one monolayer is<br />
achieved. Thus in order to get spectra of clean surfaces, vacuum of even better than<br />
10 −10 torr is mandatory. The IMFP of an electron in a solid is a strong function of<br />
its kinetic energy [3, 12, 13], and the relationship termed as the ”universal curve” as<br />
shown in Fig. 2.3.<br />
The electron kinetic energy in the range of interest between about 10 and 2000<br />
eV. The escape depth is only on the order of a few Å. From the universal curve, one<br />
can see the IMFP has a minimum value of 5 to 10 Å for the kinetic energy range
Experimental Techniques 30<br />
Figure 2.3: ”Universal curve” for electron inelastic mean free path λ.<br />
of 20 to 200 eV. And hence the surface sensitivity of photoemission spectroscopy is<br />
highest in this energy range.<br />
2.2.4 Experimental set-up<br />
The OMICRON photoemission system at Institute of Physics consists of two connected<br />
UHV chambers, a sample preparation chamber and an analysis chamber. The<br />
preparation chamber is equipped with a precision manipulator, sample heater, evaporator<br />
and a diamond file. The analysis chamber is made up of µ metal to protect low<br />
energy electrons from earth’s magnetic field and stray magnetic fields. It is equipped<br />
with a 125 mm hemispherical analyser [10] for angle integrated photoemission and a<br />
movable 65 mm hemispherical analyser mounted on a 2-axis goniometer is also there<br />
for angle resolved photoemission. Two photon sources [10, 14] (ultra-violet and x-ray)<br />
are used for photoexcitations. The analysis chamber is also equipped with a 4-axis<br />
sample manipulator-cum cryostat which can go down to 20 K. Facility for performing<br />
low energy electron diffraction (LEED) is also available in the analysis chamber. The<br />
external view of the spectrometer is shown in the Fig. 2.4. Some of the important<br />
components of the spectrometer are described below.
Experimental Techniques 31<br />
Figure 2.4: Photoemission spectrometer at Institute of Physics, Bhubaneswar.
Experimental Techniques 32<br />
Figure 2.5: Schematic diagram of the VUV source.<br />
Figure 2.6: Dimensional outline of the VUV source HIS 13. The viewport can be<br />
used for inspection of the discharge and for adjustment of the light spot position on<br />
the sample. The dimensions are in mm.<br />
Vacuum ultra-violet photon source (VUV source HIS 13)<br />
The high intensity VUV Source [14] HIS 13 can be operated with various discharge<br />
gases, such as helium, neon, argon, krypton, xenon or hydrogen. But in our laboratory<br />
we use only helium and neon gases. The operation of the lamp is based on the principle<br />
of a cold cathode capillary discharge [15]. If an increasing potential is applied between<br />
the ends of an insulating tube filled with gas at a pressure of typically 1 mbar, a
Experimental Techniques 33<br />
point will be reached when spontaneous breakthrough occurs leading to a continuous<br />
discharge. The ignition potential is about an order of magnitude higher than the<br />
operating potential necessary to maintain a continuous discharge. The schematic of<br />
the VUV source is shown in the Fig. 2.5. There is a windowless direct-sight connection<br />
between the discharge area and the target. The discharge current is electronically<br />
stabilised. The lamp is water cooled in order to allow for high discharge currents<br />
(up to 300 mA) and to reduce electrode degradation resulting in prolonged service<br />
intervals. The dimensional outline of the VUV source is shown in the Fig. 2.6<br />
X-ray source (DAR 400)<br />
The X-ray source we used is DAR 400 from OMICRON designed specifically for X-ray<br />
photoelectron spectroscopy [10]. It has twin anodes that allow either Mg Kα (1253.6<br />
eV) or Al Kα (1486.6 eV) radiation to be selected. The schematic of the X-ray source<br />
is shown in Fig. 2.7.<br />
Figure 2.7: Schematic of the X-ray source in operation<br />
Electrons are extracted from a heated filament to bombard the selected surface of<br />
an anode at high positive potential. The focus ring and the shape of the nose cone<br />
ensure that the electrons hit the anode in the right area. By switching the filament
Experimental Techniques 34<br />
used, the second face of the anode can be excited. The anode is water cooled to<br />
prevent the aluminium or magnesium surfaces from evaporating.<br />
X-rays generated in the surface of the anode pass through a thin aluminium window<br />
to the sample under analysis. The aluminium window forms a partial vacuum<br />
barrier between the source and the sample region.<br />
The hemispherical electron energy analyser (EA 125)<br />
The EA 125 energy analyser based on a 125 mm mean radius electrostatic hemispherical<br />
deflection analyser composed of two concentric hemispheres [10, 16]. The inner<br />
and outer spheres are biased positive and negative with respect to the pass energy of<br />
the analyser. The analyser disperses electrons according to the energy across the exit<br />
plane (between the two hemispheres) and focuses them in the angular dimension from<br />
the entrance to the exit plane. Slits which are variable are located at the entrance<br />
and exit of the analyser. A schematic of the EA 125 analyser is shown in Fig. 2.8.<br />
Figure 2.8: Schematic diagram of the EA 125 hemispherical analyser.<br />
The principle of operation of concentric hemispherical analyser is shown schematically<br />
in Fig. 2.9. It consists of two concentric hemispheres of radii R 1 (inner hemisphere)<br />
and R 2 (outer hemisphere) with mean radius R 0 mounted with a common<br />
centre O. A potential V is applied between the two hemispheres so that the outer is
Experimental Techniques 35<br />
negative and the inner positive with respect to V 0 which is the median equipotential<br />
surface between the hemispheres.<br />
Figure 2.9: The principle of operation of the concentric hemispherical analyser,<br />
schematic diagram.<br />
If E p is the kinetic energy of an electron travelling in an orbit of radius R 0 , then<br />
the relationship between E p and V 0 is given by the expression,<br />
eV 0 = E p ·<br />
(<br />
R2<br />
− R )<br />
1<br />
R 1 R 2<br />
(2.21)<br />
The voltages on the inner and outer hemispheres are V 1 and V 2 respectively and<br />
these are given by,<br />
V 1 = E p ·<br />
V 2 = E p ·<br />
(a) Modes of analyser operation<br />
(<br />
2 R )<br />
0<br />
− 1<br />
R 1<br />
(<br />
2 R )<br />
0<br />
− 1<br />
R 2<br />
(2.22)<br />
(2.23)<br />
The kinetic energy can be scanned either by varying the retardation ratio whilst<br />
holding the analyser pass energy constant known as constant analyzer energy (CAE)
Experimental Techniques 36<br />
mode or by varying the the pass energy E p whilst holding the retardation ratio constant<br />
known as the constant retard ratio (CRR) mode [10].<br />
i. Constant retard ratio (CRR) mode<br />
In the CRR mode electrons entering the analyser system from the sample are<br />
retarded by the lens stack by a constant proportion of their kinetic energy so that<br />
the ratio of electron kinetic energy to analyser pass energy is kept constant during a<br />
spectrum. The retard ratio k is defined as,<br />
K = E k − φ a<br />
E p<br />
∼ E k<br />
E p<br />
(2.24)<br />
where φ a is the analyser work function. Throughout the scan range the pass energy<br />
of the analyser is continuously varied to maintain a constant retard ratio. Sensitivity<br />
and resolution are proportional to the pass energy and therefore the kinetic energy<br />
in this mode. In this mode the analysed sample area and the emission angle remain<br />
almost constant throughout the whole kinetic energy range.<br />
ii. Constant analyser energy (CAE) mode<br />
In the CAE mode, the analyser pass energy is held constant, and the retarding<br />
voltage is changed thus scanning the kinetic energy of detected electrons. The resolution<br />
obtained in CAE is constant throughout the whole kinetic energy range. The<br />
sensitivity, however is inversely proportional to the kinetic energy and at low kinetic<br />
energies is improved over that of CRR. In the CAE mode, the analysed sample area<br />
and the emission angle may vary slightly with the kinetic energy, but this is dependent<br />
on the lens design.<br />
(b) Analyser resolution<br />
The analyser is a band pass energy filter for electrons at a specific energy E p<br />
and has a finite energy resolution ∆E which is dependent on the chosen mode of<br />
operation and specific operating conditions. The energy resolution of the analyser is<br />
given approximately by,
Experimental Techniques 37<br />
∆E = E p<br />
( d<br />
2R 0<br />
+ α 2 )<br />
(2.25)<br />
where d is the slit width, R 0 is the mean radius of the hemispheres and α is the<br />
half angle of electrons entering the analyser at the entrance slit.<br />
(c) The electrostatic input lens<br />
The input lens collects the electrons from the source and focuses them onto the<br />
entrance aperture of the analyser whilst simultaneously adjusting their kinetic energy<br />
to match the pass energy of the analyser. The lens is also designed to define the analysis<br />
area and angular acceptance of electrons which pass through the hemispherical<br />
analyser. The lens design employs double lens concept where two lenses are stacked<br />
one above the other.<br />
The first lens selects the analysis area (spot size) and angular acceptance. This is<br />
an Einzel lens, i.e it does not change the energy of the electrons and therefore has a<br />
constant magnification throughout the entire energy range. This lens can be operated<br />
in three discrete magnification modes: high, medium and low. In high magnification<br />
mode, the focal plane is nearer to the sample and the lens accept a wide angle of<br />
electron beams from a small region. In low magnification mode, the focal plane is<br />
farther from the sample and the lens accept only a small angle of beams but from a<br />
larger area. The medium magnification is in-between the two.<br />
The second lens retards or accelerates the electrons to match the pass energy of<br />
the analyser and uses the zoom lens function to ensure that the focal point remains<br />
on the analyser entrance aperture. The magnification of this lens varies with retard<br />
ratio as a result of the law of Helmholtz-Lagrange.<br />
The analysis area is defined by the combination of the selected analyser entrance<br />
aperture and the magnification of the entire lens. The magnification of the entire lens<br />
is a product of the magnifications of the two discrete lenses.<br />
(d) The detector<br />
A single channel electron multiplier (Channeltron) is placed across the exit plane<br />
of the analyser. The channeltron amplifies the current of a single electron by a factor<br />
of about 10 8 . The small current pulse present at the output of the channeltron is
Experimental Techniques 38<br />
passed through a vacuum feedthrough and then directly into the preamplifier. And<br />
then the signal is passed onto a pulse counter for processing and production of an<br />
electron energy spectrum.<br />
2.3 Inverse Photoemission Spectroscopy<br />
2.3.1 Historical background<br />
The inverse photoemission spectroscopy (IPES) [17, 18, 19] is a powerful technique<br />
to study the unoccupied electronic states of solids. The IPES experiment can be<br />
carried out in two different photon energy regimes. One is at higher energy range<br />
(detected photons are in x-ray range) called Bremsstrahlung isochromat spectroscopy<br />
(BIS) and the other one is at low energy region (detected photons are in the ultraviolet<br />
range) which is called UV-IPES or simply IPES. The history of IPES technique<br />
was developed in 1915 by Duane and Hunt [20] when an inverse relationship is observed<br />
between the short wave-length cutoff of emitted x-rays and incident electron<br />
energy when a solid is bombarded with electrons. Then pursued by Ohlin [21] and<br />
Nijboer [22] in 1940s and was resumed in the 1950s by Ulmer and Vernickel [23]. The<br />
IPES work in the ultra-violet region is of relatively recent origin. The experimental<br />
work was started by Dose [24] and the theoretical work by Pendry [25, 26]. The<br />
experimental advances since then have been rapid.<br />
2.3.2 Basic principle<br />
