PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

SPECTROSCOPIC INVESTIGATIONS OF THE
ELECTRONIC STRUCTURE OF Pr 1−x Ca x MnO 3
SYSTEM
A THESIS SUBMITTED TO THE
HOMI BHABHA NATIONAL INSTITUTE
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY IN SCIENCE
(PHYSICS)
MAY, 2009
By
MANAS KUMAR DALAI
INSTITUTE OF PHYSICS,
BHUBANESWAR 751 005,
INDIA

CERTIFICATE
This is to certify that the thesis entitled Spectroscopic investigations of the
electronic structure of Pr 1−x Ca x MnO 3 system, which is being submitted by
Manas Kumar Dalai in partial fulfillment of the degree of Doctor of Philosophy
in Science (Physics) of Homi Bhabha National Institute is a record of his own
research work carried out by him. He has carried out his investigations for the last
six years on the subject matter of the thesis under my supervision at Institute of
Physics, Bhubaneswar, India. To the best of my knowledge, the matter embodied
in the thesis has not been submitted for the award of any other degree.
Signature of the Candidate
Manas Kumar Dalai
Institute of Physics
Bhubaneswar
Date :
Signature of the supervisor
Dr. B. R. Sekhar
Associate Professor
Institute of Physics
Bhubaneswar
ii

iii
To My Parents

Acknowledgements
I owe a debt of gratitude to many people who made this thesis possible due to their
support, suggestions, and encouragements.
First of all, it is a great pleasure for me to express my sincere gratitude and indebtedness
to my thesis supervisor Prof. B. R. Sekhar for his invaluable guidance
and constant encouragement to complete this work. His excellent training in handling
the intricacies of various experiments and making me acquainted with the data
analysis have been an inspiration for me to pursue my career as an experimental
condensed matter physicist. I wish to thank him for giving me the complete freedom
both academically and socially during the period of my work.
I would like to thank my group members Prabir and Rupali for their untiring
support during the experiments and for many interesting discussions. I would also
like to thank our technical staff Mr. Chittaranjan Swain for his help during the
experiments. My sincere thanks go to Prof. C. Martin (Laboratoire CRISMAT,
France) for providing good quality samples for this thesis work.
I would like to thank Dr. S. R. Barman, Dr. S. Banik, Dr. A. K. Shukla and Dr.
R. Dhaka (UGC-DAE CSR, Indore) for their help and discussions for inverse photoemission
measurements. I would also like to thank Prof. Stefan Schuppler, Dr. Peter
Nagel and Dr. Michael Merz for their help and cooperation during the high resolution
photoemission and x-ray absorption measurements at WERA beamline of ANKA
synchrotron, Karlsruhe, Germany. I would like to thank Dr. Federica Bondino, Dr.
Marco Zangrando, Dr. Elena Magnano and Dr. Bryan Doyle of ELETTRA synchrotron,
Trieste for their help and technical discussions for XAS experiments.
It is a great pleasure for me to thank Prof. N. L. Saini (Departimento di fisica,
Universita di Roma, Rome, Italy) for his fruitfull discussion at different stages of
my Ph. D. career. I also thank Prof. Anil Kumar and Prof K. K. Nanda of IISc
Bangalore for discussion and suggestions during their short visit to IOP.
I would like to thank all my teachers, who had taught me at various stages of
my educational career, especially my M. Sc teachers Profs. N. Barik, L. P. Singh, L.
iv

Maharana, P. Khare, D. K. Basa, K. Maharana, S. Mohapatra and D. Behera and
B. Sc teacher Mr. Dipak Singh for their encouragement and excellent teaching. It
is my great pleasure to thank Profs. A. Khare, A. M. Jayannavar, S. G. Mishra, D.
P. Mohapatra, K. Kundu, P. V. Satyam, S. Varma, S. Mukherji, J. Maharana, G. V.
Raviprasad, A. M. Srivastava, S. M. Bhattacharjee, A. Kumar, P. Agrawal, T. Som,
G. Tripathy, S. K. Patra for their teaching, suggestions and encouragements. I thank
our director Dr. Y. P. Viyogi and ex-director Dr. R. K. Choudhury for their support
and encouragements in all respect.
I would like to thank Mr. Santosh Choudhury, Mr. Anup Behera, Mrs. Ramarani
Das, Mr. Arun Dash, Mr. Khirod Patra, and Dr. S. N. Sarangi for providing technical
help when needed. I would also like to thank the library staff Rama, Rabaneswar,
Duryodhan and Kissan for their cooperation at every stage. I thank Dipankar, Srijesh
and Dhiren for their help and cooperation at every computer related problems. I wish
to thank the hostel mess staff for their wonderful services.
I am thankful to our registrar Mr. C. B. Mishra for his imense help and cooperation
in all administrative problems. I also thank Mr. Sk. Kefaytulla and Rajesh
Mohapatra for their administrative help.
I take this oppurtunity to thank my M.Sc friends Rajesh, Tapas, Swayambhu, Hitu
and Manmohan and village friend Suda for their constant support and encouragement
in all stage of my life. A special thank to Ananta for his constructive criticism, support
and company for both academic and social activities in my Ph.D life.
It is my pleasure to thank Zashmir bhai, Satya bhai, Jhasa bhai, Satchi bhai,
Bedanga bhai, Pal bhai, Kamal bhai, Pradeep bhai, Bishnu bhai, Birendra bhai,
Sudhira bhauja and Raj babu for their encouragement and cooperation during my
stay at IOP. I would like to thank Millind, Sinu, Boby, Chandra, Hara, Amulya, Ajay,
Trilochan, Smruti, Chitrasen, Jatis, Ashutosh, Kuntala, Sadhana, Mamata, Ranjita,
Binata, Jaya, Poulomi, Saumia, Pramita, Probodh, Jay, Dipak, Sumalay, Bharat,
Anupam, Umanand, Dilip, Suchi, Srikumar, Sankha, Nabyendu, Somnath, Sandeep,
Sourabh, Subrat, Sanjay, Raghavendra, Vanaraj, and Tanmoy for making my stay at
the Institute a memorable one. I also thank Dr. V. R. R. Medicherla for his valuable
discussions.
I would like to give my special thanks to Dr. Arun Kumar Pati and Dr. Biswajit
Pradhan for making my stay in the beautiful campus of IOP an exciting, memorable
and pleasing one.
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I am extremely grateful to Mrs. Ranjana Sekhar for making my stay homely
during the course of my work.
Gratefully I would like to thank my younger sister Tuin and younger brother Tulia
for their unlimited love, affection, encouragements, assistance and support at every
stage of my life. I do not find proper words to express my sense of indebtedness to my
wife Manasmita, whose unlimited love, affection, encouragements, and support have
become a special inspiration to me during my Ph. D life. I also thank my brother
in-law Munu for his love, affection and support.
Finally, I wish to express my deep sense of gratitude to my parents and late grand
parents for their immense patience, love, affection, encouragements and support. I am
indebted to them for giving me the freedom to choose my career and for constantly
providing me the moral and emotional support at every stage of my life. It is only
because of their kind blessings that I could complete this work.
Date: 22.05.2009
(Manas Kumar Dalai)
vi

List of Publications/Preprints
[1] Valence band electronic structure of Pr 1−x Sr x MnO 3 from photoemission studies,
P. Pal, M. K. Dalai, B. R. Sekhar, S. N. Jha, S. V. N. Bhaskara Rao, N. C.
Das, C. Martin, and F. Studer; J. Phys. : Condens. Matter 17, 2993
(2005).
*[2] Electronic structure of Pr 0.67 Ca 0.33 MnO 3 near the Fermi level studied by ultraviolet
photoelectron and x-ray absorption spectroscopy, M. K. Dalai, P. Pal,
B. R. Sekhar, N. L. Saini, R. K. Singhal, K. B. Garg, B. Doyle, S. Nannarone,
C. Martin, and F. Studer; Phys. Rev. B 74, 165119 (2006).
[3] Near Fermi level electronic structure of Pr 1−x Sr x MnO 3 : Photoemission study, P.
Pal, M. K. Dalai, R. Kundu, M. Chakraborty, B. R. Sekhar, and C. Martin;
Phys. Rev. B 76, 195120 (2007).
[4] Pseudogap behavior of phase-separated Sm 1−x Ca x MnO 3 : A comparative photoemission
study with double exchange, P. Pal, M. K. Dalai, R. Kundu, B. R.
Sekhar, and C. Martin; Phys. Rev. B 77, 184405 (2008).
*[5] Electronic structure of Pr 1−x Ca x MnO 3 from photoemission and inverse photoemission
spectroscopy, M. K. Dalai, P. Pal, R. Kundu, B. R. Sekhar, S. Banik,
A. K. Shukla, S. R. Barman, and C. Martin; Submitted to Physica B.
*[6] Spectroscopic investigations of the electron-doped Ca 0.86 Pr 0.14 MnO 3 , M. K.
Dalai, P. Pal, R. Kundu, B. R. Sekhar, M. Mertz, P. Nagel, S. Schuppler,
and C. Martin; Submitted to Phys. Rev. B.
*[7] Core level electronic structure of Pr 1−x Ca x MnO 3 using x-ray photoelextron spectroscopy,
M. K. Dalai, P. Pal, R. Kundu, B. R. Sekhar, and C. Martin; To
be submitted.
vii

[8] Electron spectroscopic investigations of the paramagnetic insulator to antiferromagnetic
insulator transition in Pr 0.5 Sr 0.5 MnO 3 , P. Pal, M. K. Dalai, R.
Kundu, B. R. Sekhar, S. Schuppler, M. Merz, P. Nagel, M. Channabasappa and
A. Sundaresan; Submitted to Phys. Rev. B.
(*) indicates papers on which this thesis is based.
viii

Synopsis
The perovskite type manganites having general formula R 1−x A x MnO 3 (where R is a
trivalent rare earth element like La, Pr, Nd. and A is a divalent alkaline earth element
like Sr, Ca, Ba.) have been one of the interesting compounds among the transition
metal oxide systems due to their important physical property such as colossal magnetoresistance
(CMR) [1, 2, 3, 4]. CMR is defined as the colossal decrease of resistance
by the application of magnetic field. These materials are insulators at high temperatures
and poor metals at low temperatures. Accompanying this insulator-metal
transition is a magnetic transition from a high temperature paramagnetic phase to a
low temperature ferromagnetic phase. These transitions occurs around the same transition
temperature T C . Applying a magnetic field around T C will greatly reduce the
resistivity, i.e negative magnetoresistance effect. These materials were first described
in 1950 by Jonker and van Santen [5] in perovskite type of manganites. One year later
(1951), Zener [6, 7] explained this unusual correlation between magnetism and transport
properties by introducing a novel concept called ”Double exchange” mechanism
(DE). The pioneering work of Zener was followed by Anderson and Hasegawa [8] in
1955 and de Gennes [9] in 1960. Double exchange theory argues that the electron
hoping is related to the relative spin orientations of the neighbouring sites. Although
it qualitatively explains the connection between the insulator-metal and the magnetic
transitions, it was noticed latter by A. J. Millis et. al. [10] that it fails to explain
the quantitative change in resistivity and the very low T C . Thus it is revealed that
the double-exchange mechanism is not enough to explain the CMR effect. Several
mechanisms [11, 12, 13, 14], based on electron-phonon interactions (the polaronic
effect), electronic phase separation, charge and/or orbital ordering etc. have been
proposed to account for the discrepancy. Although a detailed picture is still unclear,
there seems to be a growing consensus that one or more of these additional effects
probably exists, with the various phenomena both competing and cooperating with
each other. This leads to a very rich and exotic behaviour, as observed in these
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manganites. This mixed valence manganites have been studied for more than five
decades but are still considered modern materials because of their wide potential for
technological application.
Experimental electronic structure studies [15, 16, 17, 18] on these materials have
contributed substantially to the understanding of their unusual behavior which could
not be fully accomodated within the framework of the DE model. The temperature
and doping dependent metal-insulator transitions in these materials are found to be
closely related to the unique electronic structure derived from the Mn 3d and O 2p
hybridized orbitals. Most of their electrical and magnetic properties originate from
the Jahn-Teller distortion in the MnO 6 octahedra around the Mn 3+ ions. The strong
interplay between spin and orbital ordering originating from the single occupancy of
the doubly degenerate e g orbitals of Mn 3+ (t 3 2ge 1 g) ions make the phase diagram of these
compounds rich with physics. Many of these compounds, for example L 1−x Ca x MnO 3
(L = La, Pr) show strong charge and/or orbital ordered (CO/OO) insulating nature
at low temperatures, especially when x equals commensurate value.
Electron spectroscopy is a very powerful experimental tool for probing the electronic
structure of these materials. For this thesis work, the electronic structure of
Pr 1−x Ca x MnO 3 series have been investigated using ultra-violet photoelectron spectroscopy
(UPS), X-ray photoelectron spectroscopy (XPS), Inverse photoemission spectroscopy
(IPES) and X-ray absorption spectroscopy (XAS). Pr 1−x Ca x MnO 3 is one of
the interesting among these materials due to their insulating phase at all temperatures
and great variety of ordered phases, that are very sensitive to the cation/anion
doping. For 0.3 ≤ x ≤ 0.8, a charge ordering of Mn 3+ and Mn 4+ was found and an
antiferromagnetic (AF) ordering can be observed with neel temperature ranging from
100 to 170 K. For x ≤ 0.25, a ferromagnetic insulating state (FMI) is observed and
with no CO, whatever the temperature. In this region the metallic state is never realized
even upon the application of magnetic field. But the charge ordered insulating
state can be melted into ferromagnetic metallic state (FMM) upon application of a
magnetic field [4]. Interestingly the x = 0.33 doping shows the coexistence of FM and
AFM phases. Around x = 0.9 a metallic cluster glass domain [19] has been observed.
Using valence band photoemission and O K edge x-ray absorption, we studied
the temperature dependent finer changes in the near E F electronic structure of
Pr 0.67 Ca 0.33 MnO 3 , which is regarded as a prototype for the electronic phase separation
models in CMR systems. With decrease in temperature the O 2p contributions
x

to the t 2g and e g spin-up states in the valence band were found to increase until T c .
Below T c , the density of states with e g spin-up symmetry increased while those with
t 2g symmetry decreased, possibly due to the change in the orbital degrees of freedom
associated with the Mn 3dO 2p hybridization in the pseudo-CE-type charge or orbital
ordering. These changes in the density of states could well be connected to the electronic
phase separation reported earlier. By combining the UPS and XAS spectra,
we derived a quantitative estimate of the charge transfer energy E CT (2.6±0.1 eV),
which is large compared to the earlier reported values in other CMR systems. Such
a large charge transfer energy was found to support the phase separation model.
For a complete understanding of the changes in the occupied and unoccupied
electronic states we performed a detailed study of the Pr 1−x Ca x MnO 3 system across
their ferromagnetic - antiferromagnetic phase boundary using UPS and IPES. We
used three compositions x = 0.2, 0.33 and 0.4. Our photoemission studies showed
that the pseudogap formation in these compositions occur over an energy scale of
0.48±0.02 eV. Here again, we have estimated the charge transfer energy in these
compositions to be of the order of ∼ 2.8±0.2 eV. We have also undertaken a detailed
study of the core level electronic structure of Pr 1−x Ca x MnO 3 ((x = 0.2, 0.33, 0.4 and
0.84) using X-ray photoelectron spectroscopy. These studies have been performed at
the Mn 2p, Ca 2p and Pr 4d levels. We have made systematic measurments of the
changes in the binding energy positions of these levels as a function of the charge
carrier concentration. We also have analyzed the line shapes of these core levels.
To a large extent the traditional models employing the charge-spin coupling, have
been able to explain the CMR in many of the hole doped compositions. But, much
less is known about the electron-doped versions of these materials in which the charge,
orbital and spin ordering add more complexity to the ferromagnetic (FM) double exchange
and the superexchange interactions. These Mn(IV) rich compositions exhibit
marked differences in the electronic and magnetic properties from their Mn(III) rich
counterparts. It has been shown that the substitution of Ca by a trivalent cation
in the G-type antiferromagnet, CaMnO 3 leads to the formation of a FM component
with a maximum for optimal electron doping. Depending on the nature of the trivalent
cation, the x opt was found to lie between 0.135 and 0.16. This FM component is
correlated to the distortion of the MnO 6 octahedra and thereby the e g band width.
With low filling of the narrow e g band, these systems have strong electron-electron
interactions and consequent charge localizations. Using high resolution photoelectron
xi

spectroscopy and O K edge x-ray absorption spectroscopy we have studied the temperature
dependence of the near Fermi level electronic structure in the electron doped
CMR, Ca 0.86 Pr 0.14 MnO 3 . We found that the temperature dependent changes in the
electronic structure is consistent with the conductivity behavior of the electron doped
systems in general. As the temperature is lowered from 293 K a feature due to the e g
states starts appearing near the E F . This increase in the DOS is a signature of the
semi-metallic behavior of the sample in the range 110 - 300 K. Surprisingly, though
the resistivity measurement on this sample showed an insulating behavior below its
transition temperature T c (110 K), the intensity of this feature further increases below
the T c . Further, we have found an increase in the e g band width (W) also at
low temperatures. It was also interesting to note that the increase in the near E F
DOS in this compound is apparently different in nature from that associated with the
pseudogap formation in the other systems like Pr 1−x Sr x MnO 3 . We have discussed our
results from the point of view of phase sepation models considering the temperature
dependent structural changes and consequent localization of the electrons in a narrow
e g band.
xii

Bibliography
[1] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, and K. Samwer; Phys. Rev.
Lett. 71, 2331 (1993).
[2] Y. Tokura, A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido and
N. Furukawa; J. Phys. Soc. Jpn. 63, 3931 (1994).
[3] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, and Y. Tokura;
Phys. Rev.B 51, 14103 (1995).
[4] Y. Tomioka, A. Asamitsu, Y. Moritomo, and Y. Tokura, J. Phys. Soc. Jpn 64,
3626 (1995).
[5] G. H. Jonker and J. H. Van Santen Physica 16, 337 (1950).
[6] C. Zener, Phys. Rev. 81, 440 (1951).
[7] C. Zener, Phys. Rev. 82, 403 (1951).
[8] P. W. Anderson and H. Hasegawa, Phys. Rev. 100, 675 (1955).
[9] P. G. de Gennes, Phys. Rev. 118, 141 (1960).
[10] A. J. Millis, B. I. Shraiman, and P. B. Lttlewood, Phys. Rev. Lett. 74, 5144
(1995).
[11] A. J. Millis, Nature 392, 147 (1998).
[12] A. Moreo, S. Yunoki, and E. Dagatto, Science 283 2034 (1999).
[13] A. Moreo, M. Mayr, A. Feiguin, S. Yunoki, and E. Dagotto, Phys. Rev. Lett. 84
5568 (2000).
xiii

[14] H. Kuwahara, Y. Tomioka, A. Asamitsu, Y. Moritomo, Y. Tokura; Science 270,
961 (1995).
[15] T. Saitoh, A. E. Bocquet, T. Mizokawa, H. Namatame, A. Fujimori, M. Abbate,
Y. Takeda, and M. Takano; Phys. Rev. B 51, 13942 (1995).
[16] D. D. Sarma, N. Shanthi, S. R. Krishnakumar, T. Saitoh, T. Mizokawa, A.
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Takeda, and M. Takano; Phys. Rev. B 53, 6873 (1996).
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U. Idzerda; Phys. Rev. Lett. 76, 4215 (1996).
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Tokura; Phys. Rev. B 56, R15513 (1997).
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(1999).
xiv

