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

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128 Chapter 9. Conclusion has shown a nonlinear response to x and has appeared only for x ≤ 0.25, presenting the effect beyond the single-site mechanism for the gap formation. (a) Yb Lu B 0.5 0.5 12 -0.5 eV (b) YbB 12 - 80 meV LuB 12 ~ 3 k T B max - 30 meV EF YbB 12 E F E F 300 K 7 K 300 K Figure 9.1: Schematic summary of the photoemission spectra of YbB12 and Yb1−xLuxB12: Kondo peaks in the 4f spectra (a) and the temperature dependence of the (pseudo)gap in the conduction-band spectra of YbB12 and LuB12 (b). As for FeSi, we have studied the response of the electronic states to temperature and Fe-site or Si-site substitution. In spite of its structureless line shape, where only one peak appears at ∼−0.4 eV (Fig. 9.2 (a)), the response of the photoemission spectrum of FeSi to temperature and substitution is different at different energy regions. Though Co substitution for the Fe site changes not only the vicinity of EF but also the broad peak due to the difference in the 3d levels of Fe and Co, it affects mainly 7 K
low-temperature and low-energy electronic structure of FeSi and recovers the DOS in the vicinity of EF , consistent with the optical conductivity and the dc conductivity. A systematic substitution effect on FeSi has been studied thoroughly in FeSi1−xAlx. The depression in the vicinity of the Fermi level (∼ −20 meV) is filled with a small amount of Al substitution (Fig. 9.2 (b)) and temperature increase. The pseudogap region (∼ −50 meV) depends on the temperature and the Al substitution (Fig. 9.2 (c)). The shoulder around ∼−100 meV, which may correspond to the Kondo peak in 4f-electron systems, depends on the Al substitution up to x = 0.30 (Fig. 9.2 (b)) and such substitution dependence of the shoulder is enhanced at higher temperature. The broad peak at ∼−400 meV is stable against both temperature and substitution. A clear difference between YbB12 and FeSi appears in the substitution dependence. The disappearance of the gap has been observed at x ∼ 0.25-0.5 in Yb1−xLuxB12 and x ∼ 0.1 in FeSi1−xAlx. These critical concentrations (xc) are consistent with the transport properties. The rapid collapse of the gap with substitution indicates the importance of the intersite interaction for the gap opening in FeSi. While the gap in Yb1−xLuxB12, which survives up to x ∼ 0.25-0.50, reflects the localized character of the 4f states, the x-nonlinearity of the substitution dependence of the photoemission spectra indicates an effect of the intersite interaction in the gap formation for YbB12. The difference in xc might be one of the origin of the relatively high residual DOS for FeSi: At and around the scraped surface, where the atomic coordination is not perfect, the gap character would be more robust for YbB12. The relation between the peak position in the photoemission spectra and Tmax, where the magnetic susceptibility shows a maximum, is different between YbB12 and FeSi. The sharp Kondo peak, which is observed in the Yb Kondo systems, does not appear in FeSi. However, the substitution dependence of the spectra of FeSi at low temperatures resembles that of YbB12 and is reminiscent of the hybridization gap model. 129
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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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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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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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References [4.1] G. Aeppli and Z. F
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References 61 [4.25] P. Weibel,M. G
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64 Chapter 5. Temperature Dependenc
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74 References [5.10] N. Shimizu, F.
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