PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

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Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 60 3.1 Introduction Recently, a lot of attention have been focused on the charge-ordered compositions of Pr 1−x Ca x MnO 3 due to their importance as possible prototypes for the electronic phase separation (PS) models [1, 2, 3, 4, 5, 6, 7, 8, 9] proposed to explain the phenomenon of colossal magnetoresistance (CMR). The PS models are qualitatively different from the double-exchange model [10, 11, 12] or those based on strong Jahn-Teller polarons [13, 14]. According to the PS model the ground state of CMR materials is comprised of coexisting nanosize clusters of metallic ferromagnetic and insulating antiferromagnetic nature [6]. The insulator-metal transition in this scenario is through current percolation. Though there have been a number of experimental studies showing the existence of phase separation, their size varies from nano- to mesoscopic scales [8, 9]. Radaelli et al. have shown the origin of mesoscopic phase separation to be the intergranular strain [9] rather than the electronic nature as in PS models. Nanosized stripes of a ferromagnetic phase [15] were reported in Pr 0.67 Ca 0.33 MnO 3 . This compound also attracted much attention earlier due to the existence of a nearly degenerate ferromagnetic metallic state and a charge-ordered antiferromagnetic insulating state with a field-induced phase transition possible between them [16]. 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 [17, 15]. The composition x=0.33 shows a coexistence of ferromagnetic and antiferromagnetic phases [18]. This makes the charge- or orbital-ordered Pr 0.67 Ca 0.33 MnO 3 a prototype for the PS scenario. One of the requisites for the existence of an electronic phase separation in manganites is a strong-coupling interaction affecting the hopping of the itinerant e g electrons. The PS is expected to occur [5] when J H /t ≫ 1, where J H is the Hund’s coupling contribution between the localized t 2g and the e g electrons and t is the hopping amplitude of the e g electrons. The PS also favors a strong electron-phonon coupling, like the influence of a strong Jahn-Teller (JT) polaron arising from the Q 2 and Q 3 JT modes. Essentially, one expects a strong localization of charge carriers to accompany the electronic separation of phases. Apart from charge, the orbital degrees of freedom also play an important role in this scenario [5]. The itinerant electron hopping term is strongly influenced by the symmetry of the orbitals (d x 2 −y 2 and d 3z 2 −r 2) hybridized with the O 2p orbitals of the MnO 6 octahedra. In comparison with the most popular CMR material La 1−x Sr x MnO 3 , the charge-ordered Pr system has an inherently reduced e g bandwidth W due to the smaller ionic radius of Pr, which also
Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 61 enhances the tendency for carrier localization. All these charge and orbital interactions are reflected in the near-E F electronic structure of these materials and could be probed using electron spectroscopic techniques. Valence band photoemission and O K x-ray absorption are two such proven tools sensitive to the changes in the lowenergy states crucial to these interactions. Although, there are many reports on the near-E F electronic structure of other CMR compounds using these techniques [19, 20, 21, 22, 23, 24, 25, 26, 27, 28] only a few studies have been reported on their charge-ordered compositions [8, 29]. One of the key energy 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 [20]. In this study we have used ultraviolet photoelectron spectroscopy and x-ray absorption spectroscopy (XAS) in order to probe the electronic structure of the occupied and unoccupied states on a well-characterized, high-quality single crystal of Pr 0.67 Ca 0.33 MnO 3 . Another part of this single crystal had earlier been used for a detailed neutron scattering experiment that indicated the possible existence of a phase separation of ferromagnetic and antiferromagnetic stripes [15]. In the present study we have analyzed the temperature-dependent changes in the near-E F electron energy states from the perspective of a phase separation. 3.2 Experimental The single-crystal sample of Pr 0.67 Ca 0.33 MnO 3 was grown by the floating zone method in a mirror furnace. The compositional homogeneity of the crystal was confirmed using energy-dispersive spectroscopic analysis. Magnetization and transport measurements on this crystal showed the transition temperatures T c and T N to be 100 and 110 K, respectively. Details of the sample preparation, magnetization, and transport measurements and structural studies are published elsewhere [15, 30]. Angle-integrated ultraviolet photoemission measurements were performed using an Omicron µ-metal UHV system equipped with a high-intensity vacuum-ultraviolet source (HIS 13) and a hemispherical electron energy analyzer (EA 125 HR). At the He I (21.2 eV) line, the photon flux was of the order of 10 16 photons/s/sr with a beam spot of 2.5 mm diameter. Fermi energies for all measurements were calibrated using a freshly evaporated Ag film on a sample holder. The total energy resolution, estimated from the
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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
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 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: Chapter 3 Electronic Structure of P
Page 85 and 86: Electronic Structure of Pr 0.67 Ca
Page 95 and 96: Bibliography [1] I. G. Deac, J. F.
Page 99 and 100: Chapter 4 Electronic Structure of P
Page 101 and 102: 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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