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

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56 Chapter 4. Evolution of Electronic States in the Kondo Alloy ... peak position in going from YbB12 to Yb0.5Lu0.5B12, ∆ is varied from 0.21 eV to 0.28 eV as shown in the lower panel of Fig. 4.6 with other parameters fixed. The increase in ∆ with Lu-substitution deduced from the AIM analysis is consistent with the recovery of the B sp DOS near EF in the wide-energy range observed in the He I spectra. The consistent changes of the Yb 4f spectra and the B sp DOS mean that the presence of the 4f-derived spectral weight near EF affects the B sp DOS, which in turn affects the 4f states on neighboring Yb atoms. That is, the 4f states at different Yb sites are interacting with each other through the hybridization, indicating that the Kondo singlet in YbB12 is not completely localized. We have also studied the effect of gap opening in the conduction band using the AIM as presented in Fig. 4.9. We opened a narrow (∼ 10 meV) square-well-shaped gap locating the Fermi level at the middle of the gap, and found that the Kondo peak become narrower with its weight (and hence ¯nf) conserved and that the high energy 4f 13 → 4f 12 structure did not change at all. Experimentally we have indeed observed the narrowing of the Kondo peak in going from Yb0.5Lu0.5B12 to YbB12 but a substantial change in ¯nf. Therefore the changes in the 4f spectral line shape with Lu-substitution does not necessarily imply the opening of a narrow gap in the B sp DOS. The absence of the narrow gap in the PES spectra in spite of the semiconducting behavior may indicate that the states near EF are localized due to disorder or that the Fermi level is located near the bottom of the band gap. According to the AIM, the shift of the Kondo peak with Lu-substitution should be accompanied by an increase of the Kondo temperature. Indeed the temperature where the magnetic susceptibilities reaches the maximum [4.11] increases by 10-20 K but to a lesser extent than the shift of the Kondo peak position. The maximum in the magnetic susceptibility of the insulating Yb1−xLuxB12 might simply have originated from missing of the Pauli paramagnetism owing to the gap opening itself. The validity of the AIM, which has only one characteristic temperature TK, remains to be checked for YbB12. 4.4 Summary In summary, we have studied how Lu-substitution into YbB12 changes the low-energy electronic structure. In going from YbB12 to LuB12, the broad dip in the B sp-derived DOS is gradually filled and the simple Fermi edge is recovered; the Kondo peak is shifted towards higher binding energy. According to the AIM analysis, the change
4.4. Summary 57 in the 4f spectral line shape is caused by the recovery of the conduction-band DOS around EF . Since the recovery of the conduction-band DOS is certainly caused by the disappearance of the Yb 4f spectral weight near EF , the present results reveal that interaction between the Yb ions is mediated by the 4f-conduction-band hybridization and therefore that there is finite interaction between the Kondo singlets in YbB12.
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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.
Page 10 and 11: 6 Chapter 1. Introduction peaks, wh
Page 12 and 13: 8 Chapter 1. Introduction only the
Page 14 and 15: 10 Chapter 1. Introduction directio
Page 16 and 17: 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 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
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
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106 Chapter 7. Temperature and Co-S
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References [7.1] G. Aeppli and Z. F
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Chapter 8 Temperature and Al-Substi
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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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