Contents
CERTIFICATE
Acknowledgements
List of Publications/Preprints
Synopsis
ii
iv
vii
ix
1 Introduction 1
1.1 An overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Colossal magnetoresistance . . . . . . . . . . . . . . . . . . . . . 3
1.3 Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 The basic electronic structure . . . . . . . . . . . . . . . . . . . . 5
1.5 Double exchange mechanism . . . . . . . . . . . . . . . . . . . . 7
1.6 Jahn-Teller effect in CMR manganites . . . . . . . . . . . . . . 8
1.7 Phase separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.8 Spin, Charge and Orbital ordering in manganites . . . . . . . 11
1.9 Pseudogap behavior in manganites . . . . . . . . . . . . . . . . 13
1.10 Basic properties, phase diagram and CMR effect in Pr 1−x Ca x MnO 3
system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2 Experimental Techniques 23
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2 Photoemission Spectroscopy . . . . . . . . . . . . . . . . . . . . . 24
2.2.1 Basic principle . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2.2 Theoretical interpretation of photoemission . . . . . . . . . . . 25
2.2.3 The electron escape depth . . . . . . . . . . . . . . . . . . . . 29
2.2.4 Experimental set-up . . . . . . . . . . . . . . . . . . . . . . . 30
xv

2.3 Inverse Photoemission Spectroscopy . . . . . . . . . . . . . . . . 38
2.3.1 Historical background . . . . . . . . . . . . . . . . . . . . . . 38
2.3.2 Basic principle . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.3.3 Basic interpretation of inverse photoemission . . . . . . . . . . 39
2.3.4 Experimental Set-up . . . . . . . . . . . . . . . . . . . . . . . 41
2.4 X-ray Absorption Spectroscopy . . . . . . . . . . . . . . . . . . . 45
2.4.1 Basic principle . . . . . . . . . . . . . . . . . . . . . . . . . . 45
2.4.2 Detection techniques . . . . . . . . . . . . . . . . . . . . . . . 47
2.5 Synchrotron Radiation . . . . . . . . . . . . . . . . . . . . . . . . 49
3 Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 59
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . 62
3.3.1 The UPS study of Pr 0.67 Ca 0.33 MnO 3 . . . . . . . . . . . . . . 62
3.3.2 The XAS study of Pr 0.67 Ca 0.33 MnO 3 . . . . . . . . . . . . . . 67
3.3.3 The Charge Transfer Energy (E CT ) . . . . . . . . . . . . . . . 69
3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4 Electronic Structure of Pr 1−x Ca x MnO 3 77
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . 80
4.3.1 The UPS study of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33 and 0.4 . . . 80
4.3.2 The IPES study of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33, 0.4 and 1 . 84
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
4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
5 Electronic Structure of Ca 0.86 Pr 0.14 MnO 3 96
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
5.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
5.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . 98
5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
6 Summary and Conclusions 108
xvi

List of Figures
1.1 Temperature dependence of the resistivity of La 1−x Sr x MnO 3 (x =
0.175) under various magnetic fields [2]. . . . . . . . . . . . . . . . . . 4
1.2 Schematic view of the cubic perovskite structure. . . . . . . . . . . . 5
1.3 Phase diagram of temperature versus tolerance factor for the system
R 0.7 A 0.3 MnO 3 , where R is a trivalent rare earth ion and A is a divalent
alkaline earth ion [12]. . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 A schematic of the electronic structure of Mn 3+ (a) and Mn 4+ (b). . . 7
1.5 Schematic features of the double exchange mechanism. . . . . . . . . 8
1.6 Schematic view of the Jahn-Teller effect for the d 4 ion. . . . . . . . . 9
1.7 Schematic representation of a macroscopic phase separated state (a),
as well as possible charge inhomogeneous states stabilized by the longrange
Coulomb interaction (spherical droplets in (b), stripes in (c))
[20]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.8 Different types of spin arrangements. . . . . . . . . . . . . . . . . . . 11
1.9 A Schematic picture of antiferromagnetic CE-type structure with the
charge and e g orbital ordering. . . . . . . . . . . . . . . . . . . . . . . 12
1.10 A schematic picture of the antiferromagnetic pseudo-CE-type structure
in the case of Pr 1−x Ca x MnO 3 (x = 0.4) [38]. . . . . . . . . . . . . . . 13
xvii

1.11 (a) DOS of the one-orbital model on a 10×10 cluster at J H = ∞ and
temperature T = 1/130 (hopping t = 1). The four lines from the top
correspond to densities 0.90, 0.92, 0.94, and 0.97. The inset has results
at 〈n〉 = 0.86, a marginally stable density at T = 0. (b) DOS of the
two-orbital model on a 20-site chain, working at 〈n〉 = 0.7, J H = 8, and
λ = 1.5. Starting from the top at ω − µ = 0, the three lines represent
temperatures 1/5, 1/10 and 1/20 respectively. Both (a) and (b) are
taken from Ref. [40]. (c) DOS using a 20-site chain of the one-orbital
model at T = 1/175, J H = 8, 〈n〉 = 0.87, and at a chemical potential
such that the system is phase separated in the absence of disorder. W
regulates the strength of the disorder, as explained in Moreo et. al.
[41] from where this figure was taken. . . . . . . . . . . . . . . . . . . 14
1.12 Schematic explanation of pseudogap formation at low electron density
[40]. In (a) A typical Monte Carlo configuration of t 2g -spin is sketched.
In (b) the corresponding e g density is shown, with electrons mostly
located in the FM regions. In (c), the effective potential felt by the
electrons is presented. A populated cluster band is formed (thick line).
In (d) the resulting DOS is shown. . . . . . . . . . . . . . . . . . . . 15
1.13 Phase diagram of Pr 1−x Ca x MnO 3 . Figure taken from ref. [44] . . . . 17
1.14 (a) Temperature dependence of the resistivity of Pr 1−x Ca x MnO 3 with
x = 0.3 at various magnetic fields. The inset is the phase diagram
in the temperature magnetic field plane. The hatched area indicates
the hysteretic region [45]. (b) The temperature dependence of the
resistivity of Pr 1−x Ca x MnO 3 (x = 0.5, 0.4, 0.35 and 0.3) at magnetic
fields of 0, 6 and 12 T [44]. . . . . . . . . . . . . . . . . . . . . . . . . 18
2.1 Principle of a modern photoemission experiment. The photon source is
either UV light or X-ray, which hit the sample surface under an angle
ψ with respect to the surface normal. The kinetic energy E kin of the
photoelectrons can be analysed by use of electrostatic analysers. . . . 24
xviii

2.2 Schematic view of the energetics of the photoemission process [3], which
gives the relation between the energy levels in a solid and the electron
energy distribution produced by photons of energy hν. The electron
energy distribution which can be measured by the analyser as a function
of kinetic energy (E kin ) is more conveniently expressed in terms of
the binding energy E B when one refers to the density of states inside
the solid. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.3 ”Universal curve” for electron inelastic mean free path λ. . . . . . . . 30
2.4 Photoemission spectrometer at Institute of Physics, Bhubaneswar. . . 31
2.5 Schematic diagram of the VUV source. . . . . . . . . . . . . . . . . . 32
2.6 Dimensional outline of the VUV source HIS 13. The viewport can be
used for inspection of the discharge and for adjustment of the light
spot position on the sample. The dimensions are in mm. . . . . . . . 32
2.7 Schematic of the X-ray source in operation . . . . . . . . . . . . . . . 33
2.8 Schematic diagram of the EA 125 hemispherical analyser. . . . . . . . 34
2.9 The principle of operation of the concentric hemispherical analyser,
schematic diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.10 Schematic diagram of inverse photoemission spectroscopy (IPES). . . 39
2.11 (a) IPES spectrometer at UGC-DAE consortium for scientific research,
Indore, (b) External view of photon detector attached to the IPES
chamber, and (c) Inside view of the chamber showing a rotatable (left)
and fixed (right) electron gun and the sample holder . . . . . . . . . . 42
2.12 A schematic diagram of the electron gun [30]. . . . . . . . . . . . . . 43
2.13 Schematic cross section of the photon detector showing the window
(1), O-ring (2), cylindrical cap (3), detector tube (4), teflon spacer (5),
anode (6), double sided DN40 CF flanges (7, 10), gas inlet and outlet
(8, 9), barrel connector (11), teflon sleeve (12), DN40 CF flange (13)
and SHV connector (14). . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.14 Schematic diagram of x-ray absorption spectroscopy . . . . . . . . . . 46
2.15 The x-ray loses its intensity via interactions with material. The absorption
coefficient decreases smoothly with higher energy, except for
special photon energies. When the photon energy reaches a critical
value for a core electron transition, the absorption coefficient increases
abruptly. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
xix

2.16 Schematic of the Si-photodiode. . . . . . . . . . . . . . . . . . . . . . 49
2.17 Schematic of the synchrotron radiation. . . . . . . . . . . . . . . . . . 50
2.18 Schematic representation of an insertion device. . . . . . . . . . . . . 51
2.19 Optical layout of the WERA soft x-ray beam line, ANKA. . . . . . . 52
2.20 Optical layout of the BEAR beam line. . . . . . . . . . . . . . . . . . 53
2.21 Optical layout of the BACH beam line. . . . . . . . . . . . . . . . . . 53
2.22 The variable included angle monochromator : the combined movement
of PM1 (plane mirror) and one of the four spherical gratings SG1(-4)
leads to the monochromatization of the light keeping both the entrance
and exit slit fixed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.1 Valence band photoemission spectra of Pr 0.67 Ca 0.33 MnO 3 taken using
He I photon energy (21.2 eV) at 77 (solid line), 110 (dotted), 150
(dashed), 220 (dot-dashed), and 300 K (double-dot dashed). All the
spectra have been normalized and shifted along y-axis by a constant
for clarity. The subbands around -1.2 (e g↑ ), -3.5 (t 2g↑ ), and -5.6 eV (e g↑
+ t 2g↑ ) are marked as A, B, and C respectively. . . . . . . . . . . . . 63
3.2 High-resolution photoemission spectra of the near-E F region of the
valence band of Pr 0.67 Ca 0.33 MnO 3 . In (a) the spectrum taken below T c
(red) is compared with the normal state (300 K) spectrum (blue). The
difference spectrum (green) obtained by subtracting the spectra at 300
K from 77 K and multiplied by 5 is also shown in the panel. Similarly,
in (b), (c), and (d) the near-E F spectra taken at 110, 150, and 220 K
are compared with that taken at 300 K. The feature in the difference
spectra corresponds to the tail feature A (e g↑ ) in Fig. 3.1. . . . . . . . 64
3.3 Temperature dependence of the area of the three valence band features
obtained from fitting the spectra with Lorentzian line shapes using a
χ 2 iterative program. We have used an integral background, which was
kept the same for all the spectra. The energy positions and FWHMs
were determined by finding the best fit common to all the spectra by the
iterative program. The final fit for all spectra at different temperatures
were obtained with the same energy positions and FWHMs. (a), (b),
and (c) correspond to the features A, B, and C positioned at -1.19,
-3.49, and -5.63 eV with FWHMs 2.56, 1.93, and 1.37 eV respectively. 65
xx

3.4 O K edge x-ray absorption spectra of Pr 0.67 Ca 0.33 MnO 3 taken at 300
(solid line), 150 (dashed line), and 95 K (dotted). The pre-edge feature
centered around 529.5 eV (marked by a box) is where most interest lies.
Since this feature consists of two peaks (a main line and a shoulder on
the low-energy side), this part of the spectrum was fitted with two
components of Lorentzian line shapes. Results of the curve fit are
given in Table 3.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.5 (a) Combined spectra from valence band photoemission (occupied energy
states) and pre-peak in O K edge x-ray absorption spectra (unoccupied
energy states) of Pr 0.67 Ca 0.33 MnO 3 taken at room temperature
(above T c ). The three features corresponding to the subbands in the
valence region are marked A, B, and C.The two peaks of the fitted O
K edge pre-peak are marked A ′ and B ′ . We used integral background
for both sides of E F . Details of the fitting are mentioned in the text.
(b) Combined spectra of Pr 0.67 Ca 0.33 MnO 3 below T c . The valence band
spectrum was taken at 77 K and O K edge XAS was taken at 95 K. . 70
3.6 Schematic diagram of the near-E F energy levels of Pr 0.67 Ca 0.33 MnO 3
in the occupied and unoccupied parts. The diagram is not drawn to
scale. Some of the e g↑ states are occupied (at the Mn 3+ sites) and some
are unoccupied (at the Mn 4+ sites). Hence the last occupied and first
unoccupied states are marked with dotted lines. . . . . . . . . . . . . 71
4.1 Phase diagram of the Pr 1−x Ca x MnO 3 system. In this study we have
used the x = 0.2, x = 0.33 and x = 0.4 compositions. (FMI = ferromagnetic
insulating, CO-AFMI = charge ordered antiferromagnetic
insulating, CG = cluster glass, T N = Neel Temperature, T C = Critical
Temperature, T CO = charge ordering temperature). . . . . . . . . . . 79
4.2 The angle integrated valence band photoemission spectra of the 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 room
temperature and 77K using He I photons (21.2 eV). . . . . . . . . . . 81
xxi

4.3 The near E F region of the valence band spectra (panel(a): 300 K,
panel(b): 77 K) of the three samples plotted against that of CaMnO 3 .
The difference spectra (lower panel) is obtained by subtracting the
spectra of CaMnO 3 from those of the three Pr 1−x Ca x MnO 3 compositions
and multiplied by 5. The difference spectra corresponds to the
density of e g states. The red and blue peaks are A’ and A” respectively.
The green line shows the spectral background used in the fitting. . . . 83
4.4 The IPES spectra of the Pr 1−x Ca x MnO 3 samples (x = 0.2, 0.33 and
0.4) and the CaMnO 3 sample taken at room temperature. The spectra
represents the unoccupied electron states of the samples. . . . . . . . 85
4.5 Panel(a) shows the IPES spectra of the near E F unoccupied states.
Panel(b) shows the difference spectra obtained by subtracting the spectra
corresponding to the x = 0.2 from those of other compositions. . . 86
4.6 The Mn 2p core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken
at room temperature (a) and 77 K (b) . . . . . . . . . . . . . . . . . . 88
4.7 The Ca 2p core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken
at room temperature (a) and 77 K (b). . . . . . . . . . . . . . . . . . 89
4.8 The Pr 4d core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken
at room temperature (a) and 77 K (b). . . . . . . . . . . . . . . . . . 90
5.1 The angle integrated valence band photoemission spectrum of Ca 0.86 Pr 0.14 MnO 3
taken using 124 eV photons. The spectra taken at different temperatures
(30 K, 70 K, 110 K, 220 K and 293 K) are normalized for their
intensities and shifted along Y-axis by a constant for clarity of presentation.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
5.2 The high resolution spectra of the near E F region of the valence band
of 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 e g band. . . . . . . . . . . . . . . . . . . . . . . . 100
5.3 The O K-edge x-ray absorption spectra of the Ca 0.86 Pr 0.14 MnO 3 taken
at 30 K, 80 K, 150 K, 200 K and 300 K. The pre-edge feature appears
between 527 - 533 eV is mostly due to the strong hybridization between
Mn 3d and O 2p orbitals, where most interest lies. Origin of other
features are given in the text. . . . . . . . . . . . . . . . . . . . . . . 101
5.4 The pre-edge peak of the O K -edge XAS spectra of Ca 0.86 Pr 0.14 MnO 3
for temperatures 30 K, 80 K, 150 K, 200 K and 300 K. . . . . . . . . 102
xxii

Chapter 1
Introduction
1

Introduction 2
1.1 An overview
The perovskite type manganites having general formula R 1−x A x MnO 3 (where R is a
trivalent rare earth element like La, Pr, Nd. and A is a divalent alkaline earth element
like Sr, Ca, Ba.) have been one of the interesting compounds among the transition
metal oxide systems due to their intriguing physical property such as colossal magnetoresistance
(CMR) [1, 2, 3]. CMR is defined as the colossal decrease of resistance by
the application of magnetic field. These materials are insulators at high temperatures
and poor metals at low temperatures. Accompanying this insulator-metal transition
is a magnetic transition from a high temperature paramagnetic phase to a low temperature
ferromagnetic phase. These transitions occurs around the same transition
temperature T C . Applying a magnetic field around T C will greatly reduce the resistivity,
i.e negative magnetoresistance effect. These materials were first described in
1950 by Jonker and van Santen [4] in perovskite type manganites.
One year later (1951), Zener [5, 6] explained this unusual correlation between
magnetism and transport properties by introducing a novel concept, so-called ”Double
exchange” mechanism (DE). The pioneering work of Zener was followed by Anderson
and Hasegawa [7] in 1955 and de Gennes [8] in 1960. Double exchange theory
argues that the electron hopping is related to the relative spin orientations of the
neighbouring sites. Although it qualitatively explains the connection between the
insulator metal and the magnetic transitions, it was noticed latter by A. J. Millis et.
al. [9] that it fails to explain the quantitative change in resistivity and the very low
T C . Thus it is believed that double-exchange mechanism is not enough to explain
the CMR effect. Several mechanisms, such as electron-phonon interactions (the polaronic
effect), electronic phase separation, charge and/or orbital ordering etc. have
been proposed to account for the discrepancy. Although the detailed picture is still
unclear, there seems to be a growing consensus that one or more of these additional
effects probably exists, with the various phenomena both competing and cooperating
with each other. This can lead to very rich and exotic behaviour, as observed in the
manganites. This mixed valence manganites have been studied for more than five
decades but are still considered modern materials because of their wide potential for
technological applications. Potential applications of the CMR effect in these manganites
include magnetic sensors, magnetoresistive read heads, magnetoresistive random
access memory (MRAM) etc.

