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

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122 Chapter 8. Temperature and Al-Substitution Dependence of the ... dependence of the optical conductivity of FeSi does not depend on a slight Al substitution. The merging point of the temperature-dependent conductivity remains ∼ 0.8 eV in the x = 0.02 and 0.05 samples while the conductivity within the gap grows with the substitution. On the other hand, the conductivity of the x = 0.10 sample is drastically different from the others: The temperature dependence mostly disappears and the shoulder of the gap edge, which is observed in the x = 0.00, 0.02, and 0.05 conductivity at ∼−0.1 eV, is smeared out. Both the PES spectra and the optical-conductivity spectra support that for x
8.4. Summary 123 temperature magnetic susceptibility4 of FeSi1−xAlx would be helpful to understand the photoemission spectra on the energy scale higher than the (pseudo)gap. Finally, we compare the present result with that of Fe1−xCoxSi. In contrast with the spectral DOS of Fe1−xCoxSi, where the temperature-dependent depression on cooling from 295 K to 18 K survives almost fully in the x = 0.10 sample, the depression appears more sensitive to the Al substitution than the Co substitution although the electrical resistivity of Fe1−xCoxSi is almost the same as that of FeSi1−xAlx [8.8]. Surviving pseudogap in Fe1−xCoxSi is remarkable because the Fe 3d states dominate the electronic structure around EF in FeSi and thus the Fe-site substitution may be expected to change the low-energy electronic states of FeSi to a larger extent than the Al-site substitution. The energy region where the pseudogap opens on cooling is ∼ 50 meV from EF , independent of a small substitution of both Al and Co. Under the present normalization, raising the temperature increases the spectral DOS in any energy position while a slight (x ≤ 0.05) Al substitution at low temperature increases the spectral DOS at EF and simultaneously decreases the DOS between − 30 meV and − 300 meV. 8.4 Summary We have studied the electronic states in FeSi1−xAlx by temperature-dependent photoemission measurements. In spite of its structureless line shape, the response of the photoemission spectrum of FeSi to temperature or substitution is different in different energy regions. The dip in the vicinity of EF (∼ −20 meV) is filled for a small amount of Al substitution or a small temperature increase. The pseudogap region (∼ −50 meV) monotonously depends on the temperature and the Al substitution. The shoulder around ∼−100 meV, which is expected to correspond to the Kondo peak in a Kondo metal, depends on the Al substitution up to x = 0.30. The broad peak at ∼− 400 meV is stable against both temperature and substitution. 4 Ohno et al. [8.8] and DiTusa et al. [8.11, 10] measured the magnetic susceptibility of FeSi1−xAlx below room temperature.
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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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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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