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

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104 Chapter 7. Temperature and Co-Substitution Dependence of the ... lowest panel of Fig. 7.6. 1 The spectral DOS thus obtained for FeSi and Fe1−xCoxSi are shown in Fig. 7.6. One can see that the energy range where the DOS vary with temperature is between EF and ∼ 50 meV below it, consistent with the data taken at room temperature and ∼10 K reported by Breuer et al. [7.8] The size of this wide dip or “pseudogap” is almost independent of the Co content. The pseudogap is wide enough compared with the instrumental resolution and therefore the present approach of dividing the spectra with the Fermi-Dirac distribution function is valid to discuss this structure. From the present PES spectra, however, one cannot conclude where the missing spectral weight in the pseudogap region is transferred. Here, it should also be noted that spectral weight around EF remains substantially high at all temperatures, contrasted with the optical conductivity data, in which almost all the spectral weight is depleted below ∼−60 meV at low temperatures. Comparing the data from single crystals and polycrystals, Breuer et al. [7.8] proposed that the surface might be metallic and contribute the observed spectral weight at and around EF . In Fig. 7.6 we note that the recovery of the DOS with temperature and with Co substitution is dependent on the energy position: we show in Fig. 7.7 the temperature dependence of the DOS at EF and 30 meV below it. Plotted are the intensities relative to the values at room temperature. The DOS at EF of pure FeSi increases with temperature more rapidly than those of Co-substituted samples. On the other hand, such a difference between the pure and Co-substituted samples is absent at −30 meV. This has already been suggested in Fig. 7.4, which shows that the Co substitution affects mainly the vicinity of EF . Here, we note that the recovery of the spectral function both at EF and 30 meV below it starts between 225 K and 150 K, low enough temperatures compared with the gap size. This is consistent with the temperature dependence of the optical conductivity, where the gap feature disappears above ∼ 200 K. The bottom panel of Fig. 7.7 shows the dc conductivity of the samples studied here normalized to the values at room temperature. (The room-temperature conductivity of pure FeSi is ∼ 75 % of those of the x = 0.05 and x = 0.10 samples.) Like the DOS at EF , the dc conductivity depends on Co content mainly below ∼ 200 K. Thus we can state 1 On the other hand, one cannot discuss sharp structures compared with the energy resolution from these DOS thus obtained although sharp structures sometimes appear as spurious features. We have made extensive simulations for various DOS line shapes and temperatures in order to see to what extent the present method is reliable in extracting the original DOS. Figure 7.6 indicates temperaturedependent spectral changes in the wide energy range but from this figure alone one cannot exclude the possibility that a sharp gap compared with the energy resolution is opened near EF .
7.3. Results and Discussion 105 DOS 297 K 225 K 150 K 75 K 18 K 293 K 225 K 150 K 75 K 18 K 298 K 225 K 150 K 75 K 18 K FeSi Fe 0.95 Co 0.05 Si Fe 0.90 Co 0.10 Si 295 K 225 K 150 K 75 K 18 K Au -100 -80 -60 -40 -20 0 20 Energy relative to E F (meV) Figure 7.6: Temperature-dependent spectral DOS of Fe1−xCoxSi and Au obtained by dividing the photoemission spectra by the Fermi-Dirac distribution function convoluted with a Gaussian function. that the low-energy (< 30 meV) or low-temperature (< 200 K) electronic structure is strongly affected by the small amount of Co substitution in contrast to the high-energy or high-temperature electronic structure. This is consistent with the recent transport and optical study of Fe1−xCoxSi [7.21]: the gap size deduced from the optical conductivity and the high-temperature (130
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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 58 and 59: 54 Chapter 4. Evolution of Electron
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Page 63 and 64: References [4.1] G. Aeppli and Z. F
Page 65: References 61 [4.25] P. Weibel,M. G
Page 68 and 69: 64 Chapter 5. Temperature Dependenc
Page 78 and 79: 74 References [5.10] N. Shimizu, F.
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
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