PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

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Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 66 three components corresponding to the three subbands contributing to this region. We used a χ 2 iterative program for fitting the different spectra with Lorentzian line shapes for A, B, and C. Except for the area of the peaks, the energy positions, widths, background, and all other parameters of all the peaks were kept the same for different temperatures. The positions and full widths at half maximum (FWHMs) of the three peaks are given in the caption of Fig. 3.3. The fitted spectra for 77 K (below T c ) and 300 K (above T c ) are shown on the left sides of Figs. 3.5(a) and 3.5(b) below. One can see from Figs. 3.3(a) - (c) and that the area of all the peaks keeps increasing as we go down in temperature T c , but below this temperature the intensities of both peaks C and B decrease. On the other hand, the intensity of A does not show any decrease across the transition [Fig. 3.3(a)]. Pr 0.67 Ca 0.33 MnO 3 shows a transition to a charge-ordered state below 220 K (T co ) [30, 37, 38]. Neutron diffraction studies [15] on another part of this single crystal have shown that this charge ordering turns into an antiferromagnetically structured pseudo-CE type charge or orbital ordering below 110 K. Also, the coexistence of ferromagnetic and antiferromagnetic phases below this transition temperature has been shown on this single crystal. In the pseudo-CE-type ordering the Mn 3d z 2 −r 2 orbitals, where the e g state is occupied, are aligned with the O 2p orbitals. Such an alignment can increase the hybridization between Mn 3d z 2 −r2 and O 2p. Furthermore, a simultaneous decrease in the hybridization strength could also be expected for the core-like in-plane t 2g and O 2p states. The results of the curve fitting (Fig. 3) of our valence band spectra reflects these changes in hybridization with temperature. The charge ordering following the decrease in temperature causes an increase in the O 2p contribution to both the t 2g and e g spin-up subbands in the valence region and hence the intensities of A, B, and C go up. As mentioned earlier, the pseudo-CE-type charge or orbital ordering below 110 K, results not only in a stronger hybridization of O 2p with the e gz 2 −r 2 ↑ states compared to that with the t 2g states but a simultaneous decrease in the latter also. This is reflected in the increase in intensity of A and decrease in the intensities of both C and B below 110 K. Though in Pr 0.67 Ca 0.33 MnO 3 there is no temperature-dependent insulator-metal transition, the change in intensities of these features across T c appears similar to the shifting of spectral weight found in the La 1−x Sr x MnO 3 system [8].
Electronic Structure of Pr 0.67 Ca 0.33 MnO 3 67 Normalized Absorption (arb. units) 530 540 550 560 Photon Energy (eV) Figure 3.4: O K edge x-ray absorption spectra of Pr 0.67 Ca 0.33 MnO 3 taken at 300 (solid line), 150 (dashed line), and 95 K (dotted). The pre-edge feature centered around 529.5 eV (marked by a box) is where most interest lies. Since this feature consists of two peaks (a main line and a shoulder on the low-energy side), this part of the spectrum was fitted with two components of Lorentzian line shapes. Results of the curve fit are given in Table 3.1. 3.3.2 The XAS study of Pr 0.67 Ca 0.33 MnO 3 In a reasonable approximation, the near-E F region of the O K x-ray absorption spectra could well represent the density of unoccupied states in many of the transition metal oxide compounds [20, 34]. In order to probe the electronic structure of the unoccupied states we have performed XAS on our Pr 0.67 Ca 0.33 MnO 3 single crystal. The O K edge XAS spectra taken at room temperature and 150 and 95 K (below T c ), shown in Fig. 3.4, have been normalized in intensity all along the region starting from 550 eV. The pre-edge feature in the O K spectra (centered around 529.5 eV) is due to the strong hybridization between Mn 3d and O 2p orbitals. The broad feature around 536.5 eV is due to the bands from hybridized Pr 5d and Ca 3d orbitals, while the structure above 540 eV is due to states like Mn 4sp and Pr 6sp, etc. These states are known to contribute least to the near-E F electronic structure of these
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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
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
Page 87: 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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PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

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