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

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64 Chapter 5. Temperature Dependence of the High-Resolution Photoemission ... the f states by photoemission spectroscopy (PES). Actually the peculiar temperature dependence of the 4f emission in Yb compounds has been studied extensively with high-resolution PES [5.3]. However, few PES studies have so far focused on the non-f conduction-band states. Schlesinger et al. remarked on the optical spectra of a “3d Kondo insulator” FeSi [5.4] that the entire conduction band, which screens all the local moments, could depend on the temperature. Such a temperature dependence could appear in the formation of the Kondo singlet irrespective of whether the system becomes a renormalized metal or insulator. The electrical resistivity of the Kondo insulator is expected to be dominated by the Kondo effect at high temperatures and to show activated behavior at low temperatures. Thus a significant temperature dependence is expected in the conduction-band states and such a temperature dependence may give a clue to understand the Kondo insulators. In the preceding work using YbB12 polycrystals, we have studied the low-temperature electronic structure [5.5] and its Lu-substitution effects [5.6] by PES. For the conduction-band states of B 2sp -Yb5d hybridized character, we have found that a broad dip or a pseudogap of ∼ 40 meV width is present and is filled gradually with Lu substitution. YbB12 has a cubic UB12-type structure [5.7] and is believed to develop a rather isotropic gap at low temperatures. In this work, we have made a PES study of YbB12 single crystals with higher energy resolution to investigate how the (pseudo)gap evolves as a function of temperature. 5.2 Experimental Single crystalline YbB12 samples were prepared using the traveling-solvent floating-zone method [5.8]. YbB12 forms by a peritectic reaction at 2200◦C, which temperature is close to the peritectic temperature of the adjacent phase YbB66 (2150◦C)[5.9]. In order to keep the temperature inhomogeneity less than 50 ◦ C at 2200 ◦ C we used an image furnace with four xenon lamps. Laue photographs and x-ray powder diffraction confirmed large samples to be single crystalline. Figure 5.1 shows the electrical resistivity and the magnetic susceptibility of the best rod grown in this way [5.8]. The resistivity increases monotonously by more than five orders of magnitude as the temperature is decreased from 300 K to 1.3 K. The activation energy (∆act) for 15 K
5.2. Experimental 65 and then rapidly decreases. The low-temperature upturn of the magnetic susceptibility is very small compared with that of polycrystals, indicating a much reduced amount of magnetic impurities. ρ (Ω cm) χ (10 -3 emu/mole) 10 1 10 0 10 -1 10 -2 10 -3 10 -4 12 10 8 6 4 2 0 0 ρ (µ Ω cm) 150 100 100 T (K) 100 150 200 250300 2.0 YbB 12 single crystal T (K) 2.2 log T 200 2.4 300 Figure 5.1: Electrical resistivity and magnetic susceptibility of the YbB12 single crystal used in the present work [5.10]. High-temperature (100
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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
Page 18 and 19: 14 References [1.14] F. Iga, M. Kas
Page 21 and 22: Chapter 2 Photoemission Spectroscop
Page 23 and 24: 2.1. General Principles 19 Photoele
Page 25 and 26: 2.1. General Principles 21 where R(
Page 27 and 28: 2.2. Experimental 23 DOS Intensity
Page 29 and 30: 2.2. Experimental 25 Figure 2.5: Il
Page 31 and 32: 2.2. Experimental 27 baking out the
Page 33: References [2.1] O. Gunnarsson and
Page 36 and 37: 32 Chapter 3. Photoemission Study o
Page 48 and 49: 44 References the Kondo peak positi
Page 50 and 51: 46 Chapter 4. Evolution of Electron
Page 63 and 64: References [4.1] G. Aeppli and Z. F
Page 65: References 61 [4.25] P. Weibel,M. G
Page 70 and 71: 66 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
Page 89 and 90: 6.3. Results and Discussion 85 dist
Page 91 and 92: 6.3. Results and Discussion 87 Apar
Page 93 and 94: 6.3. Results and Discussion 89 of E
Page 95 and 96: 6.3. Results and Discussion 91 the
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
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8.3. Results and Discussion 115 8.3
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8.3. Results and Discussion 117 Int
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8.3. Results and Discussion 119 dep
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8.3. Results and Discussion 121 σ
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8.4. Summary 123 temperature magnet
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126 References [8.14] T. Tonogai, A
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128 Chapter 9. Conclusion has shown
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130 Chapter 9. Conclusion (a) FeSi
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132 Chapter 9. Conclusion developed
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