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

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78 Chapter 6. Substitution and Temperature Dependence of the Conduction ... type temperature dependence at low temperatures is weakened with x and gradually disappears between x = 0.25 and x = 0.50. The resistivity of LuB12 is as small as 0.13 µΩ cm at 4.2 K and rises monotonously with increasing temperature. The maximum of the magnetic susceptibility (Tmax) is gradually shifted from ∼ 75 K for YbB12 to ∼ 100 K for Yb0.25Lu0.75B12 as shown in Fig. 6.2. The shift of Tmax would reflect an increase in the Kondo temperature (TK) owing to the recovery of the conduction-band density of states (DOS) with x, as already discussed in Chap. 4. The suppressed magnetic susceptibility below ∼ 60 K, which corresponds to missing Pauli paramagnetism in YbB12, is recovered with Lu substitution. ρ (µ Ω cm) 600 500 400 300 200 100 0 0 Yb 1-x Lu x B 12 x = 0.00 x = 0.25 x = 0.50 x = 0.75 x = 1.00 100 200 T (K) 300 Figure 6.1: Electrical resistivities of Yb1−xLuxB12 (x = 0.00, 0.25, 0.50, 0.75, and 1.00) single crystals [6.4]. Motivation for the temperature-dependent photoemission spectroscopy (PES) study of Yb1−xLuxB12 is the following: (i) Observation of systematic changes caused by the Lu substitution will give an important clue to understand whether the gap opening in YbB12 is better described by
6.1. Introduction 79 Figure 6.2: Magnetic susceptibility of Yb1−xLuxB12 (x = 0.00, 0.25, 0.50, 0.75, and 1.00) single crystals [6.3]. the single-site model or the itinerant picture. One can understand the mechanism of the gap opening within the single-site model if no substantial changes in the spectra appear by controlling the distances between the Kondo ions (Yb atoms) after having normalized the spectral intensities to the Yb content. In that case the depression of the spectral DOS of Yb1−xLuxB12 at the Fermi level (EF ) would be simply a linear combination of the pseudogap of YbB12 and the Fermi edge of LuB12. On the other hand, the effect of coherence in the YbB12 lattice would appears as sensitive changes in the photoemission spectra with a small Lu substitution. Note that, in the case of FeSi, only the spectral DOS in the vicinity of EF is sensitive to a small substitution, indicating that the coherence in the periodic lattice plays an important role in the low-energy properties of FeSi, as will be discussed in the following Chapters. (ii) By comparing the spectra of YbB12 with those of the substituted samples, we can discuss details of the spectral line shape of YbB12. Effects of the non-periodicity and defects in the lattice at the surface prepared by scraping would be reflected in the ultraviolet photoemission spectra. One can extract intrinsic signals of YbB12 using the spectra of the Lu-substituted samples as references. It should be remembered that the residual DOS is observed in the photoemission spectrum of YbB12 at low temperature while a fully open gap is observed by tunneling spectroscopy [6.6] and optical reflectivity
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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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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 68 and 69: 64 Chapter 5. Temperature Dependenc
Page 78 and 79: 74 References [5.10] N. Shimizu, F.
Page 81: Chapter 6 Substitution and Temperat
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 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
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Page 119 and 120: 8.3. Results and Discussion 115 8.3
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Page 130 and 131: 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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