When a beam of electrons is incident on a solid surface, after entering the system,<br />
the electrons decay either radiatively or non-radiatively to states at lower energy. If<br />
the decay is radiative the emitted photon can be detected, and gives the information<br />
about the unoccupied states of the system. So this is complementary to direct photoemission<br />
spectroscopy (PES) [17, 18, 19, 27]. A schematic of the inverse photoemission<br />
spectroscopy is shown in the Fig. 2.10<br />
The energy of the photons (hν) emitted when a beam of electrons having kinetic<br />
energy E k is incident on a solid surface and then relax to a lower energy unoccupied<br />
state (E f ), is given by the conservation of energy as,<br />
E k = hν + E f − φ, (2.26)
Experimental Techniques 39<br />
Figure 2.10: Schematic diagram of inverse photoemission spectroscopy (IPES).<br />
where φ is the work function of the material.<br />
There are two modes [18, 19, 27] used for this measurement . One is isochromat<br />
mode and the other one is spectrograph mode. In isochromat mode, the incident<br />
electron energy is ramped and the emitted photons are detected at a fixed energy.<br />
In the spectrograph mode, the energy of the incident electron remains fixed and the<br />
emitted photons are detected over a range of photon energies.<br />
2.3.3 Basic interpretation of inverse photoemission<br />
The Inverse photoemission process can be described [28] by a comparison with photoemission.<br />
Both photoemission and inverse photoemission involve the interaction<br />
between photons and electrons. Now the interaction Hamiltonian associated with the<br />
electromagnetic field can be written as,
Experimental Techniques 40<br />
H ′ =<br />
e (A.p + p.A) (2.27)<br />
2mc<br />
where A is the vector potential of the electromagnetic radiation. For photoemission<br />
the field A can be treated as a classical time-dependent perturbation, and<br />
results will be the same as a proper quantum treatment had been made. For inverse<br />
photoemission, however, this is no longer true. Since photons are created it becomes<br />
necessary to quantize the electromagnetic field. In such a treatment the classical<br />
vector potential is replaced by the field operator A(x, t) defined by [29]<br />
A(x, t) =<br />
1 ∑ ∑ 2π¯hc<br />
(V p ) 1/2 q α ω [a q,α(t)ˆε (α) e iq·x + a † q,α(t)ˆε (α) e −iq·x ], (2.28)<br />
where ˆε α is the linear polarization vector, a real unit vector whose direction depends<br />
on the photon propagation direction q. The two operators a † q,α and a q,α either<br />
create or destroy a photon in the state q, α respectively. V p is the normalization<br />
volume for the photon. For inverse photoemission the initial state consists of an electron<br />
in a continuum state |i〉≡ψ i (r) with no photons and the final state consists of<br />
an electron in a bound state |f〉≡ψ f (r) and a photon with wave vector q. Then the<br />
transition rate is given by the above two equations using the first order perturbation<br />
theory,<br />
R =<br />
2π¯h<br />
c22π¯h<br />
ω<br />
where ρ p is the photon density of states<br />
and dΩ is the solid angle of emission. Then<br />
1<br />
|〈f|ˆε.p|i〉| 2 ρ p (2.29)<br />
V p<br />
V p<br />
ω 2<br />
ρ p =<br />
(2π) 3 ¯hc 3dΩ<br />
(2.30)<br />
R = 1 e 2 ω<br />
2π ¯hc m 2 c 2 |〈f|ˆε.p|i〉|2 dΩ, (2.31)<br />
where e2<br />
¯hc = α≈ 1<br />
137<br />
To obtain a cross section, we devide by the incident electron flux<br />
j e = ¯hk 1<br />
(2.32)<br />
m V e<br />
We further assume that the continuum electron state is normalized to a box of<br />
volume V e . Then
Experimental Techniques 41<br />
( ) dσ<br />
= α ω 1<br />
dΩ 2π mc<br />
IPES 2 ¯hk |〈f|ˆε.p|i〉|2 (2.33)<br />
It is interesting to compare this cross section with that of the photoemission.<br />
In photoemission the final states are those of electrons, The density of states factor<br />
becomes<br />
ρ e = V e mk<br />
8π 3 ¯h 2 (2.34)<br />
and the incident flux is the photon flux<br />
Then<br />
j p =<br />
ω 1<br />
(2.35)<br />
8π¯hc V p<br />
( ) dσ<br />
= α k 1<br />
dΩ 2π m ¯hω |〈i|ˆε.p|f〉|2 (2.36)<br />
PES<br />
To relate the cross sections of the two processes (PES and IPES), let us suppose<br />
the matrix element in PES is same as that of the IPES. Then the ratio of two cross<br />
sections reflects only the different phase space available for photon creation or electron<br />
creation, and is given by<br />
or<br />
r =<br />
( ) dσ<br />
/<br />
dΩ<br />
IPES<br />
( ) dσ<br />
= ω2<br />
dΩ c<br />
PES<br />
2 k = q2<br />
(2.37)<br />
2 k 2<br />
r =<br />
(<br />
λe<br />
λ p<br />
) 2<br />
(2.38)<br />
where λ e and λ p are the electron and photon wavelengths respectively.<br />
And the equation (2.38) says the ratio of the inverse photoemission cross section to<br />
that of the phtoemission is the ratio of the square of the wave length of electron to that<br />
of photon. At an energy of 10 eV, characteristics of the vacuum ultraviolet region, the<br />
ratio r∼10 −5 . Therefore the signal levels in IPES is much lower comparable to PES.<br />
At higher energies ∼1000 eV, characteristics of the x-ray range, we have r∼10 −3 .<br />
2.3.4 Experimental Set-up<br />
The external view of IPES spectrometer at UGC-DAE consortium for scientific research,<br />
Indore, is shown in the Fig. 2.11.
Experimental Techniques 42<br />
Figure 2.11: (a) IPES spectrometer at UGC-DAE consortium for scientific research,<br />
Indore, (b) External view of photon detector attached to the IPES chamber, and (c)<br />
Inside view of the chamber showing a rotatable (left) and fixed (right) electron gun<br />
and the sample holder
Experimental Techniques 43<br />
The experimental UHV chamber is made of µ metal which provides effective shielding<br />
for the low energy electrons from earth’s magnetic field and stray magnetic fields.<br />
A photon detector capable of detecting fixed energy photons with well defined narrow<br />
bandpass and an electron gun producing nearly monoenergetic low energy electrons<br />
with well defined angular spread are equipped in the UHV chamber. A sample manipulator<br />
is mounted vertically which allow to move the sample both along axial and<br />
azimuthal direction.<br />
The low energy electron gun (e-gun)<br />
The e-gun used for the experiment is of Stoffel Johnson type [30] is shown schematically<br />
in Fig. 2.12. This is chosen because it has less space-charge effect and high<br />
beam current.<br />
Figure 2.12: A schematic diagram of the electron gun [30].<br />
In this type of e-gun the minimum beam size obtained is 1.1 mm with full angular<br />
convergence of 5 ◦ to 7 ◦ . It consists of a filament in a mounting block followed by<br />
three element refocusing lens. All the lens elements are with cylindrical cross-section.<br />
In the design (Fig.) L = D = 16mm, P = 1.3D and Q = 2.5D. Ratio of voltages<br />
applied on the cathode and the subsequent electrostatic lens elements is V C : V A :<br />
V F : V O = −1 : +5 : −0.9 : 0. The Kinetic energy of emitting electrons is governed by<br />
V C . The whole surface of the lenses is gold plated to obtain a uniform work function.<br />
BaO dispenser cathode is used as the electron emitter. The e-gun operates in the<br />
range of 5 − 40 eV.
Experimental Techniques 44<br />
The photon detector<br />
Gas filled band-pass Geiger-Muller (GM) type counters are used for photon detection<br />
in IPES because of their high efficiency, low cost and simple design [31, 32, 33, 34,<br />
35, 36, 37, 38]. In our IPES experiment the acetone/CaF 2 detector [39] was used.<br />
The design of the photon detector is based on the following operating principle<br />
: the high energy cut-off of the acetone/CaF 2 detector is due to the CaF 2 window<br />
that does not transmit photons with energy > 10.2, while the threshold for the<br />
photoionization of acetone at 9.7 eV sets the low energy cut-off [40]. This determines<br />
the band pass function and results [33] in a mean photon detection energy of 9.9<br />
eV with a FWHM of 0.4 eV. Thus, 9.9±0.2 eV photons can enter the detector to<br />
photoionize acetone.<br />
Figure 2.13: Schematic cross section of the photon detector showing the window (1),<br />
O-ring (2), cylindrical cap (3), detector tube (4), teflon spacer (5), anode (6), double<br />
sided DN40 CF flanges (7, 10), gas inlet and outlet (8, 9), barrel connector (11),<br />
teflon sleeve (12), DN40 CF flange (13) and SHV connector (14).<br />
The detector assembly [41] (2.13) has a modular design consisting of a 250 mm<br />
long stainless steel cylindrical tube with inner diameter 17.8 mm and outer diameter<br />
21.2 mm (Fig), welded to a double-sided DN40 CF flange (7) with a through hole equal<br />
to the inner diameter of the tube (4). Another double-sided DN40 CF flange (10)<br />
with a similar through hole has two additional 6 mm diameter through holes drilled<br />
in the radial direction. Two 6 mm diameter of stainless steel tubes of length 25 mm
Experimental Techniques 45<br />
are welded to these radial holes, and on the other end, DN16 CF flanges (8, 9) are<br />
welded to these tubes. These are used as gas inlet (8) and outlet (9). The anode (6)<br />
is attached to a SHV feedthrough (14) mounted on another DB40 CF flange (13) by<br />
a barrel connector (11). The three DN40 CF flanges (7, 10, 13) are mounted together<br />
on a linear translator. The anode is a 1.6 mm diameter electropolished stainless steel<br />
wire and is held by two perforated teflon spacers (5) in the detector tube. High voltage<br />
is applied to the anode. A 25 mm diameter, 2 mm thick CaF 2 window (1) sits on a<br />
viton O-ring (2) and is tightly pressed by a stainless steel cap (3) at the entrance of<br />
the detector. This isolates the detector from the UHV chamber. Use of viton O-ring<br />
ensures easy exchange of windows at the testing stage of the detector. The count rate<br />
increases with higher solid angle of detection. So the detector is brought as close as<br />
possible to the sample using the linear translator.<br />
2.4 X-ray Absorption Spectroscopy<br />
X-ray absorption spectroscopy (XAS) is another technique to study the unoccupied<br />
electronic states as well as the geometrical arrangements of any materials around an<br />
atom [42, 43]. But in this thesis XAS technique is used only to study the electronic<br />
structure of the materials.<br />
2.4.1 Basic principle<br />
When a beam of monochromatic x-ray enters a solid, either it will be scattered by<br />
the electrons or absorbed and excite the core electrons. In the absorption process the<br />
core electron is excited to an empty state [21, 45]. The schematic of XAS is shown in<br />
Fig. 2.14.<br />
If the energy of the x-ray is much larger than the binding energy of the core state<br />
the excited electron behaves like a free electron in the solid. If however the x-ray<br />
energy is just enough to excite a core electron, it will occupy a lowest available empty<br />
state.<br />
When a monochromatic x-ray beam of intensity I 0 goes through a material of<br />
thickness d, its intensity I will get reduced as,<br />
I = I 0 e −ρµd (2.39)
Experimental Techniques 46<br />
Figure 2.14: Schematic diagram of x-ray absorption spectroscopy<br />
where µ is the absorption coefficient of the material, which depends on the types<br />
of atoms and density ρ of the material. The absorption coefficient decreases smoothly<br />
with higher energy, except for certain photon energies where the absorption increases<br />
drastically, and gives rise to an absorption edge. Each such edge occurs when the<br />
energy of the incident photons is just sufficient to excite a core electron to an unoccupied<br />
state or into the continuum state (leave the atom). After each absorption edge<br />
the absorption coefficient continues to decrease with increasing photon energy (Fig.<br />
2.15).<br />
There are two distinguishable parts in x-ray absorption process : The low energy,<br />
near edge x-ray absorption fine structure (NEXAFS) [46, 47] or x-ray absorption<br />
near edge structure (XANES) region and the extended x-ray absorption fine structure<br />
(EXAFS) [48, 49], where the ejected electrons have high kinetic energy with single<br />
scattering. In this thesis work, only the XANES (or NEXAFS) region has been taken<br />
to study the unoccupied states.
