Thesis High-Resolution Photoemission Study of Kondo Insulators ...

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98 Chapter 7. Temperature and Co-Substitution Dependence of the ... et al.[7.4] performed the first optical study of FeSi and found that the optical gap of ∼ 60 meV was opened below ∼ 200 K, a rather low temperature compared with the gap size as presented in Fig. 7.1. This gap is confirmed by Degiorgi et al.[7.11] and Damascelli et al. [7.13] although the energy range of spectral weight redistribution remains controversial. Nyhus et al.[7.14] reported that at low temperatures the Raman intensity centered at ∼ 100 meV grows simultaneously with decreasing intensity below ∼ 60 meV. While the optical conductivity is due to direct transitions between the occupied and unoccupied states, photoemission spectroscopy (PES) involves only the occupied states. Thus PES studies are expected to yield less ambiguous information on the unusual electronic structure of FeSi. However, even more puzzling results have come out from recent PES studies: two angle-integrated PES studies showed no temperature dependence of the spectra beside a small shift away from the Fermi level (EF ) [7.6, 7]. That is, Chainani et al.[7.6] and Saitoh et al.[7.7] compared spectra taken at different temperatures and concluded that a gap as small as ∼ 5 meV is opened at low temperatures, which energy scale does not appear in any other measurements. An angle-resolved photoemission study by Park et al. [7.9] revealed a narrow band dispersing by ∼ 30 meV, indicating a strong correlation in FeSi. Saitoh et al. [7.7] attributed the difference between the angle-integrated photoemission spectra of FeSi and the band-structure density of states (DOS) [7.15] to the self-energy correction and concluded that near EF a strong band narrowing is present, consistent with the angleresolved study. Note that the dispersion of every energy band in FeSi is larger than ∼ 300 meV in the band structure calculations [7.15, 16]. Very recently, Breuer et al. [7.8] performed a partially angle-integrated photoemission study of single crystalline FeSi with higher energy resolution and found that the spectral intensity decreases between EF and 40 meV below it at low temperatures, consistent with the energy scale seen in the optical conductivity. They also stated that a full gap at EF , if existed at all, would be less than 1 meV. In this work, in order to further clarify the electronic structure of FeSi, we have studied detailed temperature dependence as well as the effect of Co substitution for Fe by high-resolution angle-integrated photoemission measurements. Because comparison of high-resolution spectra taken at different temperatures is rather difficult owing to the strong temperature dependence of the Fermi-Dirac distribution function, we have attempted to remove that effect and to extract the intrinsic temperature dependence
7.1. Introduction 99 Figure 7.1: Infrared reflectivity R, optical conductivity σ1(ω), and the integral of σ1(ω) from 0 to ω, n(ω)/m∗ for FeSi at 20 K (solid curves), 100 K (dashed curves), 150 K (dots), 200 K (dot-dashed curves), and 250 K (solid curves). The inset in panel (c) shows the difference between n(ω)/m∗ at 250 K and at 20 K (solid curve). The dotted curve has been deduced by shifting the 20 K reflectivity by 0.5 % to show the accuracy [7.4].
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Thesis High-Resolution Photoemissio
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ii CONTENTS 5.2 Experimental . . .
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2 Chapter 1. Introduction Figure 1.
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4 Chapter 1. Introduction Figure 1.
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6 Chapter 1. Introduction peaks, wh
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8 Chapter 1. Introduction only the
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10 Chapter 1. Introduction directio
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12 Chapter 1. Introduction activati
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14 References [1.14] F. Iga, M. Kas
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Chapter 2 Photoemission Spectroscop
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2.1. General Principles 19 Photoele
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2.1. General Principles 21 where R(
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2.2. Experimental 23 DOS Intensity
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2.2. Experimental 25 Figure 2.5: Il
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2.2. Experimental 27 baking out the
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References [2.1] O. Gunnarsson and
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32 Chapter 3. Photoemission Study o
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44 References the Kondo peak positi
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46 Chapter 4. Evolution of Electron
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Page 63 and 64: References [4.1] G. Aeppli and Z. F
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Page 68 and 69: 64 Chapter 5. Temperature Dependenc
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Page 83 and 84: 6.1. Introduction 79 Figure 6.2: Ma
Page 85 and 86: 6.3. Results and Discussion 81 6.3
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Page 97: 6.4. Summary 93 and consistent anal
Page 100 and 101: 96 References [6.11] M. J. Rozenber
Page 104 and 105: 100 Chapter 7. Temperature and Co-S
Page 113 and 114: References [7.1] G. Aeppli and Z. F
Page 115 and 116: Chapter 8 Temperature and Al-Substi
Page 117 and 118: 8.2. Experimental 113 and Fisk [8.2
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Page 127: 8.4. Summary 123 temperature magnet
Page 130 and 131: 126 References [8.14] T. Tonogai, A
Page 132 and 133: 128 Chapter 9. Conclusion has shown
Page 134 and 135: 130 Chapter 9. Conclusion (a) FeSi
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