PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

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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
Page 1 and 2: SPECTROSCOPIC INVESTIGATIONS OF THE
Page 3 and 4: iii To My Parents
Page 5 and 6: Maharana, P. Khare, D. K. Basa, K.
Page 7 and 8: List of Publications/Preprints [1]
Page 9: Synopsis The perovskite type mangan
Page 13 and 14: Bibliography [1] R. von Helmolt, J.
Page 15 and 16: Contents CERTIFICATE Acknowledgemen
Page 17 and 18: List of Figures 1.1 Temperature dep
Page 19 and 20: 2.2 Schematic view of the energetic
Page 21 and 22: 3.4 O K edge x-ray absorption spect
Page 23 and 24: Chapter 1 Introduction 1
Page 25 and 26: Introduction 3 The temperature and
Page 27 and 28: Introduction 5 Figure 1.2: Schemati
Page 29 and 30: Introduction 7 Figure 1.4: A schema
Page 31 and 32: Introduction 9 Figure 1.6: Schemati
Page 33 and 34: Introduction 11 Figure 1.8: Differe
Page 35 and 36: Introduction 13 Figure 1.10: A sche
Page 37 and 38: Introduction 15 Figure 1.12: Schema
Page 39 and 40: Introduction 17 Figure 1.13: Phase
Page 41 and 42: Bibliography [1] R. von Helmolt, J.
Page 43 and 44: Introduction 21 [31] Damien Saurel,
Page 45 and 46: Chapter 2 Experimental Techniques 2
Page 47 and 48: Experimental Techniques 25 Figure 2
Page 49 and 50: Experimental Techniques 27 Let us n
Page 51 and 52: Experimental Techniques 29 and G(k,
Page 53 and 54: Experimental Techniques 31 Figure 2
Page 55 and 56: Experimental Techniques 33 point wi
Page 57 and 58: Experimental Techniques 35 negative
Page 59 and 60: Experimental Techniques 37 ∆E = E
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Experimental Techniques 39 Figure 2
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Experimental Techniques 41 ( ) dσ
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Experimental Techniques 43 The expe
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Experimental Techniques 45 are weld
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Bibliography [1] H. Hertz, Ann. Phy
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Experimental Techniques 57 [35] K.
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Chapter 3 Electronic Structure of P
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Electronic Structure of Pr 0.67 Ca
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Bibliography [1] I. G. Deac, J. F.
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Chapter 4 Electronic Structure of P
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Electronic Structure of Pr 1−x Ca
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Electronic Structure of Ca 0.86 Pr
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Bibliography [1] C. Zener, Phys. Re
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Summary and Conclusions 109 In this
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