Experimental Techniques 47<br />
Figure 2.15: The x-ray loses its intensity via interactions with material. The absorption<br />
coefficient decreases smoothly with higher energy, except for special photon<br />
energies. When the photon energy reaches a critical value for a core electron transition,<br />
the absorption coefficient increases abruptly.<br />
2.4.2 Detection techniques<br />
The normal way to measure the x-ray absorption is in the transmission mode [21]. The<br />
intensity of the x-ray is measured before and after the sample and the percentage of<br />
transmitted x-rays is determined. Such transmission mode of experiment is standard<br />
for hard x-rays. But for soft x-rays they are difficult to perform because of the strong<br />
interaction of soft x-rays with matter and requires a very thin sample of 0.1 µm to<br />
obtain a detectable signal. An alternative to the tansmission mode experiment is the<br />
”Yield mode”, where the decay product of the core hole (created in x-ray absorption)<br />
will be measured. The decay of the core hole gives rise to an avalanche of electrons,<br />
photons and ions escaping from the sample surface. There are four types of detection<br />
methods [21] in yield mode XAS measurement such as Auger electron yield (or partial<br />
yield), fluorescence yield, ion yield and total electron yield.<br />
In Auger electron yield method, the electrons of a certain energy range (i.e electrons<br />
of a specific Auger decay channel of the core hole) are detected. It is found<br />
that the mean free path of a 500 eV Auger electron is of the order of 20 Å. Hence
Experimental Techniques 48<br />
the number of Auger electrons emitted is equal to the number of core holes which<br />
are created in the first 20 Å from the surface. The Auger electron yield method is a<br />
measurement of only 20 Å of material, hence the Auger electron yield is rather surface<br />
sensitive. In fluorescence yield XAS mode, the fluorescence decay of the core hole is<br />
measured instead of Auger decay electrons. The intensity of the fluorescence yield<br />
is small compared to Auger electron yield mode for the elements of atomic number<br />
Z ≤ 50. The main advantage of fluorescence yield is that the created photon has a<br />
mean free path of the same order of magnitude as the x-ray (∼ 1000 Å) and hence it<br />
probes the bulk. In ion yield mode, only the surface ions created by Auger decay are<br />
measured. So it is highly surface sensitive and is used for special surface study. The<br />
most applied detection technique for soft XAS is the total electron yield. In total<br />
electron yield mode, all escaping electrons are counted regardless of their energy. The<br />
count rate is very large in this case. This thesis contains the XAS spectra measured<br />
in total electron yield and fluorescence yield mode. In TEY mode, the electrons are<br />
detected by the hemispherical analyser, which is discussed in the last section. In the<br />
FY mode the radiative photons are detected by the Si-photodiode.<br />
Si - photodiode<br />
Silicon photodiode detectors are widely used over a broad wavelength range extending<br />
from the x-ray to the visible regions. The detector used for the XAS measurement<br />
involved in this thesis are AXUV and SXUV photodiodes [50, 51]. The photodiodes<br />
are fabricated by an ULSI (Ultra large Scale Integrated circuit) compatible process.<br />
Fig. 2.16 shows the cross section structure of the AXUV and SXUV photodiodes.<br />
The AXUV photodiode has a thin nitrided SiO 2 surface layer (approximately 7 nm<br />
thick) and 100 % internal quantum efficiency. The SiO 2 layer is known to have a<br />
fixed positive charge which repels minority carrier holes generated at the surface by<br />
strongly absorbing extreme ultra-violet (EUV) radiation. The SXUV photodiode has<br />
metal silicide surface layer which replaces the oxide layer on the AXUV photodiode<br />
and improves the radiation hardness. However, the absence of the oxide layer causes<br />
significant surface recombination in the SXUV photo diodes and reduces the internal<br />
quantum efficiency below 100 % value, that is characteristics of AXUV photodiodes.<br />
When the silicon photodiode is exposed to photons with energy greater than 1.12<br />
eV (wavelength less than 1100 nm), electron-hole pairs (carriers) are created. These<br />
photogenerated carriers are separated by the p-n junction electric field and a current
Experimental Techniques 49<br />
Figure 2.16: Schematic of the Si-photodiode.<br />
proportional to the number of electron hole pairs created flows through an external<br />
circuit. Several unique properties of the AXUV photodiodes have resulted in previously<br />
unattained stability and 100 % carrier collection efficiencies giving rise to near<br />
theoretical quantum efficiencies. The first property is the absence of a surface dead<br />
region i.e no photogenerated carrier recombination occurs in the doped n-type region<br />
and at the silicon-silicon dioxide interface. As the absorption depths for the majority<br />
of UV/EUV photons are less than 1 micrometer in silicon, the absence of a dead region<br />
yields complete collection of photogenerated carriers by an external circuit resulting<br />
into 100 % collection efficiency. The second unique property of the AXUV diodes is<br />
their extremely thin (4 to 8 nm), radiation-hard silicon dioxide junction passivating,<br />
protective entrance window. Owing to their 100 % collection efficiency and the thin<br />
entrance window, the quantum efficiency of the AXUV diodes can be approximately<br />
predicted in most of the XUV region by the theoretical expression E ph /3.65, where<br />
E ph is the photon energy in electron volts.<br />
2.5 Synchrotron Radiation<br />
The unique properties of synchrotron radiation are its continuous spectrum, high<br />
flux and brightness, and high coherence, which make it an indispensable tool in the
Experimental Techniques 50<br />
exploration of matter. Synchrotron radiation are produced when charge particles<br />
Figure 2.17: Schematic of the synchrotron radiation.<br />
moving with relativistic velocity are deflected in a magnetic field. The synchrotron<br />
radiation had been developed by taking the effect of Heinrich Hetz [1] as the electrons<br />
fly around the curves through the magnetic field emit electromagnetic radiation. In<br />
SR facilities the accelerated electrons fly in a closed orbit inside a stainless steel<br />
tube, which consists of large electromagnets at regular intervals to guide the electrons<br />
around the curves and keep them focussed in the center of the tube called the storage<br />
ring. The electrons are generated in an injection system consisting of an electron<br />
source and an accelerator, either a synchrotron or a linear accelerator. When the<br />
electrons are accelerated to the operating energy of the storage ring, they are injected<br />
into the storage ring. The bending magnets are used to deflect the electron beam.<br />
In order to compensate the energy loss of the electrons during the SR emission, the<br />
electrons are accelerated each time as they pass through the RF cavity installed inside<br />
the storage ring. A schematic of SR light source is shown in the Fig. 2.17.<br />
In the developed synchrotrons the intensity of the emitted radiation is enhanced<br />
by insertion devices (undulator or wiggler). The insertion devices are periodic arrays<br />
of permanent magnets. When electrons traverse in the periodic array of magnets are<br />
forced to undergo oscillations and radiate (Fig. 2.18 shows the schematic representation).The<br />
static magnetic field is alternating along the length of the insertion devices<br />
with a wave length λ 0 . The radiation produced by insertion devices is very intense and<br />
is guided through beam lines for various experiments. An important dimensionless<br />
parameter K which characterizes the nature of electron motion is defined as,
Experimental Techniques 51<br />
Figure 2.18: Schematic representation of an insertion device.<br />
K = eB 0λ 0<br />
(2.40)<br />
2πmc<br />
where e is the electronic charge, B 0 is the magnetic field, m is the electron rest<br />
mass and c is the speed of light.<br />
For undulator K > 1, the oscillation amplitude is bigger and the radiation contributions<br />
from each field period sum up independently leading to a broad energy spectrum.<br />
Insertion devices can provide several orders of magnitude higher flux than a simple<br />
bending magnet and are in high demand at synchrotron radiation facilities. For an<br />
undulator with N periods, the brightness can be up to N 2 more than a bending<br />
magnet.<br />
The experiments presented in this thesis were performed at different synchrotrons<br />
using monochromators of various type. The HRPES and XAS experiments were<br />
performed at the WERA soft x-ray beamline of ANKA synchrotron, Germany. At<br />
BEAR and BACH beamlines of ELETTRA synchrotron, Italy, the XAS experiments<br />
were performed.<br />
At WERA soft x-ray beamline of ANKA synchrotron, a spherical grating monochromator<br />
(DRAGON type) [21, 52] was used. The layout of the WERA soft x-ray beamline<br />
is shown in the Fig. 2.19. It consists of a pair of mirrors 1 (horizontal) and
Experimental Techniques 52<br />
Figure 2.19: Optical layout of the WERA soft x-ray beam line, ANKA.<br />
2 (vertical) to focus the incoming beam into the entrance slit of the grating box<br />
and also a pair of bendable focusing mirrors after the exit slit for optimum focus at<br />
sample position. Both entrance and exit slits are movable. The energy resolution<br />
∆E/E < 10 −4 . The photon energy range is 100 - 1500 eV. The experimental facilities<br />
associated with the WERA soft x-ray beamline are PES, PEEM, NEXAFS and<br />
SXMCD.<br />
The BEAR beamline at ELETTRA is dedicated to the study of optical, electronic<br />
and magnetic properties of materials under UHV condition. The lay out of the BEAR<br />
beam line is shown in the Fig. 2.20. The beamline optics is based on a couple of<br />
parabolic mirrors. The radiation is collimated by the first parabolic mirror (P1). The<br />
parallel beam produced by P1 reaches the monochromator stage based on the plane<br />
grating/plane mirror scheme according with the Naletto-Tondelo scheme [53]. There<br />
are three gratings (G-NIM, G-1200 and G1800) associated with the monochromator.<br />
Three gratings cover 5 - 1600 eV photon energy range. The G-NIM (1200 1/mm)<br />
works at near normal incidence to cover the 5 - 45 eV region. The G1200 (1200 1/mm)<br />
and G1800 (1800 1/mm) work in the 45 - 1600 eV energy range. The monochromatic<br />
radiation is focused onto the exit slit by a second parabolic mirror (P2). The elliptical<br />
re-focusing mirror (REFO) brings the beam at the sample position. The experimental
Experimental Techniques 53<br />
Figure 2.20: Optical layout of the BEAR beam line.<br />
facilities available at BEAR beamline are ARPES, XAS, photoelectron diffraction and<br />
specular and diffuse reflectivity.<br />
The BACH beam line is designed to deliver a photon beam with high intensity<br />
and brilliance in the soft x-rays energy range 35 -1600 eV with full control of the<br />
polarization of the light. The goal of this beam line is to perform the electron and<br />
photon spectroscopy experiment with polarization dependence. The optical layout of<br />
the BACH beam line [54, 55] is shown in the Fig. 2.21.<br />
Figure 2.21: Optical layout of the BACH beam line.
Experimental Techniques 54<br />
Figure 2.22: The variable included angle monochromator : the combined movement<br />
of PM1 (plane mirror) and one of the four spherical gratings SG1(-4) leads to the<br />
monochromatization of the light keeping both the entrance and exit slit fixed.<br />
The radiation source is based on two APPLE-II elliptical undulators that are<br />
used alternatively to optimize the flux. The photon dispersion system is based on the<br />
Padmore ”variable angle spherical grating monochromator (VASGM)” scheme and it<br />
includes four different interchangeable spherical gratings (Fig. 2.22). The first three<br />
gratings works in the energy ranges 35 - 200 eV, 200 - 500 eV and 500 - 1600 eV<br />
respectively. The fourth grating operates in the 400 - 1600 eV range providing higher<br />
flux allowing fluorescence and x-ray scattering experiments. Two refocusing sections,<br />
based on plane elliptical mirrors in Kirkpatrick - Baez configuration provide nearly<br />
aberration free spots on the samples. A third branch line after the monochromator will<br />
host a microscope based on Fresnel zone plate focusing. The following experimental<br />
facilities are associated with the BACH beam line ; XPS, UPS, XAS, XMCD, XMLD,<br />
SXES and RIXS.
