PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

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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
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: Introduction 11 Figure 1.8: Differe
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 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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