PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

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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
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SPECTROSCOPIC INVESTIGATIONS OF THE
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iii To My Parents
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Maharana, P. Khare, D. K. Basa, K.
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List of Publications/Preprints [1]
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Synopsis The perovskite type mangan
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to the t 2g and e g spin-up states
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Bibliography [1] R. von Helmolt, J.
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Contents CERTIFICATE Acknowledgemen
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List of Figures 1.1 Temperature dep
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2.2 Schematic view of the energetic
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3.4 O K edge x-ray absorption spect
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Chapter 1 Introduction 1
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Introduction 3 The temperature and
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Introduction 5 Figure 1.2: Schemati
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Introduction 7 Figure 1.4: A schema
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Introduction 9 Figure 1.6: Schemati
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Introduction 11 Figure 1.8: Differe
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Introduction 13 Figure 1.10: A sche
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Introduction 15 Figure 1.12: Schema
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Introduction 17 Figure 1.13: Phase
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Bibliography [1] R. von Helmolt, J.
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Introduction 21 [31] Damien Saurel,
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Chapter 2 Experimental Techniques 2
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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
Page 63 and 64: Experimental Techniques 41 ( ) dσ
Page 65 and 66: Experimental Techniques 43 The expe
Page 67 and 68: Experimental Techniques 45 are weld
Page 77 and 78: Bibliography [1] H. Hertz, Ann. Phy
Page 79 and 80: Experimental Techniques 57 [35] K.
Page 81 and 82: Chapter 3 Electronic Structure of P
Page 83 and 84: Electronic Structure of Pr 0.67 Ca
Page 95 and 96: Bibliography [1] I. G. Deac, J. F.
Page 99: Chapter 4 Electronic Structure of P
Page 103 and 104: Electronic Structure of Pr 1−x Ca
Page 119 and 120: Electronic Structure of Ca 0.86 Pr
Page 127 and 128: Bibliography [1] C. Zener, Phys. Re
Page 131 and 132: Summary and Conclusions 109 In this
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PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

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