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Chapter 3<br />
Electronic Structure of<br />
Pr 0.67 Ca 0.33 MnO 3<br />
59
Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 60<br />
3.1 Introduction<br />
Recently, a lot of attention have been focused on the charge-ordered compositions of<br />
Pr 1−x Ca x MnO 3 due to their importance as possible prototypes for the electronic phase<br />
separation (PS) models [1, 2, 3, 4, 5, 6, 7, 8, 9] proposed to explain the phenomenon<br />
of colossal magnetoresistance (CMR). The PS models are qualitatively different from<br />
the double-exchange model [10, 11, 12] or those based on strong Jahn-Teller polarons<br />
[13, 14]. According to the PS model the ground state of CMR materials is comprised<br />
of coexisting nanosize clusters of metallic ferromagnetic and insulating antiferromagnetic<br />
nature [6]. The insulator-metal transition in this scenario is through current<br />
percolation. Though there have been a number of experimental studies showing the<br />
existence of phase separation, their size varies from nano- to mesoscopic scales [8, 9].<br />
Radaelli et al. have shown the origin of mesoscopic phase separation to be the intergranular<br />
strain [9] rather than the electronic nature as in PS models. Nanosized<br />
stripes of a ferromagnetic phase [15] were reported in Pr 0.67 Ca 0.33 MnO 3 . This compound<br />
also attracted much attention earlier due to the existence of a nearly degenerate<br />
ferromagnetic metallic state and a charge-ordered antiferromagnetic insulating state<br />
with a field-induced phase transition possible between them [16]. With slight variations<br />
in the Ca doping the Pr 1−x Ca x MnO 3 system turns ferromagnetic (x=0.2) or<br />
antiferromagnetic (x=0.4) at low temperatures [17, 15]. The composition x=0.33<br />
shows a coexistence of ferromagnetic and antiferromagnetic phases [18]. This makes<br />
the charge- or orbital-ordered Pr 0.67 Ca 0.33 MnO 3 a prototype for the PS scenario.<br />
One of the requisites for the existence of an electronic phase separation in manganites<br />
is a strong-coupling interaction affecting the hopping of the itinerant e g electrons.<br />
The PS is expected to occur [5] when J H /t ≫ 1, where J H is the Hund’s coupling<br />
contribution between the localized t 2g and the e g electrons and t is the hopping amplitude<br />
of the e g electrons. The PS also favors a strong electron-phonon coupling,<br />
like the influence of a strong Jahn-Teller (JT) polaron arising from the Q 2 and Q 3<br />
JT modes. Essentially, one expects a strong localization of charge carriers to accompany<br />
the electronic separation of phases. Apart from charge, the orbital degrees of<br />
freedom also play an important role in this scenario [5]. The itinerant electron hopping<br />
term is strongly influenced by the symmetry of the orbitals (d x 2 −y 2 and d 3z 2 −r 2)<br />
hybridized with the O 2p orbitals of the MnO 6 octahedra. In comparison with the<br />
most popular CMR material La 1−x Sr x MnO 3 , the charge-ordered Pr system has an<br />
inherently reduced e g bandwidth W due to the smaller ionic radius of Pr, which also
Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 61<br />
enhances the tendency for carrier localization. All these charge and orbital interactions<br />
are reflected in the near-E F electronic structure of these materials and could<br />
be probed using electron spectroscopic techniques. Valence band photoemission and<br />
O K x-ray absorption are two such proven tools sensitive to the changes in the lowenergy<br />
states crucial to these interactions. Although, there are many reports on<br />
the near-E F electronic structure of other CMR compounds using these techniques<br />
[19, 20, 21, 22, 23, 24, 25, 26, 27, 28] only a few studies have been reported on their<br />
charge-ordered compositions [8, 29]. One of the key energy terms these spectroscopies<br />
could show earlier was the charge transfer energy E CT which is intimately related to<br />
the electron-electron and electron-lattice interactions [20].<br />
In this study we have used ultraviolet photoelectron spectroscopy and x-ray absorption<br />
spectroscopy (XAS) in order to probe the electronic structure of the occupied<br />
and unoccupied states on a well-characterized, high-quality single crystal of<br />
Pr 0.67 Ca 0.33 MnO 3 . Another part of this single crystal had earlier been used for a detailed<br />
neutron scattering experiment that indicated the possible existence of a phase<br />
separation of ferromagnetic and antiferromagnetic stripes [15]. In the present study<br />
we have analyzed the temperature-dependent changes in the near-E F electron energy<br />
states from the perspective of a phase separation.<br />
3.2 Experimental<br />
The single-crystal sample of Pr 0.67 Ca 0.33 MnO 3 was grown by the floating zone method<br />
in a mirror furnace. The compositional homogeneity of the crystal was confirmed using<br />
energy-dispersive spectroscopic analysis. Magnetization and transport measurements<br />
on this crystal showed the transition temperatures T c and T N to be 100 and 110<br />
K, respectively. Details of the sample preparation, magnetization, and transport measurements<br />
and structural studies are published elsewhere [15, 30]. Angle-integrated<br />
ultraviolet photoemission measurements were performed using an Omicron µ-metal<br />
UHV system equipped with a high-intensity vacuum-ultraviolet source (HIS 13) and<br />
a hemispherical electron energy analyzer (EA 125 HR). At the He I (21.2 eV) line,<br />
the photon flux was of the order of 10 16 photons/s/sr with a beam spot of 2.5 mm<br />
diameter. Fermi energies for all measurements were calibrated using a freshly evaporated<br />
Ag film on a sample holder. The total energy resolution, estimated from the
Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 62<br />
width of the Fermi edge, was about 80 meV. The single-crystalline samples were repeatedly<br />
scraped using a diamond file inside the chamber with a base vacuum of ∼<br />
1.0 × 10 −10 mbar. Scraping was repeated until negligible intensity was found for<br />
the bump around 9.5 eV, which is a signature of surface contamination [19]. For<br />
the temperature-dependent measurements, the sample was cooled by pumping liquid<br />
nitrogen through the sample manipulator fitted with a cryostat. Sample temperatures<br />
were measured using a silicon diode sensor touching the bottom of the sample<br />
holder. XAS measurements were performed using the BEAR [31] and BACH [32]<br />
beamlines associated with ELETTRA at Trieste, Italy. At the BEAR beamline we<br />
used monochromatized radiation from a bending magnet in order to record the O K<br />
edge spectra at room temperature and 150 K in fluorescence detection mode on a<br />
freshly scraped surface of the single crystal. The energy resolution was around 0.2<br />
eV in the case of these two spectra. The O K edge spectra at 95 K was recorded at<br />
the BACH beamline, using the total electron yield mode. Before the measurements,<br />
the sample surface was scraped inside the UHV chamber (∼ 1.0 × 10 −10 mbar) using<br />
a diamond file. At this beamline we used radiation from an undulator, monochromatized<br />
using a spherical grating. The resolution at the O Kedge for this measurement<br />
was better than 0.1 eV.<br />
3.3 Results and Discussion<br />
3.3.1 The UPS study of Pr 0.67 Ca 0.33 MnO 3<br />
The angle-integrated valence band photoemission spectra of Pr 0.67 Ca 0.33 MnO 3 taken<br />
at different temperatures below and above T c are shown in Fig. 3.1. Intensities of<br />
all the spectra were normalized and shifted along the y axis by a constant for clarity.<br />
The spectra, dominated by the states due to the Mn 3d-O 2p hybridized orbitals, look<br />
similar to those reported earlier on the La 1−x Sr x MnO 3 system [19, 20, 21, 22, 23]. The<br />
origin of the two prominent features, one at ∼ -3.5 eV (marked B) and another at<br />
∼ -5.6 eV (marked C) below E F , are now well known from earlier experiments and<br />
band structure calculations [23, 24, 33] on similar systems. While the feature at -3.5<br />
eV is mainly due to the t 2g↑ states of the MnO 6 octahedra, the -5.6 eV subband<br />
has contributions from both t 2g and e g states. More important contributors to the<br />
properties of these systems are the states nearer to E F , which appear as a tail at<br />
∼ -1.2 eV (marked A) from the chemical potential. The intensity of this feature
Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 63<br />
C<br />
B<br />
Intensity (arb. units)<br />
A<br />
-10 -8 -6 -4 -2 0<br />
Energy Relative to E F<br />
(eV)<br />
Figure 3.1: Valence band photoemission spectra of Pr 0.67 Ca 0.33 MnO 3 taken using He<br />
I photon energy (21.2 eV) at 77 (solid line), 110 (dotted), 150 (dashed), 220 (dotdashed),<br />
and 300 K (double-dot dashed). All the spectra have been normalized and<br />
shifted along y-axis by a constant for clarity. The subbands around -1.2 (e g↑ ), -3.5<br />
(t 2g↑ ), and -5.6 eV (e g↑ + t 2g↑ ) are marked as A, B, and C respectively.
Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 64<br />
300 K<br />
77 K<br />
5 X Difference<br />
(a)<br />
300 K<br />
110 K<br />
5 X Difference<br />
(b)<br />
Intensity (arb. units)<br />
-1 -0.5 0 0.5 1 1.5<br />
300 K<br />
220 K<br />
5 X Difference<br />
(d)<br />
-1 -0.5 0 0.5 1 1.5<br />
300 K<br />
150 K<br />
5 X Difference<br />
(c)<br />
-1 -0.5 0 0.5 1 1.5<br />
Binding Energy (eV)<br />
-1 -0.5 0 0.5 1 1.5<br />
Figure 3.2: High-resolution photoemission spectra of the near-E F region of the valence<br />
band of Pr 0.67 Ca 0.33 MnO 3 . In (a) the spectrum taken below T c (red) is compared with<br />
the normal state (300 K) spectrum (blue). The difference spectrum (green) obtained<br />
by subtracting the spectra at 300 K from 77 K and multiplied by 5 is also shown in<br />
the panel. Similarly, in (b), (c), and (d) the near-E F spectra taken at 110, 150, and<br />
220 K are compared with that taken at 300 K. The feature in the difference spectra<br />
corresponds to the tail feature A (e g↑ ) in Fig. 3.1.<br />
is quite small compared to B and C and is a signature of the insulating nature<br />
of this material. Earlier photoemission experiments on La 1−x Sr x MnO 3 also have<br />
shown that the intensity of this tail feature A is quite small [8, 20, 21, 22, 23].<br />
The presence of the feature A is clear from Fig. 3.2, where we have shown the<br />
near-E F region of the valence band spectra taken with a higher resolution. Here,<br />
the spectra taken at different temperatures have been compared with those at room<br />
temperature. The figure also shows the difference spectra obtained by subtracting<br />
the room-temperature spectra from spectra taken at low temperatures. The feature<br />
in the difference spectra corresponds to A, which originates from the e g↑ states [22].<br />
In order to estimate the relative changes in all the three features (A, B, and C)<br />
due to temperature, we have carefully fitted the whole valence band spectrum with
Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 65<br />
0.014<br />
(a)<br />
0.012<br />
Normalized Intensity<br />
0.01<br />
0.6<br />
0.55<br />
0.5<br />
0.45<br />
(b)<br />
(c)<br />
0.4<br />
100 150 200 250 300<br />
Temperature (K)<br />
Figure 3.3: Temperature dependence of the area of the three valence band features<br />
obtained from fitting the spectra with Lorentzian line shapes using a χ 2 iterative<br />
program. We have used an integral background, which was kept the same for all the<br />
spectra. The energy positions and FWHMs were determined by finding the best fit<br />
common to all the spectra by the iterative program. The final fit for all spectra at<br />
different temperatures were obtained with the same energy positions and FWHMs.<br />
(a), (b), and (c) correspond to the features A, B, and C positioned at -1.19, -3.49,<br />
and -5.63 eV with FWHMs 2.56, 1.93, and 1.37 eV respectively.
Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 66<br />
three components corresponding to the three subbands contributing to this region.<br />
We used a χ 2 iterative program for fitting the different spectra with Lorentzian line<br />
shapes for A, B, and C. Except for the area of the peaks, the energy positions,<br />
widths, background, and all other parameters of all the peaks were kept the same for<br />
different temperatures. The positions and full widths at half maximum (FWHMs)<br />
of the three peaks are given in the caption of Fig. 3.3. The fitted spectra for 77<br />
K (below T c ) and 300 K (above T c ) are shown on the left sides of Figs. 3.5(a) and<br />
3.5(b) below. One can see from Figs. 3.3(a) - (c) and that the area of all the peaks<br />
keeps increasing as we go down in temperature T c , but below this temperature the<br />
intensities of both peaks C and B decrease. On the other hand, the intensity of A<br />
does not show any decrease across the transition [Fig. 3.3(a)].<br />
Pr 0.67 Ca 0.33 MnO 3 shows a transition to a charge-ordered state below 220 K (T co )<br />
[30, 37, 38]. Neutron diffraction studies [15] on another part of this single crystal<br />
have shown that this charge ordering turns into an antiferromagnetically structured<br />
pseudo-CE type charge or orbital ordering below 110 K. Also, the coexistence of<br />
ferromagnetic and antiferromagnetic phases below this transition temperature has<br />
been shown on this single crystal. In the pseudo-CE-type ordering the Mn 3d z 2 −r 2<br />
orbitals, where the e g state is occupied, are aligned with the O 2p orbitals. Such an<br />
alignment can increase the hybridization between Mn 3d z 2 −r2 and O 2p. Furthermore,<br />
a simultaneous decrease in the hybridization strength could also be expected for the<br />
core-like in-plane t 2g and O 2p states. The results of the curve fitting (Fig. 3) of<br />
our valence band spectra reflects these changes in hybridization with temperature.<br />
The charge ordering following the decrease in temperature causes an increase in the<br />
O 2p contribution to both the t 2g and e g spin-up subbands in the valence region and<br />
hence the intensities of A, B, and C go up. As mentioned earlier, the pseudo-CE-type<br />
charge or orbital ordering below 110 K, results not only in a stronger hybridization of<br />
O 2p with the e gz 2 −r 2 ↑ states compared to that with the t 2g states but a simultaneous<br />
decrease in the latter also. This is reflected in the increase in intensity of A and<br />
decrease in the intensities of both C and B below 110 K. Though in Pr 0.67 Ca 0.33 MnO 3<br />
there is no temperature-dependent insulator-metal transition, the change in intensities<br />
of these features across T c appears similar to the shifting of spectral weight found in<br />
the La 1−x Sr x MnO 3 system [8].
Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 67<br />
Normalized Absorption (arb. units)<br />
530 540 550 560<br />
Photon Energy (eV)<br />
Figure 3.4: O K edge x-ray absorption spectra of Pr 0.67 Ca 0.33 MnO 3 taken at 300<br />
(solid line), 150 (dashed line), and 95 K (dotted). The pre-edge feature centered<br />
around 529.5 eV (marked by a box) is where most interest lies. Since this feature<br />
consists of two peaks (a main line and a shoulder on the low-energy side), this part<br />
of the spectrum was fitted with two components of Lorentzian line shapes. Results<br />
of the curve fit are given in Table 3.1.<br />
3.3.2 The XAS study of Pr 0.67 Ca 0.33 MnO 3<br />
In a reasonable approximation, the near-E F region of the O K x-ray absorption<br />
spectra could well represent the density of unoccupied states in many of the transition<br />
metal oxide compounds [20, 34]. In order to probe the electronic structure of the<br />
unoccupied states we have performed XAS on our Pr 0.67 Ca 0.33 MnO 3 single crystal.<br />
The O K edge XAS spectra taken at room temperature and 150 and 95 K (below<br />
T c ), shown in Fig. 3.4, have been normalized in intensity all along the region starting<br />
from 550 eV. The pre-edge feature in the O K spectra (centered around 529.5 eV) is<br />
due to the strong hybridization between Mn 3d and O 2p orbitals. The broad feature<br />
around 536.5 eV is due to the bands from hybridized Pr 5d and Ca 3d orbitals, while<br />
the structure above 540 eV is due to states like Mn 4sp and Pr 6sp, etc. These<br />
states are known to contribute least to the near-E F electronic structure of these
Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 68<br />
Table 3.1: χ 2 iterative fitting parameters for the pre-edge peak in O K XAS. The<br />
intensities of B ′ and A ′ at low temperature are normalized with respect to the total<br />
intensity of the pre-peak at room temperature. We have used an integral background,<br />
which was kept the same for all the spectra. The energy positions and FWHMs were<br />
determined by finding the best fit common to the three spectra by a χ 2 iterative<br />
program. The final fit for all the spectra from different temperatures were obtained<br />
with the same energy positions and FWHMs.<br />
Temperature (K)<br />
O K edge x-ray absorption spectra<br />
B ′ A ′<br />
Position FWHM Normalized Position FWHM Normalized<br />
95 (eV) (eV) area (eV) (eV) area<br />
2.11 1.52 0.72 1.48 0.97 0.29<br />
150 2.11 1.52 0.72 1.48 0.97 0.26<br />
300 2.12 1.52 0.73 1.47 0.97 0.26<br />
transition metal-oxide compounds. The assignments of the features mentioned above<br />
are consistent with the band structure calculations on similar systems [34, 35].<br />
The pre-edge feature in the O K edge spectra carries a substantial amount of<br />
physics involved in the properties of these materials. It has earlier been shown that<br />
the pre-peak in the O K edge spectra of different CMR materials consists of two lines,<br />
one of which appears as a shoulder on the low-energy side of the other [21, 25, 27, 28].<br />
The intensity of this shoulder was found to increase as the material goes across the<br />
insulator-metal transition [27, 28]. Since our Pr 0.67 Ca 0.33 MnO 3 sample is an insulator<br />
at all temperatures the pre-peak in our O K edge spectra (Fig. 3.4) does not show any<br />
splitting, though the presence of the shoulder is visible as an asymmetry on the lowenergy<br />
side of this peak. This shoulder feature is due to the first available unoccupied<br />
states and in XAS corresponds to the addition of an electron to the e g↑ state of the<br />
crystal-field-split MnO 6 octahedra. The main feature in the pre-peak arises from the<br />
t 2g↓ states [21]. For a quantitative estimate of the temperature-dependent changes in<br />
the intensities of the two features we have fitted the pre-peak using two components of<br />
Lorentzian line shapes. Here also the data fitting was done using χ 2 iterative method,<br />
keeping the energy positions and widths the same for all temperatures. Results of the<br />
data fit are shown in Table 3.1. Fitted spectra for temperatures above (300 K) and<br />
below Tc (95 K) are shown on the right side of Figs 3.5(a) and 3.5(b). Though very<br />
small, the intensity of the feature A ′ shows a slight increase below T c . The best fit
Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 69<br />
gives a 0.6±0.1 eV difference between the energy positions of A ′ and B ′ . Following a<br />
different assignment of origin to the pre-peak features Dessau et al. have interpreted<br />
this energy difference as a measure of the Jahn-Teller distortion (4E JT ) [26].<br />
3.3.3 The Charge Transfer Energy (E CT )<br />
Having shown the temperature-dependent changes in the UPS and XAS results in<br />
Figs. 3.2 and 3.4, we present a combined picture of them in order to derive some insights<br />
into the charge transfer energy involved in the properties of the Pr 0.67 Ca 0.33 MnO 3<br />
sample. The combined spectra of the valence band from photoemission and the preedge<br />
region of the O K edge XAS, presented in Figs. 3.5(a) and 3.5(b) give the density<br />
of both occupied and unoccupied states above and below the Curie temperature (T c ).<br />
A schematic of these energy levels is shown in Fig. 3.6. The hole or electron doping<br />
(value of x) causes the symmetry of the last occupied and first unoccupied bands to<br />
be e gz 2 −r 2 ↑. In Fig. 3.5 these states are marked by A and A ′ . In order to scale the left<br />
and right sides of E F , the O K edge spectra have been shifted such that the energy<br />
of the first unoccupied state (1.4 eV from E F ) found from inverse photoemission [36]<br />
coincides with that of A ′ . We have used integral backgrounds for both sides of E F<br />
and, as mentioned earlier, all the fitting parameters were kept the same including<br />
the energy positions for the high and low temperatures. The important parameter<br />
which can be derived from this combined spectrum is the charge transfer energy E CT ,<br />
which is the energy required for an e g electron to hop between the Mn 3+ (t 3 2ge 1 g) and<br />
Mn 4+ (t 3 2g) sites. In our spectra (Fig. 3.5) E CT is the energy difference between A<br />
and A ′ which corresponds to the last occupied and first unoccupied states. The value<br />
of E CT is 2.6±0.1 eV from our spectra, which is higher than the value (1.5±0.4 eV)<br />
determined indirectly by Park et al. for the La 1−x Ca x MnO 3 system [20]. The larger<br />
value of E CT shows the existence of a stronger charge localization in the charge- or<br />
orbital-ordered Pr 0.67 Ca 0.33 MnO 3 compound.<br />
The value of E CT indicating a strong charge localization, found in our Pr 0.67 Ca 0.33<br />
MnO 3 has significant implications for the models proposed to explain the CMR effect<br />
in manganites. Charge localization can result from strong-coupling interactions<br />
of the e g electron with the core-like t 2g or the Q 2 and Q 3 Jahn-Teller modes. Both<br />
these interactions influence the electron hopping term (t). The former interaction,<br />
ferromagnetic in nature, is the Hund’s coupling J H . A large value of J H favors<br />
the electronic separation of phases. A strong electron-lattice coupling results from
Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 70<br />
E CT<br />
(a)<br />
Intensity (arb. units)<br />
C<br />
B<br />
A’<br />
B’<br />
A<br />
-5 0 5 10<br />
Energy Relative to E F<br />
(eV)<br />
(b)<br />
B<br />
E CT<br />
B’<br />
Intensity (arb. units)<br />
C<br />
A’<br />
A<br />
-5 0 5 10<br />
Energy Relative to E F<br />
(eV)<br />
Figure 3.5: (a) Combined spectra from valence band photoemission (occupied energy<br />
states) and pre-peak in O K edge x-ray absorption spectra (unoccupied energy states)<br />
of Pr 0.67 Ca 0.33 MnO 3 taken at room temperature (above T c ). The three features corresponding<br />
to the subbands in the valence region are marked A, B, and C.The two<br />
peaks of the fitted O K edge pre-peak are marked A ′ and B ′ . We used integral background<br />
for both sides of E F . Details of the fitting are mentioned in the text. (b)<br />
Combined spectra of Pr 0.67 Ca 0.33 MnO 3 below T c . The valence band spectrum was<br />
taken at 77 K and O K edge XAS was taken at 95 K.
Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 71<br />
2 2<br />
e g x − y<br />
J ex<br />
2 2<br />
e g z − r<br />
e g x 2 − y 2<br />
t 2g<br />
e z 2 2<br />
g − r<br />
2 2<br />
e g z − r<br />
E F<br />
t 2g<br />
Figure 3.6: Schematic diagram of the near-E F energy levels of Pr 0.67 Ca 0.33 MnO 3 in<br />
the occupied and unoccupied parts. The diagram is not drawn to scale. Some of the<br />
e g↑ states are occupied (at the Mn 3+ sites) and some are unoccupied (at the Mn 4+<br />
sites). Hence the last occupied and first unoccupied states are marked with dotted<br />
lines.<br />
the cooperative Jahn-Teller distortions of the MnO 6 octahedra. These distortions,<br />
particularly the Q 2 and Q 3 vibrational modes of the oxygen ions are stronger in<br />
the case of Pr-containing manganites compared to La-containing ones. Park et al.<br />
[20] have reported a smaller charge transfer energy (E CT = 1.5 eV) in the case of<br />
La 1−x Ca x MnO 3 and have attributed it to small polarons (Anderson localization) induced<br />
from the ionic size difference between Mn 3+ and Mn 4+ . A small-polaronic<br />
model may not be able to explain the high value of E CT found in Pr 0.67 Ca 0.33 MnO 3 .<br />
On the other hand, a strong charge localization is expected in the case of the PS<br />
model in which the ground state is described as a mixture of ferromagnetic and antiferromagnetic<br />
regions. It is also possible that this large charge localization is due to<br />
the Zener polaron proposed by Aladine et al [39]. In the Zener polaron picture, the<br />
Mn ions of adjacent MnO 6 octahedra form a dimer with variations in the Mn-O-Mn<br />
bond angle. The pseudo-CE-type charge ordering in Pr 1−x Ca x MnO 3 favors such regular<br />
distortions in the MnO 6 octahedra. Further studies using electron spectroscopic<br />
techniques on samples in the vicinity of the charge-ordered compositions may be able<br />
to differentiate between these possible driving mechanisms behind the strong charge
Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 72<br />
localization.<br />
3.4 Conclusions<br />
Using valence band photoemission and O K edge x-ray absorption, we have probed<br />
the electronic structure of Pr 0.67 Ca 0.33 MnO 3 , which is regarded as a prototype for the<br />
electronic phase separation models in CMR systems. With decrease in temperature<br />
the O 2p contributions to the t 2g and e g spin-up states in the valence band were<br />
found to increase until T c . Below T c , the density of states with e g spin-up symmetry<br />
increased while those with t 2g symmetry decreased, possibly due to the change in<br />
the orbital degrees of freedom associated with the Mn 3d - O 2p hybridization in the<br />
pseudo-CE-type charge or orbital ordering. These changes in the density of states<br />
could well be connected to the electronic phase separation reported earlier. Our<br />
quantitative estimate of the charge transfer energy (E CT ) is 2.6±0.1 eV, which is<br />
large compared to the earlier reported values in other CMR systems. Such a large<br />
charge transfer energy may support the phase separation model.