Introduction 3
The temperature and doping dependent metal-insulator transitions in these materials
are found to be closely related to the unique electronic structure derived from
the Mn 3d and O 2p hybridized orbitals of MnO 6 octahedra. The 3d orbitals of Mn
in the MnO 6 octahedra, which are split by the crystal field into t 2g and e g states,
are further split by the Jahn-Teller distortion. With charge carrier doping, many of
them show well defined charge ordering (CO) and/or orbital ordering (OO) at low
temperatures, especially when x equals a commensurate value (e.g x = 0.5). The CO
state where the Mn 3+ (t 3 2ge 1 g) and Mn 4+ (t 3 2g) ions arranged like in a checkerboard was
found to bear a strong influence on the one electron band width (W) of the e g band
and the transfer interaction of the e g holes (electrons).
The work presented in this thesis consists of the investigations of the electronic
structure of Pr 1−x Ca x MnO 3 using different spectroscopic techniques as photoemission
spectroscopy, inverse photoemision spectroscopy and x-ray absorption spectroscopy
etc. The thesis is structured as follows : Following this overview, the Chapter 1 gives
a general idea about the physical properties (structural, electronic and magnetic) and
the mechanisms associated with the CMR manganites. Chapter 2 describes the principles
and operations of the experimental techniques used for this thesis work. Chapter
3 presents the electronic structure of Pr 0.67 Ca 0.33 MnO 3 at different temperatures
using ultra-violet photoelectron and x-ray absorption spectroscopy. The electronic
structure of the Pr 1−x Ca x MnO 3 series at different temperatures using ultra-violet
photoemission and inverse photoemission spectroscopy are discussed in Chapter 4.
In Chapter 5, the electronic structure of Ca 0.86 Pr 0.14 MnO 3 at different temperatures
using high resolution photoemission and x-ray absorption spectroscopy are discussed.
Chapter 6 gives the summary and conclusions of this thesis.
1.2 Colossal magnetoresistance
Colossal magnetoresistance is a property of some materials, which is the gigantic
decrease of resistance by the application of magnetic field. The magnetoresistance
can be defined as,
MR =
R(H) − R(0)
, (1.1)
R(0)
where R(H) and R(0) are the resistances with and without magnetic field H respectively.
Expressing the results as a percentage (i.e multiplying by an additional factor
100), it has been shown by Jin et. al. [10] that the MR value as large as -100,000

Introduction 4
Figure 1.1: Temperature dependence of the resistivity of La 1−x Sr x MnO 3 (x = 0.175)
under various magnetic fields [2].
% near 77 K. Which corresponds to thousand-fold magnetoresistance. The temperature
dependent resistivity of La 1−x Sr x MnO 3 (x = 0.175) single crystal under several
magnetic fields are shown in the Fig. 1.1.
1.3 Crystal structure
The CMR samples used in this thesis work are of perovskite type (ABO 3 ) structure.
The general formula for perovskite type manganites is R 1−x A x MnO 3 , where R is the
trivalent rare earth element such as La, Pr, Nd, and Sm etc. and A is the divalent
alkaline earth ion such as Sr, Ca, and Ba etc. An ideal cubic perovskite structure is
shown in the Fig. 1.2, which contains a MnO 6 octahedron, with one Mn ion at the
center and oxygen ions at each of the six corners. The trivalent or divalent ions (R
or A) are situated at the corners of the cube. The stability of the cubic perovskite
structure depends strongly on the size of the cations (R, A or Mn). If there is a
size mismatch occurs between the cations, then the cubic perovskite structure gets

Introduction 5
Figure 1.2: Schematic view of the cubic perovskite structure.
distorted. This size mismatch is governed by a factor called tolerance factor (t), and
is defined by Goldschmidt [11] as,
t = (r A + r O )
√
2(rMn + r O ) , (1.2)
where r A , r Mn and r O are the ionic radii of the A-site(R or A), Mn- and O- atoms
respectively. For cubic perovskite structure t = 1, where the Mn-O-Mn bond angles
are relatively straight. Due to the ionic size mismatch the Mn-O-Mn bond angles
deviate from 180 o and lead to distorted perovskite structure. When t decreases (0.96 <
t < 1), the cubic structure transforms to rhombohedral and then to the orthorhombic
structure (t < 0.96). The properties of manganites strongly depend on the tolerance
factor. In 1995 Hwang et. al. [12] has reported the structure-property relationship
as a function of tolerance factor for 30 % dopant concentration with a variety of
rare earth and alkaline earth ions, shown in the Fig. 1.3. It shows the presence
of three regimes: a paramagnetic insulator at high temperature, a low-temperature
ferromagnetic metal at large tolerance factor and a low-temperature ferromagnetic
insulator at small tolerance factor.
1.4 The basic electronic structure
In manganites the electronically active orbitals are the Mn 3d orbitals. In the cubic
lattice, the five-fold degenerate 3d-orbitals of an isolated Mn atom or ion are split into

Introduction 6
Figure 1.3: Phase diagram of temperature versus tolerance factor for the system
R 0.7 A 0.3 MnO 3 , where R is a trivalent rare earth ion and A is a divalent alkaline earth
ion [12].
a triple degenerate (lower energy levels) d xy , d yz , and d zx orbitals, called t 2g states
and a double degenerate (higher energy levels) d x 2 −y 2 and d 3z 2 −r 2 orbitals, called e g
states [13]. Energy separation between the t 2g states and e g states is 10Dq due to the
crystal field splitting of MnO 6 octahedra. The schematic of the electronic structure
from ionic point of view are shown in the Fig. 1.4. Since the e g orbitals point towards
the six O atoms, they are expected to have a larger hybridization with the O 2p
states (p x , p y , and p z ) and hence are more delocalized than the t 2g states. So the e g
state electrons play an important role of conduction electrons in this type of crystals.
In Mn 3+ ions three electrons occupy the t 2g states and one electron occupies the e g
states. The strong Hund’s coupling (J H ) in these systems favors the alignment of
the e g electron spins with the core-like t 2g spins. When the manganites are doped
with holes through chemical substitution, the Mn 4+ ions are formed. In Mn 4+ ions,
there are only three electrons occupying the t 2g orbitals. When the excess electron is
present in case of Mn 3+ ion, a further distortion known as the Jahn-teller distortion
may lower the crystal symmetry and break the degeneracy of the e g states. This can
be energetically favorable as the excess electron will fill the lower energy orbital while
the upper energy orbital remains empty.

Introduction 7
Figure 1.4: A schematic of the electronic structure of Mn 3+ (a) and Mn 4+ (b).
1.5 Double exchange mechanism
The starting point for understanding the properties of manganites is the so-called
”double” exchange (DE) mechanism and was first proposed by C. Zener [5, 6] in
1951, which says the simultaneous hopping of e g elctrons from Mn 3+ sites to O 2−
sites and then from O 2− to the empty e g orbitals of Mn 4+ sites. This transfer of e g
electron from Mn 3+ to Mn 4+ by DE is the basic mechanism of electrical conduction
in manganites. In 1955, the DE theory was expanded by P. W. Anderson, and H.
Hasegawa [7], where the hopping probability (t ij ) of the e g electron from Mn 3+ to
neighbouring Mn 4+ is,
t ij = t cos( θ ij
), (1.3)
2
where θ ij is the angle between the Mn spins in the case of strong Hund’s coupling.
The concept of double exchange mechanism is illustrated in Fig. 1.5. For parallel spin
configuration (θ ij = 0) the hopping probability is maximum and for antiparallel spin
(θ ij = 180 o ), the hopping probability is zero, that means the hopping cancels. Thus it
predicts maximum conductivity at ferromagnetic state where as insulating behaviour
at antiferromagnetic state. This provides the qualitative explanation for the close
connection between the insulator-metal transition and the magnetic transition. It
also explains why the external magnetic field will increase the conductivity since θ ij
will be reduced as the external magnetic field polarizes the core (t 2g ) spins of the
Mn ions. Using mean-field approximations, P. -G. degennes [8] in 1960 suggested
that the interpolation between the antiferromagnetic state of the undoped limit and
the ferromagnetic state at finite hole density, where the DE mechanism works, occurs

Introduction 8
Figure 1.5: Schematic features of the double exchange mechanism.
through a ”canted” state, similar as the state produced by a magnetic field acting over
an antiferromagnetic state. Although DE mechanism gives an intuitive explanation
for the coupling between the spin and charge degrees of freedom, it fails to predict
the correct magnitude of the change in resistivity. For paramagnetic case (T > T C ),
θ ij = 90 o , and so the hopping probability will be reduced to cos ( 90o ) = 0.7 times
2
that in the ferromagnetic case. While experimentally it is seen that the conductivity
decreases across the ferro - para transition may be many orders of magnitude. Which
says that some other mechanisms beyond DE are necessary to explain the conductivity
change across the transition.
1.6 Jahn-Teller effect in CMR manganites
A. J. Millis et. al. [9] in 1995 pointed out that, the DE model alone is not enough to
explain the CMR effect in manganites, where he argued that DE produces wrong T C
by a large factor and the resistivity that grows with reducing temperature (insulating
behaviour) even below T C . By taking these reasons Millis et. al. [9] concluded that
the model based only on a large J H is not adequate for the manganites and claimed
that the necessity of Jahn-Teller phonons to explain the change in curvature of the
resistivity close to T C . The e g electrons on a given MnO 6 octahedron are strongly
coupled to the distortions of the O 6 octahedron. In the pseudocubic materials such
as La 1−x Ca x MnO 3 the strong coupling is due in part to the Jahn-Teller effect, the

Introduction 9
Figure 1.6: Schematic view of the Jahn-Teller effect for the d 4 ion.
essence of which is that a local distortion in which some Mn-O bonds get shorter and
other gets longer breaks the local cubic symmetry and therefore splits the degeneracy
of the e g level on that site (Fig. 1.6). If only one electron is present then it will in
the absence of hybridization reside in the lower level and gain energy. For example,
a distortion in which the Mn-O bond get longer in the ±z direction and get shorter
in the ±x,y directions raise the energy of the d x 2 −y2 orbital and lowers the energy of
the d 3z 2 −r 2 orbital (Q 3 mode). Millis et. al. [14, 15, 17, 18] argued that the physics of
manganites is dominated by the interplay between a strong electron-phonon coupling
and the large Hund’s coupling effect that optimizes the electronic kinetic energy by the
generation of a FM-phase. The large value of electron-phonon coupling in manganites
is clear in the regime below x = 0.2, where a static JT distortion plays a key role in the
physics of the material and a dynamical JT effect may persists at higher hole densities
[16], without leading to long-range order but producing important fluctuations that
localize electrons by splitting the degenerate e g levels at a given MnO 6 octahedron.
Millis et. al [14] argued that the ratio λ eff = E JT
t eff
(coupling strength) dominates
the physics of the problem. Here E JT is the static trapping energy at a given octahedron,
and t eff is an effective hopping that is temperature dependent following the
standard DE discussion. In this context it was conjectured that when the temperature
is larger than T C the effective coupling λ eff could be above the critical value that
leads to the insulating behavior due to electron localization, while it becomes smaller

Introduction 10
Figure 1.7: Schematic representation of a macroscopic phase separated state (a), as
well as possible charge inhomogeneous states stabilized by the long-range Coulomb
interaction (spherical droplets in (b), stripes in (c)) [20].
than the critical value below T C , thus inducing metallic behavior. The calculations
were carried out using classical phonons and t 2g spins. The results of Millis et. al.
[14] for T C and the resistivity at a fixed density n = 1 when plotted as a function of
λ eff had formal similarities with experimental results (which are produced as a function
of density). In particular, if λ eff is tuned to be very close to the metal-insulator
transition, the resistivity naturally strongly depends on even small external magnetic
fields.
1.7 Phase separation
Another possibly complementary route to understand the CMR effect in manganites
is the phase separation. It has been recently predicted from computational studies
of realistic models that an electronic phase separation can occur in manganites in a
certain range of doping [19]. In particular at low doping level and at low temperature,
a phase separation between hole-poor antiferromagnetic (AFM) regions and hole-rich
ferromagnetic (FM) regions is energetically more favourable than the homogeneous
canted AFM phase. The energy of charge carrier is minimal for FM ordering. With
a density insufficient for establishing the FM ordering in the entire sample, the carriers
concentrate into droplets or stripes which become FM inside insulating AFM
matrix [20](Fig. 1.7). The fluctuations length scales range from nm to µm. This
point towards the conduction in metallic phase and the CMR effect is argued as a

Introduction 11
Figure 1.8: Different types of spin arrangements.
result of percolation [21, 22, 23]. On increasing the doping level the FM regions become
connected to each other and a low electrical resistance is achieved above the
percolation threshold. There are many reports about the coexistence of FM clusters
in the CO AFM insulating phase of La 1−x Ca x MnO 3 using different experimental
techniques [23, 24, 25, 26, 27]. And very recently the neutron diffraction and the
inelastic neutron scattering studies of Pr 1−x Ca x MnO 3 have shown the coexistence of
ferromagnetic fluctuations in antiferromagnetic domains [28, 29, 30, 31].
1.8 Spin, Charge and Orbital ordering in manganites
The unusual properties of manganites challenge the current topic of research in the
field of strongly correlated electrons system due to their strong spin, charge and
orbital degrees of freedom. The strong interplay between spin, charge and orbital
ordering originating from the single occupancy of the doubly degenerate e g orbitals
of Mn 3+ (t 3 2g e1 g ) ions make the phase diagram of these compounds rich with physics.
With charge carrier doping, some of them for example, La 1−x Ca x Mon 3 system show
strong charge and/or orbital ordered (CO/OO) insulating nature at low temperatures,
especially when x equals a commensurate value. The CO state where the Mn 3+ (t 3 2ge 1 g)
and Mn 4+ (t 3 2g ) are arranged like a checkerboard influences strongly the one electron
bandwidth of the e g band [32] and the transfer interaction of the e g holes (electrons).
There are various types of ordering occurs across the phase diagram of manganites,
which are described below.
The lattice distortion in manganites is due to the J-T interaction acting on the
Mn 3+ ion, which lifts the degenerate e g orbitals in the cubic electronic field. The J-T

Introduction 12
Figure 1.9: A Schematic picture of antiferromagnetic CE-type structure with the
charge and e g orbital ordering.
distorted Mn 3+ O 6 in LaMnO 3 form an orthorhombic structure (space group P bnm )
where the e g charge occupies alternating 3d 3x 2 −r 2 and 3d 3y 2 −r2 orbitals. The magnetic
structure of LaMnO 3 was found to be of A-type antiferromagnetic, where all Mn 3+
moments lying in c-plane are aligned ferromagnetically and the the FM layer stacks
antiferromagnetically along the c-axis by the weaker superexchange interaction along
this axis. The A-type AFM ordering exists below 140 K [33]. Different types of spin
arrangements are shown in the Fig. 1.8. Upon substituting divalent cations such
as Ca, Sr, Pb etc. to the parent compound like LaMnO 3 , the magnetic structure
changes from A-type AFM to the FM with the rotation of the spin direction from
the b-axis in the c-plane towards the c-axis associated with the metallic conduction,
which was interpreted successfully in terms of DE mechanism at an early stage of the
investigation. Further increase of doping, the charge ordered (CO) state near x = 0.5
is observed. For example in La 1−x Ca x MnO 3 (x = 0.5), the ordering becomes C-E type
(Fig. 1.9), where the Mn 3+ and Mn 4+ line up forming alternate stripes in the C plane
and stack also along the c-axis. This phenomenon in manganites was first observed
by Wollan and Koehler [34] in the neutron powder diffraction of La 0.5 Ca 0.5 MnO 3 .
Later these charge-ordering phases are observed in the low band width compound
Pr 1−x Ca x MnO 3 with a broad doping region 0.3 < x < 0.75 [28, 35, 36, 37, 38].
The CO state for Pr 0.6 Ca 0.4 MnO 3 is called pseudo-CE type ordering [38] is shown

Introduction 13
Figure 1.10: A schematic picture of the antiferromagnetic pseudo-CE-type structure
in the case of Pr 1−x Ca x MnO 3 (x = 0.4) [38].
in Fig. 1.10, where the arrangement of spin is canted along the c-axis while the
CE-type ordering is kept within the orthorhombic ab-plane. For the ground state
of R 1−x A x MnO 3 , i.e AMnO 3 (x = 1), the magnetic structure is called G-type AFM
[34, 39] ordering in which the spins on all Mn 4+ sites are antiparallel to their nearest
neighbours.
1.9 Pseudogap behavior in manganites
The conducting properties of the materials depend on the finer changes associated
with the density of states (DOS) near the Fermi level. If there are no allowed states
at the Fermi level a gap forms by making the system insulating and finite density of
states makes the system metallic. A Pseudogap exists in the mixed-phase regimes of
manganites, which suggests that the prominent depletion of spectral weight (density
of states) at the chemical potential, but not exactly zero (few states associated with
it). This feature is similar to that extensively discussed in high T c superconductor.
The calculations in the pseudogap context in manganites was carried out by Moreo
et. al. [40, 41] using both one- and two-orbital models (Fig. 1.11). The density of
states of the one-orbital model on 2D cluster varying the electronic density slightly
below 〈n〉 = 1.0 is shown in the Fig. 1.11(a). At zero temperature, this density
regime is unstable due to phase separation, but at the temperature of simulation
those densities still correspond to stable states, but with a dynamical mixture of

Introduction 14
Figure 1.11: (a) DOS of the one-orbital model on a 10×10 cluster at J H = ∞ and
temperature T = 1/130 (hopping t = 1). The four lines from the top correspond to
densities 0.90, 0.92, 0.94, and 0.97. The inset has results at 〈n〉 = 0.86, a marginally
stable density at T = 0. (b) DOS of the two-orbital model on a 20-site chain, working
at 〈n〉 = 0.7, J H = 8, and λ = 1.5. Starting from the top at ω − µ = 0, the three
lines represent temperatures 1/5, 1/10 and 1/20 respectively. Both (a) and (b) are
taken from Ref. [40]. (c) DOS using a 20-site chain of the one-orbital model at
T = 1/175, J H = 8, 〈n〉 = 0.87, and at a chemical potential such that the system is
phase separated in the absence of disorder. W regulates the strength of the disorder,
as explained in Moreo et. al. [41] from where this figure was taken.

Introduction 15
Figure 1.12: Schematic explanation of pseudogap formation at low electron density
[40]. In (a) A typical Monte Carlo configuration of t 2g -spin is sketched. In (b) the
corresponding e g density is shown, with electrons mostly located in the FM regions.
In (c), the effective potential felt by the electrons is presented. A populated cluster
band is formed (thick line). In (d) the resulting DOS is shown.
AFM and FM features. A clear minimum in the density of states at the chemical
potential can be observed. Very similar results also appear in 1D simulations [40].
Fig. 1.11(b) shows the results for two-orbitals and a large electron-phonon coupling,
at a fixed density and changing temperature. Clearly a pseudogap develop in the
system as a precursor of the phase separation that is reached as the temperature is
further reduced. Similar results have been obtained in other parts of parameter space,
as long as the system is near unstable phase separated regimes. A pseudogap appears
also in cases where disorder is added to the system. Fig. 1.11(c) shows the results
for the case where a random on-site energy is added to the one orbital model.
Moreo et. al. [40] explained this phenomenon tentatively for the case of ”without
disorder”. A schematic of explanation is shown in the Fig. 1.12. In Fig. 1.12(a) a
typical mixed phase FM-AFM state is sketched. In the FM regions, the e g electrons
improve their kinetic energy, and thus they prefer to be located in those regions as
shown in Fig. 1.12(b). The FM domains act as effective attractive potentials for
electrons, as shown in Fig. 1.12(c). When other electrons are added FM clusters
are created and new occupied levels appear below the chemical potential, creating
a pseudogap [Fig. 1.12(d)]. These results are compatible with the photoemission

Introduction 16
experiments by Dessau et. al. [42] for bilayer manganites (La 2−2x Sr 1+2x Mn 2 O 7 , x =
0.4). In this experiment it was observed that the low-temperature ferromagnetic
state was very different from a prototypical metal. Its resistivity is unusually high,
the width of ARPES features are anomalously broad, and they don’t sharpen as they
approach the Fermi momentum. In addition, the centroid of the near E F peak never
approach close than approximately 0.65 eV to the Fermi level. This implies that
even in the expected metallic regime, the density of states at the Fermi level is very
small. Dessau et. al. [42] refers to these results as the formation of a pseudogap. This
effect is present both in ferromagnetic and paramagnetic regimes. Scanning tunneling
microscopy data of single crystals and thin films of hole doped manganites by Biswas
et. al. [43] showed a rapid variation in the density of states for the temperatures
near the Curie temperature, such that below T c a finite density of states is observed
at the Fermi level while above T c a hard gap opens up. This result suggests that
the presence of a gap or pseudogap is not just a feature of bilayers, but it appears in
other manganites as well.
1.10 Basic properties, phase diagram and CMR
effect in Pr 1−x Ca x MnO 3 system
The Pr 1−x Ca x MnO 3 is a low band width manganite. The band width W is smaller
than in other manganites like La 1−x Sr x MnO 3 and La 1−x Ca x MnO 3 system. In the lowbandwidth
compounds, a charge-ordered (CO) state is stabilized in the vicinity of x =
0.5, while manganite with large W (La 1−x Sr x MnO 3 as example) present a metallic
phase at this concentration. The Pr 1−x Ca x MnO 3 presents a stable CO state in a
broad density region between x = 0.3 and 0.75, as showed by Jirak et. al [38]. Part of
the phase diagram is shown in the Fig. 1.13 [44]. A ferromagnetic insulating (FMI)
state exists in the range from x = 0.15 to 0.30. For x≥0.30, an antiferromagnetic CO
state is stabilized. Neutron diffraction studies by Jirak et. al. [38] showed that at
all densities between 0.30 and 0.75, the arrangement of charge/spin/orbital order of
this state is similar to the CE-state (Fig. 1.9). However, certainly the hole density is
changing with x, and as consequence the CE-state can not be perfect at all densities
but electrons have to be added or removed from the structure. Jirak et. al. [38]
discussed a ”pseudo”-CE-type structure for x = 0.4 that has the proper density.
The effect of magnetic fields on the CO-state of Pr 1−x Ca x MnO 3 is remarkable. The

Introduction 17
Figure 1.13: Phase diagram of Pr 1−x Ca x MnO 3 . Figure taken from ref. [44]
resistivity vs. temperature graph of Pr 1−x Ca x MnO 3 (x = 0.3) under various magnetic
fields [45] is shown in the Fig. 1.14(a). At low temperatures, changes in resistivity
by several orders of magnitude can be observed. The stabilization of metallic state is
realized upon the application of magnetic field. This state is ferromagnetic according
to magnetization measurements, and thus it is curious to observe that a state not
present at zero field in the phase diagram, is nevertheless stabilized at finite fields.
The magnetic field effects on the other compositions of Pr 1−x CaxMnO 3 (x = 0.35,
0.4 and 0.5) are also shown in the Fig. 1.14(b). The shape of the curves in Fig. 1.14
present a large magnetoresistance effect, and a possible origin based on percolation
between the CO- and FM-phase. Tomioka et. al. [46] showed that, as x grows away
from x = 0.3, larger fields are needed to destabilize the charge-ordered state at low
temperatures (e.g., 27 T at x = 0.50 compared with 4 T at x = 0.30).
This thesis is based on the study of electronic structure of Pr 1−x Ca x MnO 3 series
using different spectroscopic techniques.

