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

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102 Chapter 7. Temperature and Co-Substitution Dependence of the ... Intensity x = 0.00 x = 0.10 x = 0.05 Fe 1-x Co x Si 18 K hν = 21.2 eV (0.10) - (0.00) difference × 5 (0.05) - (0.00) -0.6 -0.4 -0.2 0.0 Energy relative to E F (eV) Figure 7.4: Upper panel: photoemission spectra of Fe1−xCoxSi near EF at 18 K. Lower panel: the difference spectra between pure and substituted samples. a small amount of Co substitution on this energy scale. Figure 7.4 shows changes induced by Co substitution near EF . We have subtracted the spectra of pure FeSi from those of Co-substituted samples as shown in the lower panel. The obtained difference spectra indicates that the DOS around EF in FeSi is increased upon Co substitution, corresponding to the substitution-induced metallic behavior. Apart from the vicinity of EF there is no clear structure in the difference spectra. We also note that the position of the broad peak at ∼−0.3 eV in pure FeSi is shifted by ∼ 0.1 eV towards higher binding energy in the Co-substituted samples. The temperature dependence of the spectra near EF is presented in Fig. 7.5. Normalized at higher binding energies (< −0.1 eV), the midpoint of the leading edge in FeSi appears to be shifted by ∼ 5 meV away from EF at low temperatures as reported previously [7.6, 7], clearly different from the spectra of Au shown in the same figure. The temperature-dependent spectra of Fe1−xCoxSi show a similar shift of the leading
7.3. Results and Discussion 103 Intensity Intensity Intensity 18 K 75 K 150 K 225 K 297 K 18 K 75 K 150 K 225 K 298 K 18 K 75 K 150 K 225 K 295 K FeSi Fe 0.9 Co 0.1 Si Au -40 -20 0 20 40 Energy relative to E F (meV) Figure 7.5: Photoemission spectra of FeSi, Fe0.9Co0.1Si and Au taken at different temperatures. edge. However, the edge at 18 K appears again closer to EF than that at 75 K, questioning the simple interpretation of the leading edge shift with temperature. Another possibility is a reduction of the DOS at EF with decreasing temperature, which could lead to the apparent shift of the leading edge as observed experimentally. The difficulty to distinguish between the two possible scenarios of the spectral changes with temperature comes from the difficulty to extract changes in the DOS itself apart from the temperature dependence of the Fermi-Dirac distribution function. Therefore, we have divided the photoemission spectra by the Fermi-Dirac distribution function (convoluted with a Gaussian corresponding to the instrumental resolution) [7.20]. This method gives reliable results in cases where the DOS has only broad structures near EF : application of this method to the spectra of Au yields a reasonably flat DOS as shown in the
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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 56 and 57: 52 Chapter 4. Evolution of Electron
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 81 and 82: Chapter 6 Substitution and Temperat
Page 83 and 84: 6.1. Introduction 79 Figure 6.2: Ma
Page 85 and 86: 6.3. Results and Discussion 81 6.3
Page 87 and 88: 6.3. Results and Discussion 83 DOS
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Page 93 and 94: 6.3. Results and Discussion 89 of E
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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 102 and 103: 98 Chapter 7. Temperature and Co-Su
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
Page 119 and 120: 8.3. Results and Discussion 115 8.3
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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
Page 136: 132 Chapter 9. Conclusion developed
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