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Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 76<br />
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Chapter 4<br />
Electronic Structure of<br />
Pr 1−x Ca x MnO 3<br />
77
Electronic Structure of Pr 1−x Ca x MnO 3 78<br />
4.1 Introduction<br />
Mixed phase manganite systems with coexisting ferromagnetic (FM) and antiferromagnetic<br />
(AFM) domains have been attracting considerable attention recently due to<br />
their importance in understanding the role of spin, charge, and orbital ordering in the<br />
phenomena of colossal magneto resistance (CMR). Theoretical models, particularly<br />
those [1, 2, 3] based on electronic phase separation (PS) exploits this character of the<br />
CMR compounds in explaining the insulator-metal (IM) transitions found in these<br />
systems. The electronic PS scenario is phenomenologically different from the double<br />
exchange (DE) mechanism [4, 5, 6] or those based on strong Jahn-Teller polarons<br />
[7, 8]. In this scenario, the IM transition is driven by current percolation through<br />
FM metallic domains embedded in an AFM insulating background [2]. Though, there<br />
have been many structural studies [9, 10] showing the co-existence of such magnetic<br />
domains, the reported sizes of those vary from nano - to mesoscopic scales [11, 12]. In<br />
manganites, the electronic PS is expected to occur in systems in which the itinerant<br />
e g electrons have strong Hund’s coupling interaction with the localized t 2g electrons<br />
of the MnO 6 octahedra in their structure.<br />
Some of the charge ordered (CO) compositions of the Pr 1−x Ca x MnO 3 system are<br />
considered to be the prototypes for the PS models owing to their inherently reduced<br />
e g bandwidth W. Neutron diffraction studies have shown the co-existence of nanosized<br />
FM and AFM stripes [10, 12] in Pr 0.67 Ca 0.33 MnO 3 . It was also found that a fieldinduced<br />
transition possible between their nearly degenerate FM metallic and chargeordered<br />
AFM insulating phases [12, 13]. With slight variations in the Ca doping<br />
the Pr 1−x Ca x MnO 3 system turns ferromagnetic (x = 0.2) or antiferromagnetic (x =<br />
0.4) at low temperatures [10, 14]. The composition x = 0.33 shows a co-existence of<br />
ferromagnetic and antiferromagnetic phases [15]. The energy scales involved in the<br />
IM transitions in this FM-AFM system are of importance for the understanding of<br />
the physics behind the phenomena of CMR.<br />
In this chapter we have compared the binding energy positions of some of the near<br />
Fermi level (E F ) occupied and unoccupied states, crucial to the electrical and magnetic<br />
transitions of the Pr 1−x Ca x MnO 3 system using the complementary techniques<br />
of direct and inverse photoelectron spectroscopies. Electron spectroscopic techniques<br />
have been quite successful in probing the near E F low energy states involved in the<br />
charge and orbital interactions of CMR materials. Although, there are many reports<br />
on the electronic structure of other CMR compounds using some of these techniques
Electronic Structure of Pr 1−x Ca x MnO 3 79<br />
Figure 4.1: Phase diagram of the Pr 1−x Ca x MnO 3 system. In this study we have used<br />
the x = 0.2, x = 0.33 and x = 0.4 compositions. (FMI = ferromagnetic insulating,<br />
CO-AFMI = charge ordered antiferromagnetic insulating, CG = cluster glass, T N =<br />
Neel Temperature, T C = Critical Temperature, T CO = charge ordering temperature).<br />
[16, 17, 18, 19, 20, 21, 22] only a few studies have dealt with the energy scales of the<br />
electron interactions in charge ordered compositions [23, 24]. One of the key terms,<br />
these spectroscopies could show earlier, was the charge transfer energy E CT which is<br />
intimately related to the electron-electron and electron-lattice interactions [23, 24].<br />
Here we have analyzed the temperature dependent changes shown by three compositions<br />
across the charge ordered FM-AFM phase boundary of the Pr 1−x Ca x MnO 3<br />
system using ultraviolet photoelectron spectroscopy (UPS) and inverse photoelectron<br />
spectroscopy (IPES). Also we have studied different core levels of Pr 1−x Ca x MnO 3<br />
using x-ray photoelectron spectroscopy (XPS).<br />
4.2 Experimental<br />
Single crystals of Pr 1−x Ca x MnO 3 with x = 0.2, 0.33 and 0.4 were grown by the floating<br />
zone method in a mirror furnace. The compositional homogeneity of the crystals<br />
had been confirmed using energy dispersive spectroscopic (EDS) analysis. The<br />
crystals were characterized by using magnetization, electrical transport and neutron<br />
diffraction measurements. Details of the sample preparation and characterization
Electronic Structure of Pr 1−x Ca x MnO 3 80<br />
measurements are published elsewhere [10, 25]. There are also three polycrystalline<br />
samples (PrMnO 3 , CaMnO 3 and Pr 0.16 Ca 0.14 MnO 3 ) used for different studies. Which<br />
were prepared by the standard solid state reaction method. The details of the sample<br />
preparation is published elsewhere [26]. Consolidated results of these studies are<br />
presented in the phase diagram shown in Fig. 4.1 [27]. Angle integrated ultraviolet<br />
photoemission measurements were performed by using an Omicron mu-metal ultra<br />
high vacuum system equipped with a high intensity vacuum ultraviolet source (HIS<br />
13) and a hemispherical electron energy analyzer (EA 125 HR). At the He I (21.2<br />
eV) line, the photon flux was of the order of 10 16 photons/sec/steradian with a beam<br />
spot of 2.5 mm diameter. The Fermi energies (E F ) for all measurements were calibrated<br />
by using a freshly evaporated Ag film on a sample holder. The total energy<br />
resolution, estimated from the width of the Fermi edge, was about 80 meV. The XPS<br />
measurements were performed using a twin anode Mg/Al x-ray source (DAR 400) in<br />
the same chamber as of UPS. In our measurements we have used the Mg Kα line with<br />
photon energy 1253.6 eV and the resolution was about 1.0 eV. The samples were repeatedly<br />
scraped by using a diamond file inside the chamber under a base vacuum of<br />
∼ 1.0 x 10 −10 mbar. The negligible intensity found for the ∼ 9.5 eV bump [16] in the<br />
UPS spectra ensures the cleanliness of our sample surface. For the temperature dependent<br />
measurements, the samples were cooled by pumping liquid nitrogen through<br />
the sample manipulator fitted with a cryostat. The sample temperatures were measured<br />
by using a silicon diode sensor touching the bottom of the sample holder. The<br />
IPES measurements were performed by using an electrostatically focused electron gun<br />
of Stoffel Johnson design and an acetone gas filled band pass Geiger Muller counter<br />
with a CaF 2 window [28, 29]. These measurements were carried out in the isochromat<br />
mode in which the photon energy is fixed at 9.9 eV and kinetic energy of the incident<br />
electrons is varied in 0.05 eV steps. Overall resolution of the IPES measurements<br />
were estimated [29] to be approximately 0.55 eV.<br />
4.3 Results and Discussion<br />
4.3.1 The UPS study of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33 and 0.4<br />
Neutron diffraction studies [27] on Pr 1−x Ca x MnO 3 have shown that the compositions<br />
with x = 0.2 and x = 0.4 are FM and AFM respectively (Fig. 4.1) while the x = 0.33<br />
has co-existing FM and AFM phases [14, 15]. The ground state of x = 0.33 has been
Electronic Structure of Pr 1−x Ca x MnO 3 81<br />
identified to be an inhomogeneous state below T CO . The x = 0.4 is above the critical<br />
doping (x c ) found in the Pr 1−x Ca x MnO 3 system and is homogeneous in nature.<br />
(a) 300 K<br />
C<br />
B<br />
(b) 77 K<br />
C<br />
B<br />
Intensity (arb. units)<br />
A<br />
1.0<br />
0.4<br />
0.33<br />
Intenisty (arb. units)<br />
A<br />
1.0<br />
0.4<br />
0.33<br />
0.2<br />
0.2<br />
0.0<br />
0.0<br />
10<br />
8<br />
6<br />
4<br />
2<br />
0<br />
10<br />
8<br />
6<br />
4<br />
2<br />
0<br />
Binding Energy (eV)<br />
Binding Energy (eV)<br />
Figure 4.2: The angle integrated valence band photoemission spectra of the<br />
Pr 1−x Ca x MnO 3 (x = 0.0, 0.2, x = 0.33, x = 0.4 and x = 1.0) samples taken at<br />
room temperature and 77K using He I photons (21.2 eV).<br />
Figure 4.2 shows the angle integrated valence band photoemission spectra of the<br />
Pr 1−x Ca x MnO 3 (x = 0.2, x = 0.33 and x = 0.4) samples taken at room temperature<br />
and 77K. The figure also shows the spectra of the end compositions of this system (x<br />
= 0.0 and x = 1.0). It may be noted from the phase diagram that the compositions<br />
with x = 0.2, x = 0.33 and x = 0.4 are PM insulators at 300K and FM/AFM insulators<br />
at 77K. Intensities of all the spectra shown in both the panels were normalized<br />
and shifted along the ordinate axis by a constant value for clarity. All the features<br />
seen in the spectra are dominated by the states due to the Mn 3d - O 2p hybridized<br />
orbitals. The two prominent features, one at ∼ 3.5 eV (marked B) and another at ∼<br />
5.6 eV (marked C), are primarily due to the O 2p nonbonding states and the Mn 3d<br />
- O 2p bonding states [30]. The feature closest to the E F , which appear as a tail at
Electronic Structure of Pr 1−x Ca x MnO 3 82<br />
∼ 1 eV (marked A) is due to the e g states of the MnO 6 octahedra [18, 24, 31, 32].<br />
The intensity of this feature is low due to the insulating nature of all our samples.<br />
Presence of the feature A is clear from Fig. 4.3(a) and 4.3(b) where we have shown<br />
the photoemission spectra from the near E F region of the x = 0.2, x = 0.33 and x = 0.4<br />
samples along with that of CaMnO 3 , taken at temperatures above and below the T CO .<br />
The lower panels of the figures show the difference spectra obtained by subtracting the<br />
spectra of CaMnO 3 from those of the different Pr 1−x Ca x MnO 3 compositions. Since,<br />
CaMnO 3 does not have any electrons in its e g levels, the difference spectra should<br />
correspond to the density of the e g electrons in the Pr 1−x Ca x MnO 3 samples. The<br />
shape of the difference spectra shows that it consists of two features, similar to those<br />
reported by Ebata et al. [33]. We have fitted all the difference spectra corresponding<br />
to the three compositions at two different temperatures with a mixture of Lorentzian<br />
and Gaussian line shapes using a χ 2 iterative programme. We used a fixed width<br />
for all the fitted lines marked A’ for both the temperatures in order to see the finer<br />
changes associated with the A”. The energy positions of the A’ and A” and the<br />
width of the A” were determined by the iterative programme. Table 4.2 shows the<br />
results of the fitting. At room temperature, the changes across the three compositions<br />
are negligible except that the sample with x = 0.33 has a larger width for its near<br />
E F feature marked A”. At 77K, both A’ and A” have been found to be shifted<br />
slightly towards the E F . Further, at this temperature also, the x = 0.33 shows a<br />
wider A”. The width of the near E F feature is coupled to the conductivity of these<br />
samples. Earlier experiments using valence band photoemission have shown that<br />
a finite number of states get transferred from A” to A’ when some CMR systems<br />
go across the temperature - composition phase diagram [31, 33, 34] resulting in the<br />
formation of pseudogaps at the E F . The energy scales involved in such shifts of DOS<br />
are of importance to the theoretical models, particularly for those based on electronic<br />
phase separation. Formation of pseudogaps are a generic feature of mixed phase<br />
CMR systems [31, 35] with FM electron rich phases embedded in an insulating AFM<br />
matrix. The feature A’ in our spectra could possibly be ascribed to the e g electrons<br />
in the AFM phases while the A” to those in the FM phases. The difference in the<br />
binding energy positions of A’ and A” (0.47±0.01 eV) gives an approximate measure<br />
of the energy difference between the e g electronic states in these two phases. In other<br />
words, this gives the energy scale over which the pseudogap forms. The increased<br />
width of the feature A” of x = 0.33 at temperatures above and below the T CO may
Electronic Structure of Pr 1−x Ca x MnO 3 83<br />
(a)<br />
300 K 300 K 300 K<br />
Intensity (arb. units)<br />
A’<br />
A’<br />
A’<br />
A’’ A’’ A’’<br />
0.2 - CaMnO3 0.33 - CaMnO3 0.4 - CaMnO3<br />
1.5 1 0.5 0 1.5 1 0.5 0<br />
1.5<br />
1<br />
0.5<br />
0<br />
Binding Energy (eV)<br />
(b)<br />
77 K 77 K<br />
77 K<br />
Intensity (arb. units)<br />
A’<br />
A’<br />
A’<br />
A’’<br />
A’’<br />
A’’<br />
0.2 - CaMnO3 0.33 - CaMNO3 0.4 - CaMnO3<br />
1.5<br />
1<br />
0.5<br />
0<br />
1.5<br />
1<br />
0.5<br />
0<br />
1.5<br />
1<br />
0.5<br />
0<br />
Binding Energy (eV)<br />
Figure 4.3: The near E F region of the valence band spectra (panel(a): 300 K,<br />
panel(b): 77 K) of the three samples plotted against that of CaMnO 3 . The difference<br />
spectra (lower panel) is obtained by subtracting the spectra of CaMnO 3 from<br />
those of the three Pr 1−x Ca x MnO 3 compositions and multiplied by 5. The difference<br />
spectra corresponds to the density of e g states. The red and blue peaks are A’ and<br />
A” respectively. The green line shows the spectral background used in the fitting.
Electronic Structure of Pr 1−x Ca x MnO 3 84<br />
Table 4.2: Results of the fitting of the difference spectra using a χ 2 iterative programme.<br />
We have used a mixture of Lorentzian and Gaussian line shapes for fitting<br />
the various difference spectra corresponding to the three compositions at 300K and<br />
77K. In order to see the finer changes in the near E F feature A”, we used a fixed<br />
width for all the fitted lines marked A’. The energy positions of the A’ and A” and the<br />
width of the A” were determined by the iterative programme. At both the temperatures<br />
the sample with x = 0.33 shows a larger full width at half maximum (FWHM)<br />
for its A”. At 77K, both A’ and A” have been found to be shifted slightly towards<br />
the E F .<br />
Temperature (K)<br />
300<br />
77<br />
x<br />
A’ A”<br />
Position (eV) FWHM (eV) Position (eV) FWHM (eV)<br />
0.2 0.97 0.5 0.49 0.5<br />
0.33 0.96 0.5 0.49 0.65<br />
0.4 0.96 0.5 0.49 0.59<br />
0.2 0.92 0.5 0.45 0.49<br />
0.33 0.93 0.5 0.45 0.53<br />
0.4 0.92 0.5 0.46 0.5<br />
indicate the formation of FM clusters in the AFM insulating background. From the<br />
fitting of the difference spectra we feel that the increase of the width of A” is largely<br />
due to the formation of a tail towards the E F , which makes it difficult to estimate<br />
the energy difference between A’ and A” accurately.<br />
4.3.2 The IPES study of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33, 0.4<br />
and 1<br />
Fig. 4.4 shows the IPES spectra of the Pr 1−x Ca x MnO 3 samples (x = 0.2, 0.33 and<br />
0.4) and the CaMnO 3 sample taken at room temperature. The spectra have been<br />
normalized for their intensities and shifted along the vertical axis by a constant for<br />
clarity. The spectra shows two broad features; one between 5 to 12 eV (marked Q)<br />
and another at around 2.4 eV (marked P). For the x = 0.2, 0.33, and 0.4, the feature<br />
appearing at higher energy is mostly due to Pr 5d and Ca 3d orbitals, whereas for<br />
the CaMnO 3 it is due only to the Ca 3d, 4s orbitals. The feature P is due to the<br />
hybridized Mn 3d - O 2p states. These assignments of the features are consistent<br />
with band structure calculations and other studies on similar systems [24, 36, 37].<br />
Similar energy positions were reported earlier for La 1−x Sr x MnO 3 by Chainani et al.