Introduction 18
Figure 1.14: (a) Temperature dependence of the resistivity of Pr 1−x Ca x MnO 3 with
x = 0.3 at various magnetic fields. The inset is the phase diagram in the temperature
magnetic field plane. The hatched area indicates the hysteretic region [45]. (b) The
temperature dependence of the resistivity of Pr 1−x Ca x MnO 3 (x = 0.5, 0.4, 0.35 and
0.3) at magnetic fields of 0, 6 and 12 T [44].

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Introduction 21
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Introduction 22
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Chapter 2
Experimental Techniques
23

Experimental Techniques 24
2.1 Introduction
Spectroscopic methods are some of the most powerful techniques for studying the
electronic structure of solids. This chapter is intended to describe the details of the
different spectroscopic techniques which have been used in this thesis work.
2.2 Photoemission Spectroscopy
2.2.1 Basic principle
The principle of photoemission spectroscopy is based on the principle of photoelectric
effect. The photoelectric effect was first discovered by the German Physicist Heinrich
Hertz [1] in 1887 and latter explained by Albert Einstein [2] in 1905 as a manifestation
of the quantum nature of light.
Figure 2.1: Principle of a modern photoemission experiment. The photon source is
either UV light or X-ray, which hit the sample surface under an angle ψ with respect
to the surface normal. The kinetic energy E kin of the photoelectrons can be analysed
by use of electrostatic analysers.
When light is incident on a sample, electron can absorb the photon and escape
from the sample with a maximum kinetic energy as shown in the Fig. 2.1. If a photon
having energy hν is absorbed by the electron of binding energy E B , then the kinetic
energy E kin of the electron is given by,
E kin = hν − E B − φ, (2.1)
where φ is the work function of the material. The resulting photoelectron spectra
is a plot of the number of emitted electrons versus their kinetic energy, which
gives the information about the electronic structure of the material. Fig. 2.2 shows

Experimental Techniques 25
Figure 2.2: Schematic view of the energetics of the photoemission process [3], which
gives the relation between the energy levels in a solid and the electron energy distribution
produced by photons of energy hν. The electron energy distribution which
can be measured by the analyser as a function of kinetic energy (E kin ) is more conveniently
expressed in terms of the binding energy E B when one refers to the density
of states inside the solid.
schematically how the energy level diagram and the energy distribution of photoemitted
electrons relate to each other.
2.2.2 Theoretical interpretation of photoemission
There have been many important theoretical studies to describe and analyse the spectral
features in photoemission process [3, 4, 5, 6]. The most general and widely applied
theoretical description of the photoemission spectrum is based on using Fermi’s golden
rule as a result of perturbation theory in first order. The photo current produced in
the photoemission spectroscopy experiment is due to the excitation of electrons from
the initial states i with wave function ψ i to the final states f with wave function ψ f
by the photon field having vector potential A. According to the Fermi’s Golden Rule,

Experimental Techniques 26
the transition probability can be written as,
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)
where m fi is the square of the transition matrix element. To discuss about the
transition matrix element, one has to take certain assumptions of the wave functions
contained in it. In the simplest approximation one can take a one electron view for
the initial and final state wave functions. The final state is in addition with a free
electron having kinetic energy E kin . The initial state wave function is then written as
a product of the orbital φ k from which the electron is excited and the wave function
of the remaining electrons ψi,R(N k −1), assuming that system under consideration has
N electrons, where the index k indicates that the electron k has been left out and R
refers to the remaining. Now one can write,
ψ i (N) = Cφ i,k ψ k i,R(N − 1) (2.3)
where C is the operator that antisymmetrizes the wave function properly.
Similarly the final state can be written as a product of the wave function of the
photoemitted electron φ f,Ekin and that of the remaining N −1 electrons ψf,R k (N −1),
ψ f (N) = Cφ f,Ekin ψf,R k (N − 1) (2.4)
So the transition matrix element can be written as,
〈ψ f |r|ψ i 〉 = 〈φ f,Ekin |r|φ i,k 〉〈ψf,R(N k − 1)|ψi,R(N k − 1)〉 (2.5)
Thus the matrix element is the product of one electon matrix element and the
(N − 1) electron overlap integral. Let us assume that the remaining orbitals (called
the passive orbitals) are the same in the final state as they were in the initial state,
meaning ψf,R(N k − 1) = ψi,R(N k − 1), which renders the overlap integral unity and
the transition matrix element is just the one-electron matrix element. Under this
assumption [7] the PES experiment measures the negative Hartree-Fock orbital energy
of the orbital k meaning,
which is sometimes called Koopman’s binding energy.
E B,k ≃ − ǫ k , (2.6)

Experimental Techniques 27
Let us now assume that the final state with N − 1 electrons has s excited states
with the wave function ψf,s k and energy E s (N − 1).
The transition matrix element can be calculated by summing over all the possible
excited final states yielding,
〈ψ f |r|ψ i 〉 = 〈φ f,Ekin |r|φ i,k 〉 ∑ s
c s (2.7)
with
c s = 〈ψf,s k (N − 1)|ψk i,R (N − 1)〉 (2.8)
Here |c s | 2 is the probability that removal of electron from orbital φ k of the N
electron ground state leaves the system in the excited state s of the N − 1 electron
system. For strongly correlated systems many of the c s ’s are nonzero. In terms of PE
spectrum this means that for s = k one has the so-called main line and for the other
non-zero c s additional so-called satellite lines occur. If the correlations are weak, then
ψ k f,s(N − 1)≃ψ k i,R(N − 1) (2.9)
meaning that c s 2 ≃ 1 for s = k and c s 2 ≃ 0 for s ≠ k, i.e one has only one peak.
Now the photocurrent I detected in a PE experiment as,
I∝ ∑ |〈φ f,Ekin |r|φ i,k 〉| 2∑
f,i,k
s
|c s | 2 δ(E f,kin + E s (N − 1) − E 0 (N) − hν, (2.10)
where E 0 (N) is the ground state energy of the N-electron system. The photo
current thus consists of the lines created by photoionizing the various orbitals k where
each line can be accompanied by satellites according to the number of excited states
s created in the photoexcitation of that particular orbital k.
The above discussion is very convenient for atoms and molecules [4]. However,
it is not always easy to apply for solids. For solids, a slightly different formalism
has been developed [3, 8, 9] and hence the above equation can be rewritten in the
following form
I∝ ∑ f,i,k|〈φ f,Ekin |r|φ i,k 〉| 2 A(k, E) (2.11)
where A(k, E) is the so-called spectral function for the wave number k and energy
E. The spectral function can be related to the single particle Green’s function by

Experimental Techniques 28
A(k, E) = π −1 Im{G(k, E)} (2.12)
Here A(k, E) describes the probability of removing (below the E F ) or adding
(above the E F ) an electron with the energy E and the wave vector k from (to) the
N-electron system (Interacting).
For a non interacting system with the one electron energy Ek,
0
G 0 (K, E) =
where ǫ is very small
and
1
(2.13)
E − Ek 0 − iǫ,
A 0 (k, E) = δ(E − E 0 k) (2.14)
which means the spectral function is a δ-function at E = Ek 0 . This result is
identical to that of equation (2.6) as in a noninteracting system the binding energy
is by definition the Koopman’s binding energy.
In an interacting electron system the electron energy gets renormalized by the
so-called self energy Σ(k, E) = Re{Σ(k, E)} + iIm{Σ(k, E)}, yielding
and
G(K, E) =
1
E − E 0 k − Σ(k, E) (2.15)
A(k, E) = 1 π
Im{Σ(k, E)}
[E − E 0 k − Re{Σ(k, E)}]2 + [Im{Σ(k, E)}] 2 (2.16)
If we assume that the spectrum of the interacting system is not too different
from that of the noninteracting system (Im{Σ(k, E)}), then the poles in the Green’s
function of the interacting one occur at
with the condition
E 1 k = Re{E1 k } + iIm{E1 k } (2.17)
E 1 k − E0 k − Σ(k, E1 k ) = 0 (2.18)
This allows to separate the Greens function into two parts, a pole part and a
non-singular term G inc , i.e,

Experimental Techniques 29
and
G(k, E) =
Z k
E − (Re{E 1 k } + iIm{E1 k }) + G inc (2.19)
A(k, E) = 1 π
Z k Im{E 1 k }
(E − Re{E 1 k })2 + (Im{E 1 k })2 + A inc (2.20)
where Z k is a number smaller than one. The first term in (2.19) is similar to
the result for the noninteracting system, and one can therefore say that it describes
a slightly modified electron (with renormalized mass) which is generally called a
quasiparticle. The first term in (2.19) is called the coherent part of the Green’s
function, it gives rise to the coherent part of the spectrum. And the second term in
(2.19) is called the incoherent part of the Green’s function, which gives rise to the
incoherent part of the spectrum. In a very approximate way one can identify the
coherent part of the spectrum with the main line and the incoherent one with the
satellites in equation (2.20) [3].
2.2.3 The electron escape depth
In photoemission spectroscopy, when electrons are impinged on, or emitted from a
sample surface, the fundamental phenomenon occurs is inelastic scattering of electrons.
The escape depth of electron from the sample surface is depend on the inelastic
mean free path (IMFP) λ, which is the average distance between the scattering
events. λ is small for low energy electrons in solids and hence gives rise to surface
sensitivity. So ultra - high vacuum (UHV) condition is necessary to study the solid
surfaces [3, 10, 11]. Because a monolayer is adsorbed on the surface of the sample
in about 2.5 Langmuirs (1L = 10 −6 torr.s). At a pressure of 10 −9 torr it therefore
takes about 1000 seconds, or less than an hour, until a coverage of one monolayer is
achieved. Thus in order to get spectra of clean surfaces, vacuum of even better than
10 −10 torr is mandatory. The IMFP of an electron in a solid is a strong function of
its kinetic energy [3, 12, 13], and the relationship termed as the ”universal curve” as
shown in Fig. 2.3.
The electron kinetic energy in the range of interest between about 10 and 2000
eV. The escape depth is only on the order of a few Å. From the universal curve, one
can see the IMFP has a minimum value of 5 to 10 Å for the kinetic energy range

Experimental Techniques 30
Figure 2.3: ”Universal curve” for electron inelastic mean free path λ.
of 20 to 200 eV. And hence the surface sensitivity of photoemission spectroscopy is
highest in this energy range.
2.2.4 Experimental set-up
The OMICRON photoemission system at Institute of Physics consists of two connected
UHV chambers, a sample preparation chamber and an analysis chamber. The
preparation chamber is equipped with a precision manipulator, sample heater, evaporator
and a diamond file. The analysis chamber is made up of µ metal to protect low
energy electrons from earth’s magnetic field and stray magnetic fields. It is equipped
with a 125 mm hemispherical analyser [10] for angle integrated photoemission and a
movable 65 mm hemispherical analyser mounted on a 2-axis goniometer is also there
for angle resolved photoemission. Two photon sources [10, 14] (ultra-violet and x-ray)
are used for photoexcitations. The analysis chamber is also equipped with a 4-axis
sample manipulator-cum cryostat which can go down to 20 K. Facility for performing
low energy electron diffraction (LEED) is also available in the analysis chamber. The
external view of the spectrometer is shown in the Fig. 2.4. Some of the important
components of the spectrometer are described below.

Experimental Techniques 31
Figure 2.4: Photoemission spectrometer at Institute of Physics, Bhubaneswar.

Experimental Techniques 32
Figure 2.5: Schematic diagram of the VUV source.
Figure 2.6: Dimensional outline of the VUV source HIS 13. The viewport can be
used for inspection of the discharge and for adjustment of the light spot position on
the sample. The dimensions are in mm.
Vacuum ultra-violet photon source (VUV source HIS 13)
The high intensity VUV Source [14] HIS 13 can be operated with various discharge
gases, such as helium, neon, argon, krypton, xenon or hydrogen. But in our laboratory
we use only helium and neon gases. The operation of the lamp is based on the principle
of a cold cathode capillary discharge [15]. If an increasing potential is applied between
the ends of an insulating tube filled with gas at a pressure of typically 1 mbar, a

Experimental Techniques 33
point will be reached when spontaneous breakthrough occurs leading to a continuous
discharge. The ignition potential is about an order of magnitude higher than the
operating potential necessary to maintain a continuous discharge. The schematic of
the VUV source is shown in the Fig. 2.5. There is a windowless direct-sight connection
between the discharge area and the target. The discharge current is electronically
stabilised. The lamp is water cooled in order to allow for high discharge currents
(up to 300 mA) and to reduce electrode degradation resulting in prolonged service
intervals. The dimensional outline of the VUV source is shown in the Fig. 2.6
X-ray source (DAR 400)
The X-ray source we used is DAR 400 from OMICRON designed specifically for X-ray
photoelectron spectroscopy [10]. It has twin anodes that allow either Mg Kα (1253.6
eV) or Al Kα (1486.6 eV) radiation to be selected. The schematic of the X-ray source
is shown in Fig. 2.7.
Figure 2.7: Schematic of the X-ray source in operation
Electrons are extracted from a heated filament to bombard the selected surface of
an anode at high positive potential. The focus ring and the shape of the nose cone
ensure that the electrons hit the anode in the right area. By switching the filament

Experimental Techniques 34
used, the second face of the anode can be excited. The anode is water cooled to
prevent the aluminium or magnesium surfaces from evaporating.
X-rays generated in the surface of the anode pass through a thin aluminium window
to the sample under analysis. The aluminium window forms a partial vacuum
barrier between the source and the sample region.
The hemispherical electron energy analyser (EA 125)
The EA 125 energy analyser based on a 125 mm mean radius electrostatic hemispherical
deflection analyser composed of two concentric hemispheres [10, 16]. The inner
and outer spheres are biased positive and negative with respect to the pass energy of
the analyser. The analyser disperses electrons according to the energy across the exit
plane (between the two hemispheres) and focuses them in the angular dimension from
the entrance to the exit plane. Slits which are variable are located at the entrance
and exit of the analyser. A schematic of the EA 125 analyser is shown in Fig. 2.8.
Figure 2.8: Schematic diagram of the EA 125 hemispherical analyser.
The principle of operation of concentric hemispherical analyser is shown schematically
in Fig. 2.9. It consists of two concentric hemispheres of radii R 1 (inner hemisphere)
and R 2 (outer hemisphere) with mean radius R 0 mounted with a common
centre O. A potential V is applied between the two hemispheres so that the outer is

Experimental Techniques 35
negative and the inner positive with respect to V 0 which is the median equipotential
surface between the hemispheres.
Figure 2.9: The principle of operation of the concentric hemispherical analyser,
schematic diagram.
If E p is the kinetic energy of an electron travelling in an orbit of radius R 0 , then
the relationship between E p and V 0 is given by the expression,
eV 0 = E p ·
(
R2
− R )
1
R 1 R 2
(2.21)
The voltages on the inner and outer hemispheres are V 1 and V 2 respectively and
these are given by,
V 1 = E p ·
V 2 = E p ·
(a) Modes of analyser operation
(
2 R )
0
− 1
R 1
(
2 R )
0
− 1
R 2
(2.22)
(2.23)
The kinetic energy can be scanned either by varying the retardation ratio whilst
holding the analyser pass energy constant known as constant analyzer energy (CAE)

Experimental Techniques 36
mode or by varying the the pass energy E p whilst holding the retardation ratio constant
known as the constant retard ratio (CRR) mode [10].
i. Constant retard ratio (CRR) mode
In the CRR mode electrons entering the analyser system from the sample are
retarded by the lens stack by a constant proportion of their kinetic energy so that
the ratio of electron kinetic energy to analyser pass energy is kept constant during a
spectrum. The retard ratio k is defined as,
K = E k − φ a
E p
∼ E k
E p
(2.24)
where φ a is the analyser work function. Throughout the scan range the pass energy
of the analyser is continuously varied to maintain a constant retard ratio. Sensitivity
and resolution are proportional to the pass energy and therefore the kinetic energy
in this mode. In this mode the analysed sample area and the emission angle remain
almost constant throughout the whole kinetic energy range.
ii. Constant analyser energy (CAE) mode
In the CAE mode, the analyser pass energy is held constant, and the retarding
voltage is changed thus scanning the kinetic energy of detected electrons. The resolution
obtained in CAE is constant throughout the whole kinetic energy range. The
sensitivity, however is inversely proportional to the kinetic energy and at low kinetic
energies is improved over that of CRR. In the CAE mode, the analysed sample area
and the emission angle may vary slightly with the kinetic energy, but this is dependent
on the lens design.
(b) Analyser resolution
The analyser is a band pass energy filter for electrons at a specific energy E p
and has a finite energy resolution ∆E which is dependent on the chosen mode of
operation and specific operating conditions. The energy resolution of the analyser is
given approximately by,

Experimental Techniques 37
∆E = E p
( d
2R 0
+ α 2 )
(2.25)
where d is the slit width, R 0 is the mean radius of the hemispheres and α is the
half angle of electrons entering the analyser at the entrance slit.
(c) The electrostatic input lens
The input lens collects the electrons from the source and focuses them onto the
entrance aperture of the analyser whilst simultaneously adjusting their kinetic energy
to match the pass energy of the analyser. The lens is also designed to define the analysis
area and angular acceptance of electrons which pass through the hemispherical
analyser. The lens design employs double lens concept where two lenses are stacked
one above the other.
The first lens selects the analysis area (spot size) and angular acceptance. This is
an Einzel lens, i.e it does not change the energy of the electrons and therefore has a
constant magnification throughout the entire energy range. This lens can be operated
in three discrete magnification modes: high, medium and low. In high magnification
mode, the focal plane is nearer to the sample and the lens accept a wide angle of
electron beams from a small region. In low magnification mode, the focal plane is
farther from the sample and the lens accept only a small angle of beams but from a
larger area. The medium magnification is in-between the two.
The second lens retards or accelerates the electrons to match the pass energy of
the analyser and uses the zoom lens function to ensure that the focal point remains
on the analyser entrance aperture. The magnification of this lens varies with retard
ratio as a result of the law of Helmholtz-Lagrange.
The analysis area is defined by the combination of the selected analyser entrance
aperture and the magnification of the entire lens. The magnification of the entire lens
is a product of the magnifications of the two discrete lenses.
(d) The detector
A single channel electron multiplier (Channeltron) is placed across the exit plane
of the analyser. The channeltron amplifies the current of a single electron by a factor
of about 10 8 . The small current pulse present at the output of the channeltron is

