PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

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Introduction 10 Figure 1.7: Schematic representation of a macroscopic phase separated state (a), as well as possible charge inhomogeneous states stabilized by the long-range Coulomb interaction (spherical droplets in (b), stripes in (c)) [20]. than the critical value below T C , thus inducing metallic behavior. The calculations were carried out using classical phonons and t 2g spins. The results of Millis et. al. [14] for T C and the resistivity at a fixed density n = 1 when plotted as a function of λ eff had formal similarities with experimental results (which are produced as a function of density). In particular, if λ eff is tuned to be very close to the metal-insulator transition, the resistivity naturally strongly depends on even small external magnetic fields. 1.7 Phase separation Another possibly complementary route to understand the CMR effect in manganites is the phase separation. It has been recently predicted from computational studies of realistic models that an electronic phase separation can occur in manganites in a certain range of doping [19]. In particular at low doping level and at low temperature, a phase separation between hole-poor antiferromagnetic (AFM) regions and hole-rich ferromagnetic (FM) regions is energetically more favourable than the homogeneous canted AFM phase. The energy of charge carrier is minimal for FM ordering. With a density insufficient for establishing the FM ordering in the entire sample, the carriers concentrate into droplets or stripes which become FM inside insulating AFM matrix [20](Fig. 1.7). The fluctuations length scales range from nm to µm. This point towards the conduction in metallic phase and the CMR effect is argued as a
Introduction 11 Figure 1.8: Different types of spin arrangements. result of percolation [21, 22, 23]. On increasing the doping level the FM regions become connected to each other and a low electrical resistance is achieved above the percolation threshold. There are many reports about the coexistence of FM clusters in the CO AFM insulating phase of La 1−x Ca x MnO 3 using different experimental techniques [23, 24, 25, 26, 27]. And very recently the neutron diffraction and the inelastic neutron scattering studies of Pr 1−x Ca x MnO 3 have shown the coexistence of ferromagnetic fluctuations in antiferromagnetic domains [28, 29, 30, 31]. 1.8 Spin, Charge and Orbital ordering in manganites The unusual properties of manganites challenge the current topic of research in the field of strongly correlated electrons system due to their strong spin, charge and orbital degrees of freedom. The strong interplay between spin, charge and orbital ordering originating from the single occupancy of the doubly degenerate e g orbitals of Mn 3+ (t 3 2g e1 g ) ions make the phase diagram of these compounds rich with physics. With charge carrier doping, some of them for example, La 1−x Ca x Mon 3 system show strong charge and/or orbital ordered (CO/OO) insulating nature at low temperatures, especially when x equals a commensurate value. The CO state where the Mn 3+ (t 3 2ge 1 g) and Mn 4+ (t 3 2g ) are arranged like a checkerboard influences strongly the one electron bandwidth of the e g band [32] and the transfer interaction of the e g holes (electrons). There are various types of ordering occurs across the phase diagram of manganites, which are described below. The lattice distortion in manganites is due to the J-T interaction acting on the Mn 3+ ion, which lifts the degenerate e g orbitals in the cubic electronic field. The J-T
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: Introduction 9 Figure 1.6: Schemati
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 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
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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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