Electronic Structure of Pr 1−x Ca x MnO 3 85<br />
Q<br />
x=1<br />
P<br />
x=0.4<br />
Intensity (arb. units)<br />
x=0.33<br />
x=0.2<br />
0 2.5 5 7.5 10 12.5 15<br />
Energy Relative to Fermi Energy (eV)<br />
Figure 4.4: The IPES spectra of the Pr 1−x Ca x MnO 3 samples (x = 0.2, 0.33 and 0.4)<br />
and the CaMnO 3 sample taken at room temperature. The spectra represents the<br />
unoccupied electron states of the samples.<br />
[38], though another study [39] showed the feature P to be more closer to the E F .<br />
Again, in the IPES spectra also, the feature appearing near to E F is important to<br />
properties of these compounds. In Fig. 4.5 we have shown the spectra of the near E F<br />
region containing this feature along with the difference spectra (Fig. 4.5(b)). One can<br />
see clearly that the intensity of P increases with the Ca concentration x due to the<br />
increase of unoccupied states in the Mn 3d orbitals with hole doping. From a linear<br />
extrapolation of the rising band edge in the spectra shown in the Fig. 4.5(a) we have<br />
estimated the band gap to be ∼ 0.7 eV for the three compositions (0.2, 0.33, and 0.4).<br />
At this temperature, the value of the gap does not seem to vary with the charge carrier<br />
concentration, whereas the parent compound (Ca x MnO 3 ) shows a smaller value (∼<br />
0.67 eV). In case of La 1−x Sr x MnO 3 , Chainani et al. [38] has shown the value of the<br />
band gap to be ∼ 0.6 eV. The larger values shown by our Pr 1−x Ca x MnO 3 samples<br />
could be attributed to their insulating nature. In an alternate approach, one can also<br />
estimate the band gap from the difference spectra shown in the Fig. 4.5(b). This<br />
gives a some what larger value; i.e ∼ 0.86 eV for the x = 0.4 and 0.33 and ∼ 0.82 for
Electronic Structure of Pr 1−x Ca x MnO 3 86<br />
(a)<br />
(b)<br />
Intensity (arb. units)<br />
x=1<br />
x=0.4<br />
1 - 0.2<br />
0.4 - 0.2<br />
x=0.33<br />
x=0.2<br />
0.33 - 0.2<br />
-1 0 1 2 3 4<br />
0 1 2 3 4<br />
Energy Relative to Fermi Energy (eV)<br />
Figure 4.5: Panel(a) shows the IPES spectra of the near E F unoccupied states.<br />
Panel(b) shows the difference spectra obtained by subtracting the spectra corresponding<br />
to the x = 0.2 from those of other compositions.<br />
the Ca x MnO 3 .<br />
Band structure calculations on similar CMR systems [12, 32] have unambiguously<br />
shown that both the highest occupied and lowest unoccupied states in these systems<br />
are of Mn 3d e g spin-up symmetry. As mentioned earlier, within this frame work the<br />
features marked A’ and A” in Fig. 4.3 and the feature P in Fig. 4.4 correspond to the<br />
e g spin-up states. Combining the results presented in Table I and these two figures<br />
one can estimate the energy required by an e g electron to hop from a Mn 3+ site to the<br />
neighboring Mn 4+ site. If we consider the centroids of the A’ in Fig. 4.3 and the P in<br />
Fig. 4.4 we arrive at a value of ∼ 3.3±0.2 eV, which is large compared to the earlier<br />
reported value [24]. With the A” of Fig. 4.3 and P of Fig. 4.4 we get ∼ 2.8±0.2<br />
eV which is close to the earlier report (2.6±0.1 eV). Due to the large widths and<br />
long tails of the A” and P towards the E F we expect that this value of the energy be<br />
close to its upper bound. Energy required for the transfer of e g electrons between the<br />
Mn sites again is an important parameter for the phase separation models to work in
Electronic Structure of Pr 1−x Ca x MnO 3 87<br />
these systems.<br />
4.3.3 The core levels of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33, 0.4,<br />
and 0.84<br />
The Mn2p core level spectra of Pr 1−x Ca x MnO 3 (x = 0.2, 0.33, 0.4, and 0.84) taken<br />
at room temperature as well as at 77 K are shown in Fig. 4.6. All the spectra were<br />
normalized and shifted along the Y axis by a constant for clarity. The two prominent<br />
peaks at around ∼ 641.5 and ∼ 653.5 eV are due to the spin orbit split 2p 3/2 and 2p 1/2 .<br />
These spectra look similar to those reported earlier on the similar system [17, 33]. In<br />
Fig. 4.6, the Mn 2p peak positions shift towards the higher binding energy for x =<br />
0.4 and 0.84 at room temperature (a) as well as at 77 K (b). But the binding energy<br />
shift is higher incase of room temperature spectra as compared to that of 77 K. The<br />
Mn LMV Auger peak is observed on the higher binding energy side of 2p 1/2 peak<br />
for all four concentrations at room temperature, whereas at 77 K this Auger peak is<br />
absent for x = 0.2, and 0.3. This Mn LMV was earlier observed in other hole doped<br />
manganites [17].<br />
The Ca 2p spectra of Pr 1−x Ca x MnO 3 (x = 0.2, 0.33, 0.4 and 0.84) both at room<br />
temperature as well as 77 K are shown in the Fig. 4.7. The Ca 2p 3/2 and Ca 2p 1/2 ,<br />
due to spin orbit splitting, are clearly distinguishable at around ∼ 345.5 and ∼ 349<br />
eV respectively. It is clear from the Fig. 4.7 (a) that, at room temperature the peak<br />
positions shift towards the lower binding energy when we move from x = 0.2 to x =<br />
0.84. Also the same kind of shift is observed for 77 K spectra (Fig. 4.7 (b)) with a<br />
lesser magnitude.<br />
The Pr 4d spectra of Pr 1−x Ca x MnO 3 (x = 0.2, 0.33 and 0.4) taken at room<br />
temperature as well as 77 K are shown in Fig. 4.8. The Pr 4d peaks come around<br />
116 eV binding energy. The peak positions shift towards the lower binding energy<br />
at room temperature (Fig. 4.8 (a)) when we move from x = 0.2 to 0.4. Here also<br />
the similar kind of shift is observed for 77 K spectra (Fig. 4.8 (b)) with a lesser<br />
magnitude.<br />
As mentioned above the binding energy shifts with respect to the value of x have<br />
been observed at the Ca 2p, Pr 4d and Mn 2p core levels. Incase of Mn 2p level, the<br />
binding energy shift is to the higher energy side while the other two levels shift to lower<br />
energy. The magnitude of the binding energy shifts are found to be higher at room<br />
temperature compared to 77 K. The shift to higher binding energy observed in Mn
Electronic Structure of Pr 1−x Ca x MnO 3 88<br />
Mn LMV<br />
Temp = 300 K<br />
2p 3/2<br />
Intensity (arb. units)<br />
0.84<br />
0.4<br />
0.33<br />
Mn 2p<br />
0.2<br />
670<br />
660<br />
2p 1/2<br />
640<br />
650<br />
640<br />
Binding energy (eV)<br />
(a)<br />
Mn LMV<br />
Temp = 77 K<br />
2p 2p 1/2 3/2<br />
Intensity (arb. units)<br />
0.84<br />
0.4<br />
0.33<br />
Mn 2p<br />
0.2<br />
670<br />
660<br />
650<br />
Binding Energy (eV)<br />
(b)<br />
Figure 4.6: The Mn 2p core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken at<br />
room temperature (a) and 77 K (b) .
Electronic Structure of Pr 1−x Ca x MnO 3 89<br />
Ca 2p<br />
Temp = 300 K<br />
2p 3/2<br />
2p<br />
1/2<br />
Intensity (arb. units)<br />
0.84<br />
0.4<br />
0.33<br />
0.2<br />
352<br />
348 344<br />
Binding Energy (eV)<br />
(a)<br />
340<br />
Ca 2p<br />
2p 1/2<br />
2p 3/2<br />
Temp = 77 K<br />
Intensity (arb. units)<br />
0.84<br />
0.4<br />
0.33<br />
0.2<br />
352<br />
348 344<br />
Binding Energy (eV)<br />
(b)<br />
340<br />
Figure 4.7: The Ca 2p core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken at<br />
room temperature (a) and 77 K (b).
Electronic Structure of Pr 1−x Ca x MnO 3 90<br />
Pr 4d<br />
Temp = 300 K<br />
Intensity (arb. unis)<br />
0.4<br />
0.3<br />
0.2<br />
130<br />
120 110<br />
Binding Energy (eV)<br />
(a)<br />
100<br />
Pr 4d<br />
Temp = 77 K<br />
Intensity (arb. units)<br />
0.4<br />
0.33<br />
0.2<br />
130<br />
120 110<br />
Binding Energy (eV)<br />
(b)<br />
100<br />
Figure 4.8: The Pr 4d core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken at<br />
room temperature (a) and 77 K (b).
Electronic Structure of Pr 1−x Ca x MnO 3 91<br />
2p could be due to the increase in the effective Mn valence state from Mn 3+ towards<br />
Mn 4+ . The binding energy shift incase of Ca 2p and Pr 4d core levels could possibly<br />
due to the shift in the chemical potential [33, 40]. The low temperature data shows<br />
a smaller value for this chemical potential shift. Such a suppression of the chemical<br />
potential shift have been observed earlier in other transition metal oxides [41, 42] is<br />
mostly due to the formation of stripes. According to the phase separation model, a<br />
pinning of the chemical potential is consequence of the mixed phase nature of these<br />
materials. The numerical studies [1, 3] have indeed shown the pinning of chemical<br />
potential in the composition range, where the phase separation is realized. These<br />
results also support the model of phase separation in the Pr 1−x Ca x MnO 3 system.<br />
4.4 Conclusions<br />
Using UV photoelectron spectroscopy and inverse photoelectron spectroscopy we have<br />
deduced some of the important energy scales in the Pr 1−x Ca x MnO 3 system. Our<br />
photoemission studies across the FM-AFM phase boundary show that the pseudogap<br />
formation in these compounds occur over an energy scale of 0.47±0.01 eV. From a<br />
combined analysis of the photoemission and inverse photoemission spectra we also<br />
have found the energy required for the transfer of Mn 3d e g electrons between the adjacent<br />
Mn sites to be of the order of ∼ 2.8±0.2 eV. Though an accurate measurement<br />
of these energies are not possible due to the inherent limitations of the compounds<br />
and the technique, our results give an upper bound of them. Further, we have discussed<br />
our results from a possible phase separation scenario. From the core level<br />
study of Pr 1−x Ca x MnO 3 , we observed a suppression of chemical potential shift for 77<br />
K spectra compared to that of room temperature. These results support the model<br />
of phase separation in Pr 1−x Ca x MnO 3 system.
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Chapter 5<br />
Electronic Structure of<br />
Ca 0.86 Pr 0.14 MnO 3<br />
96
Electronic Structure of Ca 0.86 Pr 0.14 MnO 3 97<br />
5.1 Introduction<br />
The strong interplay among charge, spin, lattice and orbital degrees of freedom of electrons<br />
form the basis of the phenomena of colossal magnetoresistance (CMR) shown<br />
by manganites. To a large extent the traditional models [1, 2, 3] employing the<br />
charge-spin coupling, have been able to explain the CMR in many of the hole doped<br />
compositions (R 1−x A x MnO 3 with x
Electronic Structure of Ca 0.86 Pr 0.14 MnO 3 98<br />
5.2 Experimental<br />
The polycrystalline sample of Ca 0.86 Pr 0.14 MnO 3 was prepared by standard solid state<br />
reactions. Details of the sample preparation technique is published elsewhere [4]. Purity,<br />
homogeneity and composition of the sample were checked by electron diffraction<br />
and energy dispersive spectroscopic analysis. The electrical and magnetic properties<br />
of this composition has also been published earlier [4, 15]. This sample shows<br />
a metallic behavior down to ∼ 200K and then the resistivity slightly increases till<br />
the T c (110 K). Below the T C the behavior is insulating and the resistivity goes up<br />
by many orders of magnitude. Generally, the conductivity behavior of this sample is<br />
similar to that of the other electron doped systems [4, 8, 15].<br />
High resolution photoemission and x - ray absorption spectroscopy measurements<br />
were performed by using the light from the soft x - ray beam line WERA of ANKA<br />
synchrotron, Karlsruhe, Germany. We used the monochromatized radiation from<br />
an undulator. The HRPES measurments were carried out using the photon energy<br />
124 eV. The photoelectrons were collected using a Scienta SES 2002 electron energy<br />
analyzer. The energy resolution was 25 meV in case of the narrow scan (-2 - 2.5 eV)<br />
and it was 125 meV incase of wide scan (-2 - 15 eV) of the valence band. The O K XAS<br />
spectra has been taken in the fluorescence detection mode and the energy resolution<br />
was set to 150 meV. The sample surface was cut using a diamond knife under a base<br />
vacuum of ∼ 1.0 x 10 −10 mbar before taking the data for each temperature. In case of<br />
the photoemission measurements, scraping was repeated until negligible intensity was<br />
found for the bump around 9 - 10 eV, which is a signature of surface contamination<br />
[16]. For the temperature dependent measurements, the samples were cooled by<br />
pumping liquid helium through the sample manipulator fitted with a cryostat. The<br />
sample temperatures were measured by using a silicon diode sensor touching the<br />
bottom of the sample holder.<br />
5.3 Results and Discussion<br />
Figure 5.1 shows the high resolution photoemission spectra of the Ca 0.86 Pr 0.14 MnO 3 .<br />
Varius spectra, taken at 30 K, 70 K, 110 K, 220 K and 293 K show the temperature<br />
dependent changes in the valence band electronic structure. Intensities of all the<br />
spectra were normalized and shifted along the ordinate axis by a constant value for<br />
the clarity of presentation. All the features seen in the spectra are dominated by the
Electronic Structure of Ca 0.86 Pr 0.14 MnO 3 99<br />
B<br />
D<br />
C<br />
Intensity (arb. units)<br />
300 K<br />
220 K<br />
110 K<br />
A<br />
70 K<br />
30 K<br />
12<br />
10<br />
8<br />
6<br />
4<br />
2<br />
0<br />
Binding Energy (eV)<br />
Figure 5.1: The angle integrated valence band photoemission spectrum of<br />
Ca 0.86 Pr 0.14 MnO 3 taken using 124 eV photons. The spectra taken at different temperatures<br />
(30 K, 70 K, 110 K, 220 K and 293 K) are normalized for their intensities<br />
and shifted along Y-axis by a constant for clarity of presentation.