Experimental Techniques 38
passed through a vacuum feedthrough and then directly into the preamplifier. And
then the signal is passed onto a pulse counter for processing and production of an
electron energy spectrum.
2.3 Inverse Photoemission Spectroscopy
2.3.1 Historical background
The inverse photoemission spectroscopy (IPES) [17, 18, 19] is a powerful technique
to study the unoccupied electronic states of solids. The IPES experiment can be
carried out in two different photon energy regimes. One is at higher energy range
(detected photons are in x-ray range) called Bremsstrahlung isochromat spectroscopy
(BIS) and the other one is at low energy region (detected photons are in the ultraviolet
range) which is called UV-IPES or simply IPES. The history of IPES technique
was developed in 1915 by Duane and Hunt [20] when an inverse relationship is observed
between the short wave-length cutoff of emitted x-rays and incident electron
energy when a solid is bombarded with electrons. Then pursued by Ohlin [21] and
Nijboer [22] in 1940s and was resumed in the 1950s by Ulmer and Vernickel [23]. The
IPES work in the ultra-violet region is of relatively recent origin. The experimental
work was started by Dose [24] and the theoretical work by Pendry [25, 26]. The
experimental advances since then have been rapid.
2.3.2 Basic principle
When a beam of electrons is incident on a solid surface, after entering the system,
the electrons decay either radiatively or non-radiatively to states at lower energy. If
the decay is radiative the emitted photon can be detected, and gives the information
about the unoccupied states of the system. So this is complementary to direct photoemission
spectroscopy (PES) [17, 18, 19, 27]. A schematic of the inverse photoemission
spectroscopy is shown in the Fig. 2.10
The energy of the photons (hν) emitted when a beam of electrons having kinetic
energy E k is incident on a solid surface and then relax to a lower energy unoccupied
state (E f ), is given by the conservation of energy as,
E k = hν + E f − φ, (2.26)

Experimental Techniques 39
Figure 2.10: Schematic diagram of inverse photoemission spectroscopy (IPES).
where φ is the work function of the material.
There are two modes [18, 19, 27] used for this measurement . One is isochromat
mode and the other one is spectrograph mode. In isochromat mode, the incident
electron energy is ramped and the emitted photons are detected at a fixed energy.
In the spectrograph mode, the energy of the incident electron remains fixed and the
emitted photons are detected over a range of photon energies.
2.3.3 Basic interpretation of inverse photoemission
The Inverse photoemission process can be described [28] by a comparison with photoemission.
Both photoemission and inverse photoemission involve the interaction
between photons and electrons. Now the interaction Hamiltonian associated with the
electromagnetic field can be written as,

Experimental Techniques 40
H ′ =
e (A.p + p.A) (2.27)
2mc
where A is the vector potential of the electromagnetic radiation. For photoemission
the field A can be treated as a classical time-dependent perturbation, and
results will be the same as a proper quantum treatment had been made. For inverse
photoemission, however, this is no longer true. Since photons are created it becomes
necessary to quantize the electromagnetic field. In such a treatment the classical
vector potential is replaced by the field operator A(x, t) defined by [29]
A(x, t) =
1 ∑ ∑ 2π¯hc
(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)
where ˆε α is the linear polarization vector, a real unit vector whose direction depends
on the photon propagation direction q. The two operators a † q,α and a q,α either
create or destroy a photon in the state q, α respectively. V p is the normalization
volume for the photon. For inverse photoemission the initial state consists of an electron
in a continuum state |i〉≡ψ i (r) with no photons and the final state consists of
an electron in a bound state |f〉≡ψ f (r) and a photon with wave vector q. Then the
transition rate is given by the above two equations using the first order perturbation
theory,
R =
2π¯h
c22π¯h
ω
where ρ p is the photon density of states
and dΩ is the solid angle of emission. Then
1
|〈f|ˆε.p|i〉| 2 ρ p (2.29)
V p
V p
ω 2
ρ p =
(2π) 3 ¯hc 3dΩ
(2.30)
R = 1 e 2 ω
2π ¯hc m 2 c 2 |〈f|ˆε.p|i〉|2 dΩ, (2.31)
where e2
¯hc = α≈ 1
137
To obtain a cross section, we devide by the incident electron flux
j e = ¯hk 1
(2.32)
m V e
We further assume that the continuum electron state is normalized to a box of
volume V e . Then

Experimental Techniques 41
( ) dσ
= α ω 1
dΩ 2π mc
IPES 2 ¯hk |〈f|ˆε.p|i〉|2 (2.33)
It is interesting to compare this cross section with that of the photoemission.
In photoemission the final states are those of electrons, The density of states factor
becomes
ρ e = V e mk
8π 3 ¯h 2 (2.34)
and the incident flux is the photon flux
Then
j p =
ω 1
(2.35)
8π¯hc V p
( ) dσ
= α k 1
dΩ 2π m ¯hω |〈i|ˆε.p|f〉|2 (2.36)
PES
To relate the cross sections of the two processes (PES and IPES), let us suppose
the matrix element in PES is same as that of the IPES. Then the ratio of two cross
sections reflects only the different phase space available for photon creation or electron
creation, and is given by
or
r =
( ) dσ
/
dΩ
IPES
( ) dσ
= ω2
dΩ c
PES
2 k = q2
(2.37)
2 k 2
r =
(
λe
λ p
) 2
(2.38)
where λ e and λ p are the electron and photon wavelengths respectively.
And the equation (2.38) says the ratio of the inverse photoemission cross section to
that of the phtoemission is the ratio of the square of the wave length of electron to that
of photon. At an energy of 10 eV, characteristics of the vacuum ultraviolet region, the
ratio r∼10 −5 . Therefore the signal levels in IPES is much lower comparable to PES.
At higher energies ∼1000 eV, characteristics of the x-ray range, we have r∼10 −3 .
2.3.4 Experimental Set-up
The external view of IPES spectrometer at UGC-DAE consortium for scientific research,
Indore, is shown in the Fig. 2.11.

Experimental Techniques 42
Figure 2.11: (a) IPES spectrometer at UGC-DAE consortium for scientific research,
Indore, (b) External view of photon detector attached to the IPES chamber, and (c)
Inside view of the chamber showing a rotatable (left) and fixed (right) electron gun
and the sample holder

Experimental Techniques 43
The experimental UHV chamber is made of µ metal which provides effective shielding
for the low energy electrons from earth’s magnetic field and stray magnetic fields.
A photon detector capable of detecting fixed energy photons with well defined narrow
bandpass and an electron gun producing nearly monoenergetic low energy electrons
with well defined angular spread are equipped in the UHV chamber. A sample manipulator
is mounted vertically which allow to move the sample both along axial and
azimuthal direction.
The low energy electron gun (e-gun)
The e-gun used for the experiment is of Stoffel Johnson type [30] is shown schematically
in Fig. 2.12. This is chosen because it has less space-charge effect and high
beam current.
Figure 2.12: A schematic diagram of the electron gun [30].
In this type of e-gun the minimum beam size obtained is 1.1 mm with full angular
convergence of 5 ◦ to 7 ◦ . It consists of a filament in a mounting block followed by
three element refocusing lens. All the lens elements are with cylindrical cross-section.
In the design (Fig.) L = D = 16mm, P = 1.3D and Q = 2.5D. Ratio of voltages
applied on the cathode and the subsequent electrostatic lens elements is V C : V A :
V F : V O = −1 : +5 : −0.9 : 0. The Kinetic energy of emitting electrons is governed by
V C . The whole surface of the lenses is gold plated to obtain a uniform work function.
BaO dispenser cathode is used as the electron emitter. The e-gun operates in the
range of 5 − 40 eV.

Experimental Techniques 44
The photon detector
Gas filled band-pass Geiger-Muller (GM) type counters are used for photon detection
in IPES because of their high efficiency, low cost and simple design [31, 32, 33, 34,
35, 36, 37, 38]. In our IPES experiment the acetone/CaF 2 detector [39] was used.
The design of the photon detector is based on the following operating principle
: the high energy cut-off of the acetone/CaF 2 detector is due to the CaF 2 window
that does not transmit photons with energy > 10.2, while the threshold for the
photoionization of acetone at 9.7 eV sets the low energy cut-off [40]. This determines
the band pass function and results [33] in a mean photon detection energy of 9.9
eV with a FWHM of 0.4 eV. Thus, 9.9±0.2 eV photons can enter the detector to
photoionize acetone.
Figure 2.13: Schematic cross section of the photon detector showing the window (1),
O-ring (2), cylindrical cap (3), detector tube (4), teflon spacer (5), anode (6), double
sided DN40 CF flanges (7, 10), gas inlet and outlet (8, 9), barrel connector (11),
teflon sleeve (12), DN40 CF flange (13) and SHV connector (14).
The detector assembly [41] (2.13) has a modular design consisting of a 250 mm
long stainless steel cylindrical tube with inner diameter 17.8 mm and outer diameter
21.2 mm (Fig), welded to a double-sided DN40 CF flange (7) with a through hole equal
to the inner diameter of the tube (4). Another double-sided DN40 CF flange (10)
with a similar through hole has two additional 6 mm diameter through holes drilled
in the radial direction. Two 6 mm diameter of stainless steel tubes of length 25 mm

Experimental Techniques 45
are welded to these radial holes, and on the other end, DN16 CF flanges (8, 9) are
welded to these tubes. These are used as gas inlet (8) and outlet (9). The anode (6)
is attached to a SHV feedthrough (14) mounted on another DB40 CF flange (13) by
a barrel connector (11). The three DN40 CF flanges (7, 10, 13) are mounted together
on a linear translator. The anode is a 1.6 mm diameter electropolished stainless steel
wire and is held by two perforated teflon spacers (5) in the detector tube. High voltage
is applied to the anode. A 25 mm diameter, 2 mm thick CaF 2 window (1) sits on a
viton O-ring (2) and is tightly pressed by a stainless steel cap (3) at the entrance of
the detector. This isolates the detector from the UHV chamber. Use of viton O-ring
ensures easy exchange of windows at the testing stage of the detector. The count rate
increases with higher solid angle of detection. So the detector is brought as close as
possible to the sample using the linear translator.
2.4 X-ray Absorption Spectroscopy
X-ray absorption spectroscopy (XAS) is another technique to study the unoccupied
electronic states as well as the geometrical arrangements of any materials around an
atom [42, 43]. But in this thesis XAS technique is used only to study the electronic
structure of the materials.
2.4.1 Basic principle
When a beam of monochromatic x-ray enters a solid, either it will be scattered by
the electrons or absorbed and excite the core electrons. In the absorption process the
core electron is excited to an empty state [21, 45]. The schematic of XAS is shown in
Fig. 2.14.
If the energy of the x-ray is much larger than the binding energy of the core state
the excited electron behaves like a free electron in the solid. If however the x-ray
energy is just enough to excite a core electron, it will occupy a lowest available empty
state.
When a monochromatic x-ray beam of intensity I 0 goes through a material of
thickness d, its intensity I will get reduced as,
I = I 0 e −ρµd (2.39)

Experimental Techniques 46
Figure 2.14: Schematic diagram of x-ray absorption spectroscopy
where µ is the absorption coefficient of the material, which depends on the types
of atoms and density ρ of the material. The absorption coefficient decreases smoothly
with higher energy, except for certain photon energies where the absorption increases
drastically, and gives rise to an absorption edge. Each such edge occurs when the
energy of the incident photons is just sufficient to excite a core electron to an unoccupied
state or into the continuum state (leave the atom). After each absorption edge
the absorption coefficient continues to decrease with increasing photon energy (Fig.
2.15).
There are two distinguishable parts in x-ray absorption process : The low energy,
near edge x-ray absorption fine structure (NEXAFS) [46, 47] or x-ray absorption
near edge structure (XANES) region and the extended x-ray absorption fine structure
(EXAFS) [48, 49], where the ejected electrons have high kinetic energy with single
scattering. In this thesis work, only the XANES (or NEXAFS) region has been taken
to study the unoccupied states.

Experimental Techniques 47
Figure 2.15: The x-ray loses its intensity via interactions with material. The absorption
coefficient decreases smoothly with higher energy, except for special photon
energies. When the photon energy reaches a critical value for a core electron transition,
the absorption coefficient increases abruptly.
2.4.2 Detection techniques
The normal way to measure the x-ray absorption is in the transmission mode [21]. The
intensity of the x-ray is measured before and after the sample and the percentage of
transmitted x-rays is determined. Such transmission mode of experiment is standard
for hard x-rays. But for soft x-rays they are difficult to perform because of the strong
interaction of soft x-rays with matter and requires a very thin sample of 0.1 µm to
obtain a detectable signal. An alternative to the tansmission mode experiment is the
”Yield mode”, where the decay product of the core hole (created in x-ray absorption)
will be measured. The decay of the core hole gives rise to an avalanche of electrons,
photons and ions escaping from the sample surface. There are four types of detection
methods [21] in yield mode XAS measurement such as Auger electron yield (or partial
yield), fluorescence yield, ion yield and total electron yield.
In Auger electron yield method, the electrons of a certain energy range (i.e electrons
of a specific Auger decay channel of the core hole) are detected. It is found
that the mean free path of a 500 eV Auger electron is of the order of 20 Å. Hence

Experimental Techniques 48
the number of Auger electrons emitted is equal to the number of core holes which
are created in the first 20 Å from the surface. The Auger electron yield method is a
measurement of only 20 Å of material, hence the Auger electron yield is rather surface
sensitive. In fluorescence yield XAS mode, the fluorescence decay of the core hole is
measured instead of Auger decay electrons. The intensity of the fluorescence yield
is small compared to Auger electron yield mode for the elements of atomic number
Z ≤ 50. The main advantage of fluorescence yield is that the created photon has a
mean free path of the same order of magnitude as the x-ray (∼ 1000 Å) and hence it
probes the bulk. In ion yield mode, only the surface ions created by Auger decay are
measured. So it is highly surface sensitive and is used for special surface study. The
most applied detection technique for soft XAS is the total electron yield. In total
electron yield mode, all escaping electrons are counted regardless of their energy. The
count rate is very large in this case. This thesis contains the XAS spectra measured
in total electron yield and fluorescence yield mode. In TEY mode, the electrons are
detected by the hemispherical analyser, which is discussed in the last section. In the
FY mode the radiative photons are detected by the Si-photodiode.
Si - photodiode
Silicon photodiode detectors are widely used over a broad wavelength range extending
from the x-ray to the visible regions. The detector used for the XAS measurement
involved in this thesis are AXUV and SXUV photodiodes [50, 51]. The photodiodes
are fabricated by an ULSI (Ultra large Scale Integrated circuit) compatible process.
Fig. 2.16 shows the cross section structure of the AXUV and SXUV photodiodes.
The AXUV photodiode has a thin nitrided SiO 2 surface layer (approximately 7 nm
thick) and 100 % internal quantum efficiency. The SiO 2 layer is known to have a
fixed positive charge which repels minority carrier holes generated at the surface by
strongly absorbing extreme ultra-violet (EUV) radiation. The SXUV photodiode has
metal silicide surface layer which replaces the oxide layer on the AXUV photodiode
and improves the radiation hardness. However, the absence of the oxide layer causes
significant surface recombination in the SXUV photo diodes and reduces the internal
quantum efficiency below 100 % value, that is characteristics of AXUV photodiodes.
When the silicon photodiode is exposed to photons with energy greater than 1.12
eV (wavelength less than 1100 nm), electron-hole pairs (carriers) are created. These
photogenerated carriers are separated by the p-n junction electric field and a current

Experimental Techniques 49
Figure 2.16: Schematic of the Si-photodiode.
proportional to the number of electron hole pairs created flows through an external
circuit. Several unique properties of the AXUV photodiodes have resulted in previously
unattained stability and 100 % carrier collection efficiencies giving rise to near
theoretical quantum efficiencies. The first property is the absence of a surface dead
region i.e no photogenerated carrier recombination occurs in the doped n-type region
and at the silicon-silicon dioxide interface. As the absorption depths for the majority
of UV/EUV photons are less than 1 micrometer in silicon, the absence of a dead region
yields complete collection of photogenerated carriers by an external circuit resulting
into 100 % collection efficiency. The second unique property of the AXUV diodes is
their extremely thin (4 to 8 nm), radiation-hard silicon dioxide junction passivating,
protective entrance window. Owing to their 100 % collection efficiency and the thin
entrance window, the quantum efficiency of the AXUV diodes can be approximately
predicted in most of the XUV region by the theoretical expression E ph /3.65, where
E ph is the photon energy in electron volts.
2.5 Synchrotron Radiation
The unique properties of synchrotron radiation are its continuous spectrum, high
flux and brightness, and high coherence, which make it an indispensable tool in the

Experimental Techniques 50
exploration of matter. Synchrotron radiation are produced when charge particles
Figure 2.17: Schematic of the synchrotron radiation.
moving with relativistic velocity are deflected in a magnetic field. The synchrotron
radiation had been developed by taking the effect of Heinrich Hetz [1] as the electrons
fly around the curves through the magnetic field emit electromagnetic radiation. In
SR facilities the accelerated electrons fly in a closed orbit inside a stainless steel
tube, which consists of large electromagnets at regular intervals to guide the electrons
around the curves and keep them focussed in the center of the tube called the storage
ring. The electrons are generated in an injection system consisting of an electron
source and an accelerator, either a synchrotron or a linear accelerator. When the
electrons are accelerated to the operating energy of the storage ring, they are injected
into the storage ring. The bending magnets are used to deflect the electron beam.
In order to compensate the energy loss of the electrons during the SR emission, the
electrons are accelerated each time as they pass through the RF cavity installed inside
the storage ring. A schematic of SR light source is shown in the Fig. 2.17.
In the developed synchrotrons the intensity of the emitted radiation is enhanced
by insertion devices (undulator or wiggler). The insertion devices are periodic arrays
of permanent magnets. When electrons traverse in the periodic array of magnets are
forced to undergo oscillations and radiate (Fig. 2.18 shows the schematic representation).The
static magnetic field is alternating along the length of the insertion devices
with a wave length λ 0 . The radiation produced by insertion devices is very intense and
is guided through beam lines for various experiments. An important dimensionless
parameter K which characterizes the nature of electron motion is defined as,

Experimental Techniques 51
Figure 2.18: Schematic representation of an insertion device.
K = eB 0λ 0
(2.40)
2πmc
where e is the electronic charge, B 0 is the magnetic field, m is the electron rest
mass and c is the speed of light.
For undulator K > 1, the oscillation amplitude is bigger and the radiation contributions
from each field period sum up independently leading to a broad energy spectrum.
Insertion devices can provide several orders of magnitude higher flux than a simple
bending magnet and are in high demand at synchrotron radiation facilities. For an
undulator with N periods, the brightness can be up to N 2 more than a bending
magnet.
The experiments presented in this thesis were performed at different synchrotrons
using monochromators of various type. The HRPES and XAS experiments were
performed at the WERA soft x-ray beamline of ANKA synchrotron, Germany. At
BEAR and BACH beamlines of ELETTRA synchrotron, Italy, the XAS experiments
were performed.
At WERA soft x-ray beamline of ANKA synchrotron, a spherical grating monochromator
(DRAGON type) [21, 52] was used. The layout of the WERA soft x-ray beamline
is shown in the Fig. 2.19. It consists of a pair of mirrors 1 (horizontal) and

Experimental Techniques 52
Figure 2.19: Optical layout of the WERA soft x-ray beam line, ANKA.
2 (vertical) to focus the incoming beam into the entrance slit of the grating box
and also a pair of bendable focusing mirrors after the exit slit for optimum focus at
sample position. Both entrance and exit slits are movable. The energy resolution
∆E/E < 10 −4 . The photon energy range is 100 - 1500 eV. The experimental facilities
associated with the WERA soft x-ray beamline are PES, PEEM, NEXAFS and
SXMCD.
The BEAR beamline at ELETTRA is dedicated to the study of optical, electronic
and magnetic properties of materials under UHV condition. The lay out of the BEAR
beam line is shown in the Fig. 2.20. The beamline optics is based on a couple of
parabolic mirrors. The radiation is collimated by the first parabolic mirror (P1). The
parallel beam produced by P1 reaches the monochromator stage based on the plane
grating/plane mirror scheme according with the Naletto-Tondelo scheme [53]. There
are three gratings (G-NIM, G-1200 and G1800) associated with the monochromator.
Three gratings cover 5 - 1600 eV photon energy range. The G-NIM (1200 1/mm)
works at near normal incidence to cover the 5 - 45 eV region. The G1200 (1200 1/mm)
and G1800 (1800 1/mm) work in the 45 - 1600 eV energy range. The monochromatic
radiation is focused onto the exit slit by a second parabolic mirror (P2). The elliptical
re-focusing mirror (REFO) brings the beam at the sample position. The experimental

Experimental Techniques 53
Figure 2.20: Optical layout of the BEAR beam line.
facilities available at BEAR beamline are ARPES, XAS, photoelectron diffraction and
specular and diffuse reflectivity.
The BACH beam line is designed to deliver a photon beam with high intensity
and brilliance in the soft x-rays energy range 35 -1600 eV with full control of the
polarization of the light. The goal of this beam line is to perform the electron and
photon spectroscopy experiment with polarization dependence. The optical layout of
the BACH beam line [54, 55] is shown in the Fig. 2.21.
Figure 2.21: Optical layout of the BACH beam line.

