PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

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[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. <strong>Dala</strong>i, 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 ix
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: List of Publications/Preprints [1]
Page 11 and 12: to the t 2g and e g spin-up states
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
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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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