Electronic Structure of Ca 0.86 Pr 0.14 MnO 3 100<br />
Intensity (arb. units)<br />
A<br />
30 K<br />
70 K<br />
110 K<br />
220 K<br />
300 K<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
-0.2<br />
Binding Energy (eV)<br />
Figure 5.2: The high resolution spectra of the near E F region of the valence band of<br />
Ca 0.86 Pr 0.14 MnO 3 for 30 K, 70 K, 110 K, 220 K and 300 K. This feature refers to the<br />
e g band.<br />
states due to the Mn 3d - O 2p hybridization. The broad feature appearing at ∼ 6 eV<br />
(marked D) is due to the bonding states of this hybridization while its non-bonding<br />
states appear as a shoulder at ∼ 3.2 eV (marked C). The peak at ∼ 2 eV (marked B)<br />
is due to the Mn 3d t 2g states of the MnO 6 octahedra [17]. The Mn 3d e g↑ states of<br />
the octahedra appear around ∼ 0.3 eV (marked A) in the spectra. It should be noted<br />
that the intensity of the peak B decreases with decrease of temperature while that of<br />
the feature A increases. These spectral weight shifts are characteristic of many CMR<br />
systems. The e g states lying close to the E F are important for the conductivity of this<br />
material. A high resolution spectra of the near E F region is presented in figure 5.2.<br />
As the temperature is lowered from 293 K the feature A starts appearing. Further,<br />
below the transition temperature T c (110 K), the intensity of A steeply increases and<br />
goes through a maximum. The 30 K spectrum again shows a smaller intensity for<br />
this feature compared to the one at 70 k. Interestingly, below the T c , the leading<br />
edges of the different spectra move towards the E F as we go down in temperature.
Electronic Structure of Ca 0.86 Pr 0.14 MnO 3 101<br />
Mn 3d + O 2p<br />
Pr 5d, Ca 3d<br />
Mn 4sp, Pr 6sp<br />
Normalized Absorption (arb. units)<br />
300 K<br />
200 K<br />
150 K<br />
80 K<br />
30 K<br />
530 540 550 560<br />
Photon Energy (eV)<br />
Figure 5.3: The O K-edge x-ray absorption spectra of the Ca 0.86 Pr 0.14 MnO 3 taken at<br />
30 K, 80 K, 150 K, 200 K and 300 K. The pre-edge feature appears between 527 -<br />
533 eV is mostly due to the strong hybridization between Mn 3d and O 2p orbitals,<br />
where most interest lies. Origin of other features are given in the text.<br />
This shifting of the valence band edges is a clear indication of increase in the width<br />
(W) of the e g band below the T c .<br />
In order to see the corresponding changes in the near E F unoccupied states we have<br />
taken the O K edge x-ray absorption spectra (XAS) of the Ca 0.86 Pr 0.14 MnO 3 sample<br />
at different temperatures (Fig. 5.3). Here, all the spectra have been normalized to<br />
the incoming intensity and absolute cross sections and are also corrected for selfabsorption<br />
and saturation effects. The spectra shows three broad features. The one<br />
between 527 eV to 533 eV, called the pre-edge feature in the O K edge spectra, is<br />
mostly due to the strong hybridization between Mn 3d and O 2p orbitals. The feature<br />
around 536.5 eV is due to the bands from hybridized Pr 5d and Ca 3d orbitals, while
Electronic Structure of Ca 0.86 Pr 0.14 MnO 3 102<br />
Normalized Absorption (arb. units)<br />
A’<br />
B’<br />
C’<br />
30 K<br />
80 K<br />
150 K<br />
200 K<br />
300 K<br />
527 528 529 530 531 532 533 534<br />
Photon Energy (eV)<br />
Figure 5.4: The pre-edge peak of the O K -edge XAS spectra of Ca 0.86 Pr 0.14 MnO 3 for<br />
temperatures 30 K, 80 K, 150 K, 200 K and 300 K.<br />
the structure above 540 eV originates from the states like Mn 4sp and Pr 6sp, etc.<br />
These assignments of the features are consistent with the earlier experiments and<br />
band structure calculations on similar systems [21, 22, 23]. The pre-edge region of<br />
the O K XAS has earlier been shown to represent the near E F unoccupied states<br />
in these kind of compounds [21, 22, 23]. Figure 5.4 shows the pre-edge region of<br />
the XAS spectra of taken at different temperatures. The feature extending from<br />
∼ 527 to 531 eV consists of two lines. The first one is due to the O 2p orbitals<br />
hybridized with the Mn 3d e g↑ (marked A’) while the second arises from the O 2p -<br />
Mn 3d t 2g↓ hybridization (marked B’). The weak feature centered around ∼ 531.5 eV<br />
corresponds to the e g↓ (marked C’) states [23, 24, 25, 26, 27]. All the features show<br />
some temperature dependence. As the temperature is lowered from 300 K to 30 K<br />
the intensity of A’ decreases while that of B’ increases. This again shows a spectral<br />
weight shift complementary to the one observed in the photoemission spectra (Fig.
Electronic Structure of Ca 0.86 Pr 0.14 MnO 3 103<br />
5.1). Here, the spectral weight get shifted from near E F positions to higher energy<br />
as we go down in temperature. One can also see a shift in the energy position of the<br />
feature originating from the e g↓ states.<br />
Structural studies [19] have shown that the Ca 0.86 Pr 0.14 MnO 3 has an orthorhombic<br />
perovskite structure at 293 K. Below 110 K its structure changes to elongated pseudotetragonal<br />
with four short a-b plane and two long out-of-plane Mn-O-Mn bonds.<br />
This structural change can lead to an increase in the in-plane Mn 3d - O 2p hybridization<br />
strength and a consequent increase in the W. The broadening of feature A below<br />
T c in the photoemission spectra (Fig. 2) is due to this increase in W. The parent<br />
compound of our sample, CaMnO 3 has an empty e g band. In the Ca 0.86 Pr 0.14 MnO 3 ,<br />
the e g band could well be considered as rarely occupied. The spin-spin interaction of<br />
the e g electrons in this narrow band can lead to a localization of them resulting in<br />
the formation of FM clusters. Earlier Martin et. al [20] have shown the existence of<br />
such FM clusters embedded in an AFM matrix in electron-doped compositions. The<br />
near E F electronic spectra of such phase separated systems are known to display a<br />
pseudogap behavior [18]. The spectral weight shifts observed in the photoemission<br />
and O K XAS spectra of Ca 0.86 Pr 0.14 MnO 3 corroborate this behavior. The enhanced<br />
shift of electronic states from B to A observed in the photoemission spectra taken below<br />
the T c could be due to the structural changes across the transition. The changes<br />
in the topology of the MnO 6 octahedra below the T c mentioned earlier, might result<br />
in the shift of electrons from the t 2g levels to the near E F positions because of<br />
the increased strength in the in-plane Mn 3d -O 2p hybridization. The unoccupied<br />
states represented in pre-edge region of the O K XAS spectra, on the other hand,<br />
shows complementary shifts of states from the near E F to higher energy positions<br />
with decrease of temperature.<br />
5.4 Conclusions<br />
Using high resolution photoelectron spectroscopy and O K edge XAS, we have studied<br />
the temperature dependent changes in the near E F electronic structure of the electron<br />
doped CMR, Ca 0.86 Pr 0.14 MnO 3 . High resolution photoemission study show changes<br />
in the e g electron band width and a spectral weight shift across the transition which<br />
could be ascribed to the structural changes and consequent changes in the Mn 3d - O<br />
2p hybridization strengths. On the other hand, a complementary shifts of states from
Electronic Structure of Ca 0.86 Pr 0.14 MnO 3 104<br />
the near E F to higher energy positions were observed with decrease of temperature<br />
in the unoccupied states presented in the pre-edge region of O K XAS spectra.
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Chapter 6<br />
Summary and Conclusions<br />
108
Summary and Conclusions 109<br />
In this thesis the electronic structure of Pr 1−x Ca x MnO 3 series have been investigated<br />
using photoemission, inverse photoemission and x-ray absorption spectroscopy.<br />
Using photoemission (UPS and XPS), we have studied the valence band and core levels<br />
of Pr 1−x Ca x MnO 3 . The unoccupied states of Pr 1−x Ca x MnO 3 have been studied<br />
by inverse photoemission spectroscopy and x-ray absorption spectroscopy.<br />
A brief overview of the structural, transport and magnetic properties associated<br />
with the CMR manganites have been described in the introduction of this thesis.<br />
Details of the experimental techniques used to study the electronic structure of<br />
Pr 1−x Ca x MnO 3 have been described in the chapter 2.<br />
Using valence band photoemission and O K edge x-ray absorption, we studied<br />
the temperature dependent finer changes in the near E F electronic structure of<br />
Pr 0.67 Ca 0.33 MnO 3 , which is regarded as a prototype for the electronic phase separation<br />
models in CMR systems. With decrease in temperature the O 2p contributions<br />
to the t 2g and e g spin-up states in the valence band were found to increase until T c .<br />
Below T c , the density of states with e g spin-up symmetry increased while those with<br />
t 2g symmetry decreased, possibly due to the change in the orbital degrees of freedom<br />
associated with the Mn 3dO 2p hybridization in the pseudo-CE-type charge or orbital<br />
ordering. These changes in the density of states could well be connected to the electronic<br />
phase separation reported earlier. By combining the UPS and XAS spectra,<br />
we derived a quantitative estimate of the charge transfer energy E CT (2.6±0.1 eV),<br />
which is large compared to the earlier reported values in other CMR systems. Such<br />
a large charge transfer energy was found to support the phase separation model.<br />
For a complete understanding of the changes in the occupied and unoccupied<br />
electronic states we performed a detailed study of the Pr 1−x Ca x MnO 3 system across<br />
their ferromagnetic - antiferromagnetic phase boundary using UPS and IPES. We<br />
used three compositions x = 0.2, 0.33 and 0.4. Our photoemission studies showed<br />
that the pseudogap formation in these compositions occur over an energy scale of<br />
0.48±0.02 eV. Here again, we have estimated the charge transfer energy in these<br />
compositions to be of the order of ∼ 2.8±0.2 eV. The core level electronic structure<br />
of Pr 1−x Ca x MnO 3 (x = 0.2, 0.33, 0.4 and 0.84) also have been studied using X-ray<br />
photoelectron spectroscopy. These studies have been performed at the Mn 2p, Ca 2p<br />
and Pr 4d levels. From these core level studies we observed a suppression of chemical<br />
potential shift for 77 K spectra compared to that of room temperature. These results<br />
support the model of phase separation in Pr 1−x Ca x MnO 3 system.
Summary and Conclusions 110<br />
Using high resolution photoelectron spectroscopy and O K edge x-ray absorption<br />
spectroscopy we have studied the temperature dependence of the near Fermi level<br />
electronic structure in the electron doped CMR, Ca 0.86 Pr 0.14 MnO 3 . We found that<br />
the temperature dependent changes in the electronic structure is consistent with the<br />
conductivity behavior of the electron doped systems in general. As the temperature<br />
is lowered from 293 K a feature due to the e g↑ states starts appearing near the E F .<br />
The intensity of this feature further increases below the T c with a shift of spectral<br />
weight from higher binding energy to the near E F states. Further, we have found an<br />
increase in the e g↑ band width (W) also at low temperatures. On the other hand,<br />
a complementary shifts of states from the near E F to higher energy positions were<br />
observed with decrease of temperature in the unoccupied states presented in the preedge<br />
region of O K edge x-ray absorption spectra. We have discussed our results from<br />
the point of view of phase separation models considering the temperature dependent<br />
structural changes and consequent localization of the electrons in a narrow e g band.