Experimental Techniques 54
Figure 2.22: The variable included angle monochromator : the combined movement
of PM1 (plane mirror) and one of the four spherical gratings SG1(-4) leads to the
monochromatization of the light keeping both the entrance and exit slit fixed.
The radiation source is based on two APPLE-II elliptical undulators that are
used alternatively to optimize the flux. The photon dispersion system is based on the
Padmore ”variable angle spherical grating monochromator (VASGM)” scheme and it
includes four different interchangeable spherical gratings (Fig. 2.22). The first three
gratings works in the energy ranges 35 - 200 eV, 200 - 500 eV and 500 - 1600 eV
respectively. The fourth grating operates in the 400 - 1600 eV range providing higher
flux allowing fluorescence and x-ray scattering experiments. Two refocusing sections,
based on plane elliptical mirrors in Kirkpatrick - Baez configuration provide nearly
aberration free spots on the samples. A third branch line after the monochromator will
host a microscope based on Fresnel zone plate focusing. The following experimental
facilities are associated with the BACH beam line ; XPS, UPS, XAS, XMCD, XMLD,
SXES and RIXS.

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Experimental Techniques 56
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Experimental Techniques 57
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Experimental Techniques 58
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Chapter 3
Electronic Structure of
Pr 0.67 Ca 0.33 MnO 3
59

Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 60
3.1 Introduction
Recently, a lot of attention have been focused on the charge-ordered compositions of
Pr 1−x Ca x MnO 3 due to their importance as possible prototypes for the electronic phase
separation (PS) models [1, 2, 3, 4, 5, 6, 7, 8, 9] proposed to explain the phenomenon
of colossal magnetoresistance (CMR). The PS models are qualitatively different from
the double-exchange model [10, 11, 12] or those based on strong Jahn-Teller polarons
[13, 14]. According to the PS model the ground state of CMR materials is comprised
of coexisting nanosize clusters of metallic ferromagnetic and insulating antiferromagnetic
nature [6]. The insulator-metal transition in this scenario is through current
percolation. Though there have been a number of experimental studies showing the
existence of phase separation, their size varies from nano- to mesoscopic scales [8, 9].
Radaelli et al. have shown the origin of mesoscopic phase separation to be the intergranular
strain [9] rather than the electronic nature as in PS models. Nanosized
stripes of a ferromagnetic phase [15] were reported in Pr 0.67 Ca 0.33 MnO 3 . This compound
also attracted much attention earlier due to the existence of a nearly degenerate
ferromagnetic metallic state and a charge-ordered antiferromagnetic insulating state
with a field-induced phase transition possible between them [16]. With slight variations
in the Ca doping the Pr 1−x Ca x MnO 3 system turns ferromagnetic (x=0.2) or
antiferromagnetic (x=0.4) at low temperatures [17, 15]. The composition x=0.33
shows a coexistence of ferromagnetic and antiferromagnetic phases [18]. This makes
the charge- or orbital-ordered Pr 0.67 Ca 0.33 MnO 3 a prototype for the PS scenario.
One of the requisites for the existence of an electronic phase separation in manganites
is a strong-coupling interaction affecting the hopping of the itinerant e g electrons.
The PS is expected to occur [5] when J H /t ≫ 1, where J H is the Hund’s coupling
contribution between the localized t 2g and the e g electrons and t is the hopping amplitude
of the e g electrons. The PS also favors a strong electron-phonon coupling,
like the influence of a strong Jahn-Teller (JT) polaron arising from the Q 2 and Q 3
JT modes. Essentially, one expects a strong localization of charge carriers to accompany
the electronic separation of phases. Apart from charge, the orbital degrees of
freedom also play an important role in this scenario [5]. The itinerant electron hopping
term is strongly influenced by the symmetry of the orbitals (d x 2 −y 2 and d 3z 2 −r 2)
hybridized with the O 2p orbitals of the MnO 6 octahedra. In comparison with the
most popular CMR material La 1−x Sr x MnO 3 , the charge-ordered Pr system has an
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
enhances the tendency for carrier localization. All these charge and orbital interactions
are reflected in the near-E F electronic structure of these materials and could
be probed using electron spectroscopic techniques. Valence band photoemission and
O K x-ray absorption are two such proven tools sensitive to the changes in the lowenergy
states crucial to these interactions. Although, there are many reports on
the near-E F electronic structure of other CMR compounds using these techniques
[19, 20, 21, 22, 23, 24, 25, 26, 27, 28] only a few studies have been reported on their
charge-ordered compositions [8, 29]. One of the key energy terms these spectroscopies
could show earlier was the charge transfer energy E CT which is intimately related to
the electron-electron and electron-lattice interactions [20].
In this study we have used ultraviolet photoelectron spectroscopy and x-ray absorption
spectroscopy (XAS) in order to probe the electronic structure of the occupied
and unoccupied states on a well-characterized, high-quality single crystal of
Pr 0.67 Ca 0.33 MnO 3 . Another part of this single crystal had earlier been used for a detailed
neutron scattering experiment that indicated the possible existence of a phase
separation of ferromagnetic and antiferromagnetic stripes [15]. In the present study
we have analyzed the temperature-dependent changes in the near-E F electron energy
states from the perspective of a phase separation.
3.2 Experimental
The single-crystal sample of Pr 0.67 Ca 0.33 MnO 3 was grown by the floating zone method
in a mirror furnace. The compositional homogeneity of the crystal was confirmed using
energy-dispersive spectroscopic analysis. Magnetization and transport measurements
on this crystal showed the transition temperatures T c and T N to be 100 and 110
K, respectively. Details of the sample preparation, magnetization, and transport measurements
and structural studies are published elsewhere [15, 30]. Angle-integrated
ultraviolet photoemission measurements were performed using an Omicron µ-metal
UHV system equipped with a high-intensity vacuum-ultraviolet source (HIS 13) and
a hemispherical electron energy analyzer (EA 125 HR). At the He I (21.2 eV) line,
the photon flux was of the order of 10 16 photons/s/sr with a beam spot of 2.5 mm
diameter. Fermi energies for all measurements were calibrated using a freshly evaporated
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
width of the Fermi edge, was about 80 meV. The single-crystalline samples were repeatedly
scraped using a diamond file inside the chamber with a base vacuum of ∼
1.0 × 10 −10 mbar. Scraping was repeated until negligible intensity was found for
the bump around 9.5 eV, which is a signature of surface contamination [19]. For
the temperature-dependent measurements, the sample was cooled by pumping liquid
nitrogen through the sample manipulator fitted with a cryostat. Sample temperatures
were measured using a silicon diode sensor touching the bottom of the sample
holder. XAS measurements were performed using the BEAR [31] and BACH [32]
beamlines associated with ELETTRA at Trieste, Italy. At the BEAR beamline we
used monochromatized radiation from a bending magnet in order to record the O K
edge spectra at room temperature and 150 K in fluorescence detection mode on a
freshly scraped surface of the single crystal. The energy resolution was around 0.2
eV in the case of these two spectra. The O K edge spectra at 95 K was recorded at
the BACH beamline, using the total electron yield mode. Before the measurements,
the sample surface was scraped inside the UHV chamber (∼ 1.0 × 10 −10 mbar) using
a diamond file. At this beamline we used radiation from an undulator, monochromatized
using a spherical grating. The resolution at the O Kedge for this measurement
was better than 0.1 eV.
3.3 Results and Discussion
3.3.1 The UPS study of Pr 0.67 Ca 0.33 MnO 3
The angle-integrated valence band photoemission spectra of Pr 0.67 Ca 0.33 MnO 3 taken
at different temperatures below and above T c are shown in Fig. 3.1. Intensities of
all the spectra were normalized and shifted along the y axis by a constant for clarity.
The spectra, dominated by the states due to the Mn 3d-O 2p hybridized orbitals, look
similar to those reported earlier on the La 1−x Sr x MnO 3 system [19, 20, 21, 22, 23]. The
origin of the two prominent features, one at ∼ -3.5 eV (marked B) and another at
∼ -5.6 eV (marked C) below E F , are now well known from earlier experiments and
band structure calculations [23, 24, 33] on similar systems. While the feature at -3.5
eV is mainly due to the t 2g↑ states of the MnO 6 octahedra, the -5.6 eV subband
has contributions from both t 2g and e g states. More important contributors to the
properties of these systems are the states nearer to E F , which appear as a tail at
∼ -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
C
B
Intensity (arb. units)
A
-10 -8 -6 -4 -2 0
Energy Relative to E F
(eV)
Figure 3.1: Valence band photoemission spectra of Pr 0.67 Ca 0.33 MnO 3 taken using He
I photon energy (21.2 eV) at 77 (solid line), 110 (dotted), 150 (dashed), 220 (dotdashed),
and 300 K (double-dot dashed). All the spectra have been normalized and
shifted along y-axis by a constant for clarity. The subbands around -1.2 (e g↑ ), -3.5
(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
300 K
77 K
5 X Difference
(a)
300 K
110 K
5 X Difference
(b)
Intensity (arb. units)
-1 -0.5 0 0.5 1 1.5
300 K
220 K
5 X Difference
(d)
-1 -0.5 0 0.5 1 1.5
300 K
150 K
5 X Difference
(c)
-1 -0.5 0 0.5 1 1.5
Binding Energy (eV)
-1 -0.5 0 0.5 1 1.5
Figure 3.2: High-resolution photoemission spectra of the near-E F region of the valence
band of Pr 0.67 Ca 0.33 MnO 3 . In (a) the spectrum taken below T c (red) is compared with
the normal state (300 K) spectrum (blue). The difference spectrum (green) obtained
by subtracting the spectra at 300 K from 77 K and multiplied by 5 is also shown in
the panel. Similarly, in (b), (c), and (d) the near-E F spectra taken at 110, 150, and
220 K are compared with that taken at 300 K. The feature in the difference spectra
corresponds to the tail feature A (e g↑ ) in Fig. 3.1.
is quite small compared to B and C and is a signature of the insulating nature
of this material. Earlier photoemission experiments on La 1−x Sr x MnO 3 also have
shown that the intensity of this tail feature A is quite small [8, 20, 21, 22, 23].
The presence of the feature A is clear from Fig. 3.2, where we have shown the
near-E F region of the valence band spectra taken with a higher resolution. Here,
the spectra taken at different temperatures have been compared with those at room
temperature. The figure also shows the difference spectra obtained by subtracting
the room-temperature spectra from spectra taken at low temperatures. The feature
in the difference spectra corresponds to A, which originates from the e g↑ states [22].
In order to estimate the relative changes in all the three features (A, B, and C)
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
0.014
(a)
0.012
Normalized Intensity
0.01
0.6
0.55
0.5
0.45
(b)
(c)
0.4
100 150 200 250 300
Temperature (K)
Figure 3.3: Temperature dependence of the area of the three valence band features
obtained from fitting the spectra with Lorentzian line shapes using a χ 2 iterative
program. We have used an integral background, which was kept the same for all the
spectra. The energy positions and FWHMs were determined by finding the best fit
common to all the spectra by the iterative program. The final fit for all spectra at
different temperatures were obtained with the same energy positions and FWHMs.
(a), (b), and (c) correspond to the features A, B, and C positioned at -1.19, -3.49,
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
three components corresponding to the three subbands contributing to this region.
We used a χ 2 iterative program for fitting the different spectra with Lorentzian line
shapes for A, B, and C. Except for the area of the peaks, the energy positions,
widths, background, and all other parameters of all the peaks were kept the same for
different temperatures. The positions and full widths at half maximum (FWHMs)
of the three peaks are given in the caption of Fig. 3.3. The fitted spectra for 77
K (below T c ) and 300 K (above T c ) are shown on the left sides of Figs. 3.5(a) and
3.5(b) below. One can see from Figs. 3.3(a) - (c) and that the area of all the peaks
keeps increasing as we go down in temperature T c , but below this temperature the
intensities of both peaks C and B decrease. On the other hand, the intensity of A
does not show any decrease across the transition [Fig. 3.3(a)].
Pr 0.67 Ca 0.33 MnO 3 shows a transition to a charge-ordered state below 220 K (T co )
[30, 37, 38]. Neutron diffraction studies [15] on another part of this single crystal
have shown that this charge ordering turns into an antiferromagnetically structured
pseudo-CE type charge or orbital ordering below 110 K. Also, the coexistence of
ferromagnetic and antiferromagnetic phases below this transition temperature has
been shown on this single crystal. In the pseudo-CE-type ordering the Mn 3d z 2 −r 2
orbitals, where the e g state is occupied, are aligned with the O 2p orbitals. Such an
alignment can increase the hybridization between Mn 3d z 2 −r2 and O 2p. Furthermore,
a simultaneous decrease in the hybridization strength could also be expected for the
core-like in-plane t 2g and O 2p states. The results of the curve fitting (Fig. 3) of
our valence band spectra reflects these changes in hybridization with temperature.
The charge ordering following the decrease in temperature causes an increase in the
O 2p contribution to both the t 2g and e g spin-up subbands in the valence region and
hence the intensities of A, B, and C go up. As mentioned earlier, the pseudo-CE-type
charge or orbital ordering below 110 K, results not only in a stronger hybridization of
O 2p with the e gz 2 −r 2 ↑ states compared to that with the t 2g states but a simultaneous
decrease in the latter also. This is reflected in the increase in intensity of A and
decrease in the intensities of both C and B below 110 K. Though in Pr 0.67 Ca 0.33 MnO 3
there is no temperature-dependent insulator-metal transition, the change in intensities
of these features across T c appears similar to the shifting of spectral weight found in
the La 1−x Sr x MnO 3 system [8].

Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 67
Normalized Absorption (arb. units)
530 540 550 560
Photon Energy (eV)
Figure 3.4: O K edge x-ray absorption spectra of Pr 0.67 Ca 0.33 MnO 3 taken at 300
(solid line), 150 (dashed line), and 95 K (dotted). The pre-edge feature centered
around 529.5 eV (marked by a box) is where most interest lies. Since this feature
consists of two peaks (a main line and a shoulder on the low-energy side), this part
of the spectrum was fitted with two components of Lorentzian line shapes. Results
of the curve fit are given in Table 3.1.
3.3.2 The XAS study of Pr 0.67 Ca 0.33 MnO 3
In a reasonable approximation, the near-E F region of the O K x-ray absorption
spectra could well represent the density of unoccupied states in many of the transition
metal oxide compounds [20, 34]. In order to probe the electronic structure of the
unoccupied states we have performed XAS on our Pr 0.67 Ca 0.33 MnO 3 single crystal.
The O K edge XAS spectra taken at room temperature and 150 and 95 K (below
T c ), shown in Fig. 3.4, have been normalized in intensity all along the region starting
from 550 eV. The pre-edge feature in the O K spectra (centered around 529.5 eV) is
due to the strong hybridization between Mn 3d and O 2p orbitals. The broad feature
around 536.5 eV is due to the bands from hybridized Pr 5d and Ca 3d orbitals, while
the structure above 540 eV is due to states like Mn 4sp and Pr 6sp, etc. These
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
Table 3.1: χ 2 iterative fitting parameters for the pre-edge peak in O K XAS. The
intensities of B ′ and A ′ at low temperature are normalized with respect to the total
intensity of the pre-peak at room temperature. We have used an integral background,
which was kept the same for all the spectra. The energy positions and FWHMs were
determined by finding the best fit common to the three spectra by a χ 2 iterative
program. The final fit for all the spectra from different temperatures were obtained
with the same energy positions and FWHMs.
Temperature (K)
O K edge x-ray absorption spectra
B ′ A ′
Position FWHM Normalized Position FWHM Normalized
95 (eV) (eV) area (eV) (eV) area
2.11 1.52 0.72 1.48 0.97 0.29
150 2.11 1.52 0.72 1.48 0.97 0.26
300 2.12 1.52 0.73 1.47 0.97 0.26
transition metal-oxide compounds. The assignments of the features mentioned above
are consistent with the band structure calculations on similar systems [34, 35].
The pre-edge feature in the O K edge spectra carries a substantial amount of
physics involved in the properties of these materials. It has earlier been shown that
the pre-peak in the O K edge spectra of different CMR materials consists of two lines,
one of which appears as a shoulder on the low-energy side of the other [21, 25, 27, 28].
The intensity of this shoulder was found to increase as the material goes across the
insulator-metal transition [27, 28]. Since our Pr 0.67 Ca 0.33 MnO 3 sample is an insulator
at all temperatures the pre-peak in our O K edge spectra (Fig. 3.4) does not show any
splitting, though the presence of the shoulder is visible as an asymmetry on the lowenergy
side of this peak. This shoulder feature is due to the first available unoccupied
states and in XAS corresponds to the addition of an electron to the e g↑ state of the
crystal-field-split MnO 6 octahedra. The main feature in the pre-peak arises from the
t 2g↓ states [21]. For a quantitative estimate of the temperature-dependent changes in
the intensities of the two features we have fitted the pre-peak using two components of
Lorentzian line shapes. Here also the data fitting was done using χ 2 iterative method,
keeping the energy positions and widths the same for all temperatures. Results of the
data fit are shown in Table 3.1. Fitted spectra for temperatures above (300 K) and
below Tc (95 K) are shown on the right side of Figs 3.5(a) and 3.5(b). Though very
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
gives a 0.6±0.1 eV difference between the energy positions of A ′ and B ′ . Following a
different assignment of origin to the pre-peak features Dessau et al. have interpreted
this energy difference as a measure of the Jahn-Teller distortion (4E JT ) [26].
3.3.3 The Charge Transfer Energy (E CT )
Having shown the temperature-dependent changes in the UPS and XAS results in
Figs. 3.2 and 3.4, we present a combined picture of them in order to derive some insights
into the charge transfer energy involved in the properties of the Pr 0.67 Ca 0.33 MnO 3
sample. The combined spectra of the valence band from photoemission and the preedge
region of the O K edge XAS, presented in Figs. 3.5(a) and 3.5(b) give the density
of both occupied and unoccupied states above and below the Curie temperature (T c ).
A schematic of these energy levels is shown in Fig. 3.6. The hole or electron doping
(value of x) causes the symmetry of the last occupied and first unoccupied bands to
be e gz 2 −r 2 ↑. In Fig. 3.5 these states are marked by A and A ′ . In order to scale the left
and right sides of E F , the O K edge spectra have been shifted such that the energy
of the first unoccupied state (1.4 eV from E F ) found from inverse photoemission [36]
coincides with that of A ′ . We have used integral backgrounds for both sides of E F
and, as mentioned earlier, all the fitting parameters were kept the same including
the energy positions for the high and low temperatures. The important parameter
which can be derived from this combined spectrum is the charge transfer energy E CT ,
which is the energy required for an e g electron to hop between the Mn 3+ (t 3 2ge 1 g) and
Mn 4+ (t 3 2g) sites. In our spectra (Fig. 3.5) E CT is the energy difference between A
and A ′ which corresponds to the last occupied and first unoccupied states. The value
of E CT is 2.6±0.1 eV from our spectra, which is higher than the value (1.5±0.4 eV)
determined indirectly by Park et al. for the La 1−x Ca x MnO 3 system [20]. The larger
value of E CT shows the existence of a stronger charge localization in the charge- or
orbital-ordered Pr 0.67 Ca 0.33 MnO 3 compound.
The value of E CT indicating a strong charge localization, found in our Pr 0.67 Ca 0.33
MnO 3 has significant implications for the models proposed to explain the CMR effect
in manganites. Charge localization can result from strong-coupling interactions
of the e g electron with the core-like t 2g or the Q 2 and Q 3 Jahn-Teller modes. Both
these interactions influence the electron hopping term (t). The former interaction,
ferromagnetic in nature, is the Hund’s coupling J H . A large value of J H favors
the electronic separation of phases. A strong electron-lattice coupling results from

Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 70
E CT
(a)
Intensity (arb. units)
C
B
A’
B’
A
-5 0 5 10
Energy Relative to E F
(eV)
(b)
B
E CT
B’
Intensity (arb. units)
C
A’
A
-5 0 5 10
Energy Relative to E F
(eV)
Figure 3.5: (a) Combined spectra from valence band photoemission (occupied energy
states) and pre-peak in O K edge x-ray absorption spectra (unoccupied energy states)
of Pr 0.67 Ca 0.33 MnO 3 taken at room temperature (above T c ). The three features corresponding
to the subbands in the valence region are marked A, B, and C.The two
peaks of the fitted O K edge pre-peak are marked A ′ and B ′ . We used integral background
for both sides of E F . Details of the fitting are mentioned in the text. (b)
Combined spectra of Pr 0.67 Ca 0.33 MnO 3 below T c . The valence band spectrum was
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
2 2
e g x − y
J ex
2 2
e g z − r
e g x 2 − y 2
t 2g
e z 2 2
g − r
2 2
e g z − r
E F
t 2g
Figure 3.6: Schematic diagram of the near-E F energy levels of Pr 0.67 Ca 0.33 MnO 3 in
the occupied and unoccupied parts. The diagram is not drawn to scale. Some of the
e g↑ states are occupied (at the Mn 3+ sites) and some are unoccupied (at the Mn 4+
sites). Hence the last occupied and first unoccupied states are marked with dotted
lines.
the cooperative Jahn-Teller distortions of the MnO 6 octahedra. These distortions,
particularly the Q 2 and Q 3 vibrational modes of the oxygen ions are stronger in
the case of Pr-containing manganites compared to La-containing ones. Park et al.
[20] have reported a smaller charge transfer energy (E CT = 1.5 eV) in the case of
La 1−x Ca x MnO 3 and have attributed it to small polarons (Anderson localization) induced
from the ionic size difference between Mn 3+ and Mn 4+ . A small-polaronic
model may not be able to explain the high value of E CT found in Pr 0.67 Ca 0.33 MnO 3 .
On the other hand, a strong charge localization is expected in the case of the PS
model in which the ground state is described as a mixture of ferromagnetic and antiferromagnetic
regions. It is also possible that this large charge localization is due to
the Zener polaron proposed by Aladine et al [39]. In the Zener polaron picture, the
Mn ions of adjacent MnO 6 octahedra form a dimer with variations in the Mn-O-Mn
bond angle. The pseudo-CE-type charge ordering in Pr 1−x Ca x MnO 3 favors such regular
distortions in the MnO 6 octahedra. Further studies using electron spectroscopic
techniques on samples in the vicinity of the charge-ordered compositions may be able
to differentiate between these possible driving mechanisms behind the strong charge

Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 72
localization.
3.4 Conclusions
Using valence band photoemission and O K edge x-ray absorption, we have probed
the electronic structure of Pr 0.67 Ca 0.33 MnO 3 , which is regarded as a prototype for the
electronic phase separation models in CMR systems. With decrease in temperature
the O 2p contributions to the t 2g and e g spin-up states in the valence band were
found to increase until T c . Below T c , the density of states with e g spin-up symmetry
increased while those with t 2g symmetry decreased, possibly due to the change in
the orbital degrees of freedom associated with the Mn 3d - O 2p hybridization in the
pseudo-CE-type charge or orbital ordering. These changes in the density of states
could well be connected to the electronic phase separation reported earlier. Our
quantitative estimate of the charge transfer energy (E CT ) is 2.6±0.1 eV, which is
large compared to the earlier reported values in other CMR systems. Such a large
charge transfer energy may support the phase separation model.

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Chapter 4
Electronic Structure of
Pr 1−x Ca x MnO 3
77

Electronic Structure of Pr 1−x Ca x MnO 3 78
4.1 Introduction
Mixed phase manganite systems with coexisting ferromagnetic (FM) and antiferromagnetic
(AFM) domains have been attracting considerable attention recently due to
their importance in understanding the role of spin, charge, and orbital ordering in the
phenomena of colossal magneto resistance (CMR). Theoretical models, particularly
those [1, 2, 3] based on electronic phase separation (PS) exploits this character of the
CMR compounds in explaining the insulator-metal (IM) transitions found in these
systems. The electronic PS scenario is phenomenologically different from the double
exchange (DE) mechanism [4, 5, 6] or those based on strong Jahn-Teller polarons
[7, 8]. In this scenario, the IM transition is driven by current percolation through
FM metallic domains embedded in an AFM insulating background [2]. Though, there
have been many structural studies [9, 10] showing the co-existence of such magnetic
domains, the reported sizes of those vary from nano - to mesoscopic scales [11, 12]. In
manganites, the electronic PS is expected to occur in systems in which the itinerant
e g electrons have strong Hund’s coupling interaction with the localized t 2g electrons
of the MnO 6 octahedra in their structure.
Some of the charge ordered (CO) compositions of the Pr 1−x Ca x MnO 3 system are
considered to be the prototypes for the PS models owing to their inherently reduced
e g bandwidth W. Neutron diffraction studies have shown the co-existence of nanosized
FM and AFM stripes [10, 12] in Pr 0.67 Ca 0.33 MnO 3 . It was also found that a fieldinduced
transition possible between their nearly degenerate FM metallic and chargeordered
AFM insulating phases [12, 13]. With slight variations in the Ca doping
the Pr 1−x Ca x MnO 3 system turns ferromagnetic (x = 0.2) or antiferromagnetic (x =
0.4) at low temperatures [10, 14]. The composition x = 0.33 shows a co-existence of
ferromagnetic and antiferromagnetic phases [15]. The energy scales involved in the
IM transitions in this FM-AFM system are of importance for the understanding of
the physics behind the phenomena of CMR.
In this chapter we have compared the binding energy positions of some of the near
Fermi level (E F ) occupied and unoccupied states, crucial to the electrical and magnetic
transitions of the Pr 1−x Ca x MnO 3 system using the complementary techniques
of direct and inverse photoelectron spectroscopies. Electron spectroscopic techniques
have been quite successful in probing the near E F low energy states involved in the
charge and orbital interactions of CMR materials. Although, there are many reports
on the electronic structure of other CMR compounds using some of these techniques

Electronic Structure of Pr 1−x Ca x MnO 3 79
Figure 4.1: Phase diagram of the Pr 1−x Ca x MnO 3 system. In this study we have used
the x = 0.2, x = 0.33 and x = 0.4 compositions. (FMI = ferromagnetic insulating,
CO-AFMI = charge ordered antiferromagnetic insulating, CG = cluster glass, T N =
Neel Temperature, T C = Critical Temperature, T CO = charge ordering temperature).
[16, 17, 18, 19, 20, 21, 22] only a few studies have dealt with the energy scales of the
electron interactions in charge ordered compositions [23, 24]. One of the key terms,
these spectroscopies could show earlier, was the charge transfer energy E CT which is
intimately related to the electron-electron and electron-lattice interactions [23, 24].
Here we have analyzed the temperature dependent changes shown by three compositions
across the charge ordered FM-AFM phase boundary of the Pr 1−x Ca x MnO 3
system using ultraviolet photoelectron spectroscopy (UPS) and inverse photoelectron
spectroscopy (IPES). Also we have studied different core levels of Pr 1−x Ca x MnO 3
using x-ray photoelectron spectroscopy (XPS).
4.2 Experimental
Single crystals of Pr 1−x Ca x MnO 3 with x = 0.2, 0.33 and 0.4 were grown by the floating
zone method in a mirror furnace. The compositional homogeneity of the crystals
had been confirmed using energy dispersive spectroscopic (EDS) analysis. The
crystals were characterized by using magnetization, electrical transport and neutron
diffraction measurements. Details of the sample preparation and characterization

Electronic Structure of Pr 1−x Ca x MnO 3 80
measurements are published elsewhere [10, 25]. There are also three polycrystalline
samples (PrMnO 3 , CaMnO 3 and Pr 0.16 Ca 0.14 MnO 3 ) used for different studies. Which
were prepared by the standard solid state reaction method. The details of the sample
preparation is published elsewhere [26]. Consolidated results of these studies are
presented in the phase diagram shown in Fig. 4.1 [27]. Angle integrated ultraviolet
photoemission measurements were performed by using an Omicron mu-metal ultra
high vacuum system equipped with a high intensity vacuum ultraviolet source (HIS
13) and a hemispherical electron energy analyzer (EA 125 HR). At the He I (21.2
eV) line, the photon flux was of the order of 10 16 photons/sec/steradian with a beam
spot of 2.5 mm diameter. The Fermi energies (E F ) for all measurements were calibrated
by using a freshly evaporated Ag film on a sample holder. The total energy
resolution, estimated from the width of the Fermi edge, was about 80 meV. The XPS
measurements were performed using a twin anode Mg/Al x-ray source (DAR 400) in
the same chamber as of UPS. In our measurements we have used the Mg Kα line with
photon energy 1253.6 eV and the resolution was about 1.0 eV. The samples were repeatedly
scraped by using a diamond file inside the chamber under a base vacuum of
∼ 1.0 x 10 −10 mbar. The negligible intensity found for the ∼ 9.5 eV bump [16] in the
UPS spectra ensures the cleanliness of our sample surface. For the temperature dependent
measurements, the samples were cooled by pumping liquid nitrogen through
the sample manipulator fitted with a cryostat. The sample temperatures were measured
by using a silicon diode sensor touching the bottom of the sample holder. The
IPES measurements were performed by using an electrostatically focused electron gun
of Stoffel Johnson design and an acetone gas filled band pass Geiger Muller counter
with a CaF 2 window [28, 29]. These measurements were carried out in the isochromat
mode in which the photon energy is fixed at 9.9 eV and kinetic energy of the incident
electrons is varied in 0.05 eV steps. Overall resolution of the IPES measurements
were estimated [29] to be approximately 0.55 eV.
4.3 Results and Discussion
4.3.1 The UPS study of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33 and 0.4
Neutron diffraction studies [27] on Pr 1−x Ca x MnO 3 have shown that the compositions
with x = 0.2 and x = 0.4 are FM and AFM respectively (Fig. 4.1) while the x = 0.33
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
identified to be an inhomogeneous state below T CO . The x = 0.4 is above the critical
doping (x c ) found in the Pr 1−x Ca x MnO 3 system and is homogeneous in nature.
(a) 300 K
C
B
(b) 77 K
C
B
Intensity (arb. units)
A
1.0
0.4
0.33
Intenisty (arb. units)
A
1.0
0.4
0.33
0.2
0.2
0.0
0.0
10
8
6
4
2
0
10
8
6
4
2
0
Binding Energy (eV)
Binding Energy (eV)
Figure 4.2: The angle integrated valence band photoemission spectra of the
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
room temperature and 77K using He I photons (21.2 eV).
Figure 4.2 shows the angle integrated valence band photoemission spectra of the
Pr 1−x Ca x MnO 3 (x = 0.2, x = 0.33 and x = 0.4) samples taken at room temperature
and 77K. The figure also shows the spectra of the end compositions of this system (x
= 0.0 and x = 1.0). It may be noted from the phase diagram that the compositions
with x = 0.2, x = 0.33 and x = 0.4 are PM insulators at 300K and FM/AFM insulators
at 77K. Intensities of all the spectra shown in both the panels were normalized
and shifted along the ordinate axis by a constant value for clarity. All the features
seen in the spectra are dominated by the states due to the Mn 3d - O 2p hybridized
orbitals. The two prominent features, one at ∼ 3.5 eV (marked B) and another at ∼
5.6 eV (marked C), are primarily due to the O 2p nonbonding states and the Mn 3d
- 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
∼ 1 eV (marked A) is due to the e g states of the MnO 6 octahedra [18, 24, 31, 32].
The intensity of this feature is low due to the insulating nature of all our samples.
Presence of the feature A is clear from Fig. 4.3(a) and 4.3(b) where we have shown
the photoemission spectra from the near E F region of the x = 0.2, x = 0.33 and x = 0.4
samples along with that of CaMnO 3 , taken at temperatures above and below the T CO .
The lower panels of the figures show the difference spectra obtained by subtracting the
spectra of CaMnO 3 from those of the different Pr 1−x Ca x MnO 3 compositions. Since,
CaMnO 3 does not have any electrons in its e g levels, the difference spectra should
correspond to the density of the e g electrons in the Pr 1−x Ca x MnO 3 samples. The
shape of the difference spectra shows that it consists of two features, similar to those
reported by Ebata et al. [33]. We have fitted all the difference spectra corresponding
to the three compositions at two different temperatures with a mixture of Lorentzian
and Gaussian line shapes using a χ 2 iterative programme. We used a fixed width
for all the fitted lines marked A’ for both the temperatures in order to see the finer
changes associated with the A”. The energy positions of the A’ and A” and the
width of the A” were determined by the iterative programme. Table 4.2 shows the
results of the fitting. At room temperature, the changes across the three compositions
are negligible except that the sample with x = 0.33 has a larger width for its near
E F feature marked A”. At 77K, both A’ and A” have been found to be shifted
slightly towards the E F . Further, at this temperature also, the x = 0.33 shows a
wider A”. The width of the near E F feature is coupled to the conductivity of these
samples. Earlier experiments using valence band photoemission have shown that
a finite number of states get transferred from A” to A’ when some CMR systems
go across the temperature - composition phase diagram [31, 33, 34] resulting in the
formation of pseudogaps at the E F . The energy scales involved in such shifts of DOS
are of importance to the theoretical models, particularly for those based on electronic
phase separation. Formation of pseudogaps are a generic feature of mixed phase
CMR systems [31, 35] with FM electron rich phases embedded in an insulating AFM
matrix. The feature A’ in our spectra could possibly be ascribed to the e g electrons
in the AFM phases while the A” to those in the FM phases. The difference in the
binding energy positions of A’ and A” (0.47±0.01 eV) gives an approximate measure
of the energy difference between the e g electronic states in these two phases. In other
words, this gives the energy scale over which the pseudogap forms. The increased
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
(a)
300 K 300 K 300 K
Intensity (arb. units)
A’
A’
A’
A’’ A’’ A’’
0.2 - CaMnO3 0.33 - CaMnO3 0.4 - CaMnO3
1.5 1 0.5 0 1.5 1 0.5 0
1.5
1
0.5
0
Binding Energy (eV)
(b)
77 K 77 K
77 K
Intensity (arb. units)
A’
A’
A’
A’’
A’’
A’’
0.2 - CaMnO3 0.33 - CaMNO3 0.4 - CaMnO3
1.5
1
0.5
0
1.5
1
0.5
0
1.5
1
0.5
0
Binding Energy (eV)
Figure 4.3: The near E F region of the valence band spectra (panel(a): 300 K,
panel(b): 77 K) of the three samples plotted against that of CaMnO 3 . The difference
spectra (lower panel) is obtained by subtracting the spectra of CaMnO 3 from
those of the three Pr 1−x Ca x MnO 3 compositions and multiplied by 5. The difference
spectra corresponds to the density of e g states. The red and blue peaks are A’ and
A” respectively. The green line shows the spectral background used in the fitting.

