PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

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Electronic Structure of Pr 1−x Ca x MnO 3 86 (a) (b) Intensity (arb. units) x=1 x=0.4 1 - 0.2 0.4 - 0.2 x=0.33 x=0.2 0.33 - 0.2 -1 0 1 2 3 4 0 1 2 3 4 Energy Relative to Fermi Energy (eV) Figure 4.5: Panel(a) shows the IPES spectra of the near E F unoccupied states. Panel(b) shows the difference spectra obtained by subtracting the spectra corresponding to the x = 0.2 from those of other compositions. the Ca x MnO 3 . Band structure calculations on similar CMR systems [12, 32] have unambiguously shown that both the highest occupied and lowest unoccupied states in these systems are of Mn 3d e g spin-up symmetry. As mentioned earlier, within this frame work the features marked A’ and A” in Fig. 4.3 and the feature P in Fig. 4.4 correspond to the e g spin-up states. Combining the results presented in Table I and these two figures one can estimate the energy required by an e g electron to hop from a Mn 3+ site to the neighboring Mn 4+ site. If we consider the centroids of the A’ in Fig. 4.3 and the P in Fig. 4.4 we arrive at a value of ∼ 3.3±0.2 eV, which is large compared to the earlier reported value [24]. With the A” of Fig. 4.3 and P of Fig. 4.4 we get ∼ 2.8±0.2 eV which is close to the earlier report (2.6±0.1 eV). Due to the large widths and long tails of the A” and P towards the E F we expect that this value of the energy be close to its upper bound. Energy required for the transfer of e g electrons between the Mn sites again is an important parameter for the phase separation models to work in
Electronic Structure of Pr 1−x Ca x MnO 3 87 these systems. 4.3.3 The core levels of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33, 0.4, and 0.84 The Mn2p core level spectra of Pr 1−x Ca x MnO 3 (x = 0.2, 0.33, 0.4, and 0.84) taken at room temperature as well as at 77 K are shown in Fig. 4.6. All the spectra were normalized and shifted along the Y axis by a constant for clarity. The two prominent peaks at around ∼ 641.5 and ∼ 653.5 eV are due to the spin orbit split 2p 3/2 and 2p 1/2 . These spectra look similar to those reported earlier on the similar system [17, 33]. In Fig. 4.6, the Mn 2p peak positions shift towards the higher binding energy for x = 0.4 and 0.84 at room temperature (a) as well as at 77 K (b). But the binding energy shift is higher incase of room temperature spectra as compared to that of 77 K. The Mn LMV Auger peak is observed on the higher binding energy side of 2p 1/2 peak for all four concentrations at room temperature, whereas at 77 K this Auger peak is absent for x = 0.2, and 0.3. This Mn LMV was earlier observed in other hole doped manganites [17]. The Ca 2p spectra of Pr 1−x Ca x MnO 3 (x = 0.2, 0.33, 0.4 and 0.84) both at room temperature as well as 77 K are shown in the Fig. 4.7. The Ca 2p 3/2 and Ca 2p 1/2 , due to spin orbit splitting, are clearly distinguishable at around ∼ 345.5 and ∼ 349 eV respectively. It is clear from the Fig. 4.7 (a) that, at room temperature the peak positions shift towards the lower binding energy when we move from x = 0.2 to x = 0.84. Also the same kind of shift is observed for 77 K spectra (Fig. 4.7 (b)) with a lesser magnitude. The Pr 4d spectra of Pr 1−x Ca x MnO 3 (x = 0.2, 0.33 and 0.4) taken at room temperature as well as 77 K are shown in Fig. 4.8. The Pr 4d peaks come around 116 eV binding energy. The peak positions shift towards the lower binding energy at room temperature (Fig. 4.8 (a)) when we move from x = 0.2 to 0.4. Here also the similar kind of shift is observed for 77 K spectra (Fig. 4.8 (b)) with a lesser magnitude. As mentioned above the binding energy shifts with respect to the value of x have been observed at the Ca 2p, Pr 4d and Mn 2p core levels. Incase of Mn 2p level, the binding energy shift is to the higher energy side while the other two levels shift to lower energy. The magnitude of the binding energy shifts are found to be higher at room temperature compared to 77 K. The shift to higher binding energy observed in Mn
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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
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Introduction 15 Figure 1.12: Schema
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Introduction 17 Figure 1.13: Phase
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Bibliography [1] R. von Helmolt, J.
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Introduction 21 [31] Damien Saurel,
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Chapter 2 Experimental Techniques 2
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Experimental Techniques 25 Figure 2
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Experimental Techniques 27 Let us n
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Experimental Techniques 29 and G(k,
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Experimental Techniques 31 Figure 2
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Experimental Techniques 33 point wi
Page 57 and 58: Experimental Techniques 35 negative
Page 59 and 60: Experimental Techniques 37 ∆E = E
Page 61 and 62: Experimental Techniques 39 Figure 2
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 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 107: 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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