PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

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Electronic Structure of Pr 1−x Ca x MnO 3 84 Table 4.2: Results of the fitting of the difference spectra using a χ 2 iterative programme. We have used a mixture of Lorentzian and Gaussian line shapes for fitting the various difference spectra corresponding to the three compositions at 300K and 77K. In order to see the finer changes in the near E F feature A”, we used a fixed width for all the fitted lines marked A’. The energy positions of the A’ and A” and the width of the A” were determined by the iterative programme. At both the temperatures the sample with x = 0.33 shows a larger full width at half maximum (FWHM) for its A”. At 77K, both A’ and A” have been found to be shifted slightly towards the E F . Temperature (K) 300 77 x A’ A” Position (eV) FWHM (eV) Position (eV) FWHM (eV) 0.2 0.97 0.5 0.49 0.5 0.33 0.96 0.5 0.49 0.65 0.4 0.96 0.5 0.49 0.59 0.2 0.92 0.5 0.45 0.49 0.33 0.93 0.5 0.45 0.53 0.4 0.92 0.5 0.46 0.5 indicate the formation of FM clusters in the AFM insulating background. From the fitting of the difference spectra we feel that the increase of the width of A” is largely due to the formation of a tail towards the E F , which makes it difficult to estimate the energy difference between A’ and A” accurately. 4.3.2 The IPES study of Pr 1−x Ca x MnO 3 , x = 0.2, 0.33, 0.4 and 1 Fig. 4.4 shows the IPES spectra of the Pr 1−x Ca x MnO 3 samples (x = 0.2, 0.33 and 0.4) and the CaMnO 3 sample taken at room temperature. The spectra have been normalized for their intensities and shifted along the vertical axis by a constant for clarity. The spectra shows two broad features; one between 5 to 12 eV (marked Q) and another at around 2.4 eV (marked P). For the x = 0.2, 0.33, and 0.4, the feature appearing at higher energy is mostly due to Pr 5d and Ca 3d orbitals, whereas for the CaMnO 3 it is due only to the Ca 3d, 4s orbitals. The feature P is due to the hybridized Mn 3d - O 2p states. These assignments of the features are consistent with band structure calculations and other studies on similar systems [24, 36, 37]. Similar energy positions were reported earlier for La 1−x Sr x MnO 3 by Chainani et al.
Electronic Structure of Pr 1−x Ca x MnO 3 85 Q x=1 P x=0.4 Intensity (arb. units) x=0.33 x=0.2 0 2.5 5 7.5 10 12.5 15 Energy Relative to Fermi Energy (eV) Figure 4.4: The IPES spectra of the Pr 1−x Ca x MnO 3 samples (x = 0.2, 0.33 and 0.4) and the CaMnO 3 sample taken at room temperature. The spectra represents the unoccupied electron states of the samples. [38], though another study [39] showed the feature P to be more closer to the E F . Again, in the IPES spectra also, the feature appearing near to E F is important to properties of these compounds. In Fig. 4.5 we have shown the spectra of the near E F region containing this feature along with the difference spectra (Fig. 4.5(b)). One can see clearly that the intensity of P increases with the Ca concentration x due to the increase of unoccupied states in the Mn 3d orbitals with hole doping. From a linear extrapolation of the rising band edge in the spectra shown in the Fig. 4.5(a) we have estimated the band gap to be ∼ 0.7 eV for the three compositions (0.2, 0.33, and 0.4). At this temperature, the value of the gap does not seem to vary with the charge carrier concentration, whereas the parent compound (Ca x MnO 3 ) shows a smaller value (∼ 0.67 eV). In case of La 1−x Sr x MnO 3 , Chainani et al. [38] has shown the value of the band gap to be ∼ 0.6 eV. The larger values shown by our Pr 1−x Ca x MnO 3 samples could be attributed to their insulating nature. In an alternate approach, one can also estimate the band gap from the difference spectra shown in the Fig. 4.5(b). This gives a some what larger value; i.e ∼ 0.86 eV for the x = 0.4 and 0.33 and ∼ 0.82 for
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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
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 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 105: 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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