PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

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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.
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 and 10: Synopsis The perovskite type mangan
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: Introduction 15 Figure 1.12: Schema
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 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
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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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