Electronic Structure of Pr 1−x Ca x MnO 3 84
Table 4.2: Results of the fitting of the difference spectra using a χ 2 iterative programme.
We have used a mixture of Lorentzian and Gaussian line shapes for fitting
the various difference spectra corresponding to the three compositions at 300K and
77K. In order to see the finer changes in the near E F feature A”, we used a fixed
width for all the fitted lines marked A’. The energy positions of the A’ and A” and the
width of the A” were determined by the iterative programme. At both the temperatures
the sample with x = 0.33 shows a larger full width at half maximum (FWHM)
for its A”. At 77K, both A’ and A” have been found to be shifted slightly towards
the E F .
Temperature (K)
300
77
x
A’ A”
Position (eV) FWHM (eV) Position (eV) FWHM (eV)
0.2 0.97 0.5 0.49 0.5
0.33 0.96 0.5 0.49 0.65
0.4 0.96 0.5 0.49 0.59
0.2 0.92 0.5 0.45 0.49
0.33 0.93 0.5 0.45 0.53
0.4 0.92 0.5 0.46 0.5
indicate the formation of FM clusters in the AFM insulating background. From the
fitting of the difference spectra we feel that the increase of the width of A” is largely
due to the formation of a tail towards the E F , which makes it difficult to estimate
the energy difference between A’ and A” accurately.
4.3.2 The IPES study of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33, 0.4
and 1
Fig. 4.4 shows the IPES spectra of the Pr 1−x Ca x MnO 3 samples (x = 0.2, 0.33 and
0.4) and the CaMnO 3 sample taken at room temperature. The spectra have been
normalized for their intensities and shifted along the vertical axis by a constant for
clarity. The spectra shows two broad features; one between 5 to 12 eV (marked Q)
and another at around 2.4 eV (marked P). For the x = 0.2, 0.33, and 0.4, the feature
appearing at higher energy is mostly due to Pr 5d and Ca 3d orbitals, whereas for
the CaMnO 3 it is due only to the Ca 3d, 4s orbitals. The feature P is due to the
hybridized Mn 3d - O 2p states. These assignments of the features are consistent
with band structure calculations and other studies on similar systems [24, 36, 37].
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
Q
x=1
P
x=0.4
Intensity (arb. units)
x=0.33
x=0.2
0 2.5 5 7.5 10 12.5 15
Energy Relative to Fermi Energy (eV)
Figure 4.4: The IPES spectra of the Pr 1−x Ca x MnO 3 samples (x = 0.2, 0.33 and 0.4)
and the CaMnO 3 sample taken at room temperature. The spectra represents the
unoccupied electron states of the samples.
[38], though another study [39] showed the feature P to be more closer to the E F .
Again, in the IPES spectra also, the feature appearing near to E F is important to
properties of these compounds. In Fig. 4.5 we have shown the spectra of the near E F
region containing this feature along with the difference spectra (Fig. 4.5(b)). One can
see clearly that the intensity of P increases with the Ca concentration x due to the
increase of unoccupied states in the Mn 3d orbitals with hole doping. From a linear
extrapolation of the rising band edge in the spectra shown in the Fig. 4.5(a) we have
estimated the band gap to be ∼ 0.7 eV for the three compositions (0.2, 0.33, and 0.4).
At this temperature, the value of the gap does not seem to vary with the charge carrier
concentration, whereas the parent compound (Ca x MnO 3 ) shows a smaller value (∼
0.67 eV). In case of La 1−x Sr x MnO 3 , Chainani et al. [38] has shown the value of the
band gap to be ∼ 0.6 eV. The larger values shown by our Pr 1−x Ca x MnO 3 samples
could be attributed to their insulating nature. In an alternate approach, one can also
estimate the band gap from the difference spectra shown in the Fig. 4.5(b). This
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
(a)
(b)
Intensity (arb. units)
x=1
x=0.4
1 - 0.2
0.4 - 0.2
x=0.33
x=0.2
0.33 - 0.2
-1 0 1 2 3 4
0 1 2 3 4
Energy Relative to Fermi Energy (eV)
Figure 4.5: Panel(a) shows the IPES spectra of the near E F unoccupied states.
Panel(b) shows the difference spectra obtained by subtracting the spectra corresponding
to the x = 0.2 from those of other compositions.
the Ca x MnO 3 .
Band structure calculations on similar CMR systems [12, 32] have unambiguously
shown that both the highest occupied and lowest unoccupied states in these systems
are of Mn 3d e g spin-up symmetry. As mentioned earlier, within this frame work the
features marked A’ and A” in Fig. 4.3 and the feature P in Fig. 4.4 correspond to the
e g spin-up states. Combining the results presented in Table I and these two figures
one can estimate the energy required by an e g electron to hop from a Mn 3+ site to the
neighboring Mn 4+ site. If we consider the centroids of the A’ in Fig. 4.3 and the P in
Fig. 4.4 we arrive at a value of ∼ 3.3±0.2 eV, which is large compared to the earlier
reported value [24]. With the A” of Fig. 4.3 and P of Fig. 4.4 we get ∼ 2.8±0.2
eV which is close to the earlier report (2.6±0.1 eV). Due to the large widths and
long tails of the A” and P towards the E F we expect that this value of the energy be
close to its upper bound. Energy required for the transfer of e g electrons between the
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
these systems.
4.3.3 The core levels of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33, 0.4,
and 0.84
The Mn2p core level spectra of Pr 1−x Ca x MnO 3 (x = 0.2, 0.33, 0.4, and 0.84) taken
at room temperature as well as at 77 K are shown in Fig. 4.6. All the spectra were
normalized and shifted along the Y axis by a constant for clarity. The two prominent
peaks at around ∼ 641.5 and ∼ 653.5 eV are due to the spin orbit split 2p 3/2 and 2p 1/2 .
These spectra look similar to those reported earlier on the similar system [17, 33]. In
Fig. 4.6, the Mn 2p peak positions shift towards the higher binding energy for x =
0.4 and 0.84 at room temperature (a) as well as at 77 K (b). But the binding energy
shift is higher incase of room temperature spectra as compared to that of 77 K. The
Mn LMV Auger peak is observed on the higher binding energy side of 2p 1/2 peak
for all four concentrations at room temperature, whereas at 77 K this Auger peak is
absent for x = 0.2, and 0.3. This Mn LMV was earlier observed in other hole doped
manganites [17].
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
temperature as well as 77 K are shown in the Fig. 4.7. The Ca 2p 3/2 and Ca 2p 1/2 ,
due to spin orbit splitting, are clearly distinguishable at around ∼ 345.5 and ∼ 349
eV respectively. It is clear from the Fig. 4.7 (a) that, at room temperature the peak
positions shift towards the lower binding energy when we move from x = 0.2 to x =
0.84. Also the same kind of shift is observed for 77 K spectra (Fig. 4.7 (b)) with a
lesser magnitude.
The Pr 4d spectra of Pr 1−x Ca x MnO 3 (x = 0.2, 0.33 and 0.4) taken at room
temperature as well as 77 K are shown in Fig. 4.8. The Pr 4d peaks come around
116 eV binding energy. The peak positions shift towards the lower binding energy
at room temperature (Fig. 4.8 (a)) when we move from x = 0.2 to 0.4. Here also
the similar kind of shift is observed for 77 K spectra (Fig. 4.8 (b)) with a lesser
magnitude.
As mentioned above the binding energy shifts with respect to the value of x have
been observed at the Ca 2p, Pr 4d and Mn 2p core levels. Incase of Mn 2p level, the
binding energy shift is to the higher energy side while the other two levels shift to lower
energy. The magnitude of the binding energy shifts are found to be higher at room
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
Mn LMV
Temp = 300 K
2p 3/2
Intensity (arb. units)
0.84
0.4
0.33
Mn 2p
0.2
670
660
2p 1/2
640
650
640
Binding energy (eV)
(a)
Mn LMV
Temp = 77 K
2p 2p 1/2 3/2
Intensity (arb. units)
0.84
0.4
0.33
Mn 2p
0.2
670
660
650
Binding Energy (eV)
(b)
Figure 4.6: The Mn 2p core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken at
room temperature (a) and 77 K (b) .

Electronic Structure of Pr 1−x Ca x MnO 3 89
Ca 2p
Temp = 300 K
2p 3/2
2p
1/2
Intensity (arb. units)
0.84
0.4
0.33
0.2
352
348 344
Binding Energy (eV)
(a)
340
Ca 2p
2p 1/2
2p 3/2
Temp = 77 K
Intensity (arb. units)
0.84
0.4
0.33
0.2
352
348 344
Binding Energy (eV)
(b)
340
Figure 4.7: The Ca 2p core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken at
room temperature (a) and 77 K (b).

Electronic Structure of Pr 1−x Ca x MnO 3 90
Pr 4d
Temp = 300 K
Intensity (arb. unis)
0.4
0.3
0.2
130
120 110
Binding Energy (eV)
(a)
100
Pr 4d
Temp = 77 K
Intensity (arb. units)
0.4
0.33
0.2
130
120 110
Binding Energy (eV)
(b)
100
Figure 4.8: The Pr 4d core-level XPS (Mg Kα) spectra of Pr 1−x Ca x MnO 3 taken at
room temperature (a) and 77 K (b).

Electronic Structure of Pr 1−x Ca x MnO 3 91
2p could be due to the increase in the effective Mn valence state from Mn 3+ towards
Mn 4+ . The binding energy shift incase of Ca 2p and Pr 4d core levels could possibly
due to the shift in the chemical potential [33, 40]. The low temperature data shows
a smaller value for this chemical potential shift. Such a suppression of the chemical
potential shift have been observed earlier in other transition metal oxides [41, 42] is
mostly due to the formation of stripes. According to the phase separation model, a
pinning of the chemical potential is consequence of the mixed phase nature of these
materials. The numerical studies [1, 3] have indeed shown the pinning of chemical
potential in the composition range, where the phase separation is realized. These
results also support the model of phase separation in the Pr 1−x Ca x MnO 3 system.
4.4 Conclusions
Using UV photoelectron spectroscopy and inverse photoelectron spectroscopy we have
deduced some of the important energy scales in the Pr 1−x Ca x MnO 3 system. Our
photoemission studies across the FM-AFM phase boundary show that the pseudogap
formation in these compounds occur over an energy scale of 0.47±0.01 eV. From a
combined analysis of the photoemission and inverse photoemission spectra we also
have found the energy required for the transfer of Mn 3d e g electrons between the adjacent
Mn sites to be of the order of ∼ 2.8±0.2 eV. Though an accurate measurement
of these energies are not possible due to the inherent limitations of the compounds
and the technique, our results give an upper bound of them. Further, we have discussed
our results from a possible phase separation scenario. From the core level
study of Pr 1−x Ca x MnO 3 , we observed a suppression of chemical potential shift for 77
K spectra compared to that of room temperature. These results support the model
of phase separation in Pr 1−x Ca x MnO 3 system.

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Electronic Structure of Pr 1−x Ca x MnO 3 95
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Chapter 5
Electronic Structure of
Ca 0.86 Pr 0.14 MnO 3
96

Electronic Structure of Ca 0.86 Pr 0.14 MnO 3 97
5.1 Introduction
The strong interplay among charge, spin, lattice and orbital degrees of freedom of electrons
form the basis of the phenomena of colossal magnetoresistance (CMR) shown
by manganites. To a large extent the traditional models [1, 2, 3] employing the
charge-spin coupling, have been able to explain the CMR in many of the hole doped
compositions (R 1−x A x MnO 3 with x

Electronic Structure of Ca 0.86 Pr 0.14 MnO 3 98
5.2 Experimental
The polycrystalline sample of Ca 0.86 Pr 0.14 MnO 3 was prepared by standard solid state
reactions. Details of the sample preparation technique is published elsewhere [4]. Purity,
homogeneity and composition of the sample were checked by electron diffraction
and energy dispersive spectroscopic analysis. The electrical and magnetic properties
of this composition has also been published earlier [4, 15]. This sample shows
a metallic behavior down to ∼ 200K and then the resistivity slightly increases till
the T c (110 K). Below the T C the behavior is insulating and the resistivity goes up
by many orders of magnitude. Generally, the conductivity behavior of this sample is
similar to that of the other electron doped systems [4, 8, 15].
High resolution photoemission and x - ray absorption spectroscopy measurements
were performed by using the light from the soft x - ray beam line WERA of ANKA
synchrotron, Karlsruhe, Germany. We used the monochromatized radiation from
an undulator. The HRPES measurments were carried out using the photon energy
124 eV. The photoelectrons were collected using a Scienta SES 2002 electron energy
analyzer. The energy resolution was 25 meV in case of the narrow scan (-2 - 2.5 eV)
and it was 125 meV incase of wide scan (-2 - 15 eV) of the valence band. The O K XAS
spectra has been taken in the fluorescence detection mode and the energy resolution
was set to 150 meV. The sample surface was cut using a diamond knife under a base
vacuum of ∼ 1.0 x 10 −10 mbar before taking the data for each temperature. In case of
the photoemission measurements, scraping was repeated until negligible intensity was
found for the bump around 9 - 10 eV, which is a signature of surface contamination
[16]. For the temperature dependent measurements, the samples were cooled by
pumping liquid helium through the sample manipulator fitted with a cryostat. The
sample temperatures were measured by using a silicon diode sensor touching the
bottom of the sample holder.
5.3 Results and Discussion
Figure 5.1 shows the high resolution photoemission spectra of the Ca 0.86 Pr 0.14 MnO 3 .
Varius spectra, taken at 30 K, 70 K, 110 K, 220 K and 293 K show the temperature
dependent changes in the valence band electronic structure. Intensities of all the
spectra were normalized and shifted along the ordinate axis by a constant value for
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
B
D
C
Intensity (arb. units)
300 K
220 K
110 K
A
70 K
30 K
12
10
8
6
4
2
0
Binding Energy (eV)
Figure 5.1: The angle integrated valence band photoemission spectrum of
Ca 0.86 Pr 0.14 MnO 3 taken using 124 eV photons. The spectra taken at different temperatures
(30 K, 70 K, 110 K, 220 K and 293 K) are normalized for their intensities
and shifted along Y-axis by a constant for clarity of presentation.

Electronic Structure of Ca 0.86 Pr 0.14 MnO 3 100
Intensity (arb. units)
A
30 K
70 K
110 K
220 K
300 K
1.2
1
0.8
0.6
0.4
0.2
0
-0.2
Binding Energy (eV)
Figure 5.2: The high resolution spectra of the near E F region of the valence band of
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
e g band.
states due to the Mn 3d - O 2p hybridization. The broad feature appearing at ∼ 6 eV
(marked D) is due to the bonding states of this hybridization while its non-bonding
states appear as a shoulder at ∼ 3.2 eV (marked C). The peak at ∼ 2 eV (marked B)
is due to the Mn 3d t 2g states of the MnO 6 octahedra [17]. The Mn 3d e g↑ states of
the octahedra appear around ∼ 0.3 eV (marked A) in the spectra. It should be noted
that the intensity of the peak B decreases with decrease of temperature while that of
the feature A increases. These spectral weight shifts are characteristic of many CMR
systems. The e g states lying close to the E F are important for the conductivity of this
material. A high resolution spectra of the near E F region is presented in figure 5.2.
As the temperature is lowered from 293 K the feature A starts appearing. Further,
below the transition temperature T c (110 K), the intensity of A steeply increases and
goes through a maximum. The 30 K spectrum again shows a smaller intensity for
this feature compared to the one at 70 k. Interestingly, below the T c , the leading
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
Mn 3d + O 2p
Pr 5d, Ca 3d
Mn 4sp, Pr 6sp
Normalized Absorption (arb. units)
300 K
200 K
150 K
80 K
30 K
530 540 550 560
Photon Energy (eV)
Figure 5.3: The O K-edge x-ray absorption spectra of the Ca 0.86 Pr 0.14 MnO 3 taken at
30 K, 80 K, 150 K, 200 K and 300 K. The pre-edge feature appears between 527 -
533 eV is mostly due to the strong hybridization between Mn 3d and O 2p orbitals,
where most interest lies. Origin of other features are given in the text.
This shifting of the valence band edges is a clear indication of increase in the width
(W) of the e g band below the T c .
In order to see the corresponding changes in the near E F unoccupied states we have
taken the O K edge x-ray absorption spectra (XAS) of the Ca 0.86 Pr 0.14 MnO 3 sample
at different temperatures (Fig. 5.3). Here, all the spectra have been normalized to
the incoming intensity and absolute cross sections and are also corrected for selfabsorption
and saturation effects. The spectra shows three broad features. The one
between 527 eV to 533 eV, called the pre-edge feature in the O K edge spectra, is
mostly due to the strong hybridization between Mn 3d and O 2p orbitals. The feature
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
Normalized Absorption (arb. units)
A’
B’
C’
30 K
80 K
150 K
200 K
300 K
527 528 529 530 531 532 533 534
Photon Energy (eV)
Figure 5.4: The pre-edge peak of the O K -edge XAS spectra of Ca 0.86 Pr 0.14 MnO 3 for
temperatures 30 K, 80 K, 150 K, 200 K and 300 K.
the structure above 540 eV originates from the states like Mn 4sp and Pr 6sp, etc.
These assignments of the features are consistent with the earlier experiments and
band structure calculations on similar systems [21, 22, 23]. The pre-edge region of
the O K XAS has earlier been shown to represent the near E F unoccupied states
in these kind of compounds [21, 22, 23]. Figure 5.4 shows the pre-edge region of
the XAS spectra of taken at different temperatures. The feature extending from
∼ 527 to 531 eV consists of two lines. The first one is due to the O 2p orbitals
hybridized with the Mn 3d e g↑ (marked A’) while the second arises from the O 2p -
Mn 3d t 2g↓ hybridization (marked B’). The weak feature centered around ∼ 531.5 eV
corresponds to the e g↓ (marked C’) states [23, 24, 25, 26, 27]. All the features show
some temperature dependence. As the temperature is lowered from 300 K to 30 K
the intensity of A’ decreases while that of B’ increases. This again shows a spectral
weight shift complementary to the one observed in the photoemission spectra (Fig.

Electronic Structure of Ca 0.86 Pr 0.14 MnO 3 103
5.1). Here, the spectral weight get shifted from near E F positions to higher energy
as we go down in temperature. One can also see a shift in the energy position of the
feature originating from the e g↓ states.
Structural studies [19] have shown that the Ca 0.86 Pr 0.14 MnO 3 has an orthorhombic
perovskite structure at 293 K. Below 110 K its structure changes to elongated pseudotetragonal
with four short a-b plane and two long out-of-plane Mn-O-Mn bonds.
This structural change can lead to an increase in the in-plane Mn 3d - O 2p hybridization
strength and a consequent increase in the W. The broadening of feature A below
T c in the photoemission spectra (Fig. 2) is due to this increase in W. The parent
compound of our sample, CaMnO 3 has an empty e g band. In the Ca 0.86 Pr 0.14 MnO 3 ,
the e g band could well be considered as rarely occupied. The spin-spin interaction of
the e g electrons in this narrow band can lead to a localization of them resulting in
the formation of FM clusters. Earlier Martin et. al [20] have shown the existence of
such FM clusters embedded in an AFM matrix in electron-doped compositions. The
near E F electronic spectra of such phase separated systems are known to display a
pseudogap behavior [18]. The spectral weight shifts observed in the photoemission
and O K XAS spectra of Ca 0.86 Pr 0.14 MnO 3 corroborate this behavior. The enhanced
shift of electronic states from B to A observed in the photoemission spectra taken below
the T c could be due to the structural changes across the transition. The changes
in the topology of the MnO 6 octahedra below the T c mentioned earlier, might result
in the shift of electrons from the t 2g levels to the near E F positions because of
the increased strength in the in-plane Mn 3d -O 2p hybridization. The unoccupied
states represented in pre-edge region of the O K XAS spectra, on the other hand,
shows complementary shifts of states from the near E F to higher energy positions
with decrease of temperature.
5.4 Conclusions
Using high resolution photoelectron spectroscopy and O K edge XAS, we have studied
the temperature dependent changes in the near E F electronic structure of the electron
doped CMR, Ca 0.86 Pr 0.14 MnO 3 . High resolution photoemission study show changes
in the e g electron band width and a spectral weight shift across the transition which
could be ascribed to the structural changes and consequent changes in the Mn 3d - O
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
the near E F to higher energy positions were observed with decrease of temperature
in the unoccupied states presented in the pre-edge region of O K XAS spectra.

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Chapter 6
Summary and Conclusions
108

Summary and Conclusions 109
In this thesis the electronic structure of Pr 1−x Ca x MnO 3 series have been investigated
using photoemission, inverse photoemission and x-ray absorption spectroscopy.
Using photoemission (UPS and XPS), we have studied the valence band and core levels
of Pr 1−x Ca x MnO 3 . The unoccupied states of Pr 1−x Ca x MnO 3 have been studied
by inverse photoemission spectroscopy and x-ray absorption spectroscopy.
A brief overview of the structural, transport and magnetic properties associated
with the CMR manganites have been described in the introduction of this thesis.
Details of the experimental techniques used to study the electronic structure of
Pr 1−x Ca x MnO 3 have been described in the chapter 2.
Using valence band photoemission and O K edge x-ray absorption, we studied
the temperature dependent finer changes in the near E F electronic structure of
Pr 0.67 Ca 0.33 MnO 3 , which is regarded as a prototype for the electronic phase separation
models in CMR systems. With decrease in temperature the O 2p contributions
to the t 2g and e g spin-up states in the valence band were found to increase until T c .
Below T c , the density of states with e g spin-up symmetry increased while those with
t 2g symmetry decreased, possibly due to the change in the orbital degrees of freedom
associated with the Mn 3dO 2p hybridization in the pseudo-CE-type charge or orbital
ordering. These changes in the density of states could well be connected to the electronic
phase separation reported earlier. By combining the UPS and XAS spectra,
we derived a quantitative estimate of the charge transfer energy E CT (2.6±0.1 eV),
which is large compared to the earlier reported values in other CMR systems. Such
a large charge transfer energy was found to support the phase separation model.
For a complete understanding of the changes in the occupied and unoccupied
electronic states we performed a detailed study of the Pr 1−x Ca x MnO 3 system across
their ferromagnetic - antiferromagnetic phase boundary using UPS and IPES. We
used three compositions x = 0.2, 0.33 and 0.4. Our photoemission studies showed
that the pseudogap formation in these compositions occur over an energy scale of
0.48±0.02 eV. Here again, we have estimated the charge transfer energy in these
compositions to be of the order of ∼ 2.8±0.2 eV. The core level electronic structure
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
photoelectron spectroscopy. These studies have been performed at the Mn 2p, Ca 2p
and Pr 4d levels. From these core level studies we observed a suppression of chemical
potential shift for 77 K spectra compared to that of room temperature. These results
support the model of phase separation in Pr 1−x Ca x MnO 3 system.

Summary and Conclusions 110
Using high resolution photoelectron spectroscopy and O K edge x-ray absorption
spectroscopy we have studied the temperature dependence of the near Fermi level
electronic structure in the electron doped CMR, Ca 0.86 Pr 0.14 MnO 3 . We found that
the temperature dependent changes in the electronic structure is consistent with the
conductivity behavior of the electron doped systems in general. As the temperature
is lowered from 293 K a feature due to the e g↑ states starts appearing near the E F .
The intensity of this feature further increases below the T c with a shift of spectral
weight from higher binding energy to the near E F states. Further, we have found an
increase in the e g↑ band width (W) also at low temperatures. On the other hand,
a complementary shifts of states from the near E F to higher energy positions were
observed with decrease of temperature in the unoccupied states presented in the preedge
region of O K edge x-ray absorption spectra. We have discussed our results from
the point of view of phase separation models considering the temperature dependent
structural changes and consequent localization of the electrons in a narrow e g band.