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

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112 Chapter 8. Temperature and Al-Substitution Dependence of the ... was difficult to distinguish between the intrinsic effect of changes in the band filling on the spectral DOS of FeSi and the difference in the 3d levels of Fe and Co. Photoemission spectroscopy (PES) measurements of Si-site substituted samples would enable one to extract the effect of changes in the band filling and to discuss the origin of both the pseudogap around EF and the broad peak at ∼ 0.3 eV below EF . Figure 8.1: Hall resistance of FeSi1−xAlx. Carrier concentration calculated from RH at 4 K is shown in the inset [8.11]. Transport and thermodynamic properties of FeSi1−xAlx (0 ≤ x ≤ 0.08) have been studied by DiTusa et al. [8.10, 11]. Figure 8.1 shows the Hall resistance (RH) of FeSi1−xAlx for 0.00 ≤ x ≤ 0.08. Carrier concentration calculated from RH at 4 K, which is shown in the inset, indicates that Al substitution donates one hole per added Al in the samples. They concluded from the critical behavior of conductivity near the metalinsulator transition (σ = σ0[(n/nc) − 1] ν ) that apart from the strongly renormalized mass, the metallic behavior of doped FeSi (x ≥ 0.01) is basically the same as that of a doped band insulator in the same sense that the heavy fermion metal is a Fermi liquid with a greatly enhanced band mass. It should be remembered that in YbB12 some higher-energy structures exist in the spectral DOS other than the (pseudo)gap at EF which agrees with the transport activation energy: They are a large pseudogap, which is larger than the transport gap, the Kondo peak, and a broad peak in the conduction band DOS. In addition to the similarities between FeSi and the 4f-electron Kondo insulator pointed out by Aeppli
8.2. Experimental 113 and Fisk [8.2] for the first time, we stress another similarity between FeSi and YbB12 that almost non-dispersing bands exist just below EF [8.4]. In the band structure of FeSi calculated by Mattheiss and Hamann [8.3], a band located just below the gap has a very small dispersion of a few tens meV along the ΓX, ΓM, and ΓR directions. This is in contrast with the conduction bands just above EF , which are moderately dispersing. The narrow band on the occupied side in FeSi has been observed in recent angle-resolved PES studies [8.5, 6]. Angle-integrated PES studies of FeSi have revealed an opening of the pseudogap of 40-50 meV [8.7, 9] with a finite spectral weight at the bottom of the pseudogap. The depression of the spectral DOS in FeSi is not so sharply dipped as that in YbB12 and thus the spectral weight at EF would remain finite even if the broadening due to instrumental resolution has been removed. Breuer et al. [8.7] stated based on their high-resolution (∆E ∼ 5 meV) photoemission data that a full gap at EF , if opens, should be less than 1 meV. In this work, we have made a detailed study of substitution effect on the electronic structure of FeSi. Systematic substitution effects have been observed not only in the pseudogap around EF but also on a higher energy scale, which was not possible to identify in the spectra of Fe1−xCoxSi because of the differences between Fe 3d- and Co 3d-related structures. 8.2 Experimental FeSi1−xAlx (x = 0.00, 0.02, 0.05, 0.10, and 0.30) polycrystals were prepared by arc melting Fe, Si, and Al in an argon atmosphere [8.8]. An excess Si and Al of ∼ 5% was incorporated in the starting stoichiometry. The samples were checked to be single phases by powder x-ray diffraction analysis. The electrical resistivity of the prepared samples are shown in Fig. 8.2 [8.8]. The x = 0.02, 0.05, and 0.10 samples exhibit semiconducting behavior above the resistivity maxima at ∼ 35 K, ∼ 70 K, and ∼ 95 K, respectively, with almost the same activation energy as that of pure FeSi [8.8]. At lower temperature, the resistivity decreases on cooling down as in a metal. The resistivity of the x = 0.30 sample decreases monotonously on cooling below room temperature. We have performed PES measurements on these samples using a newly developed PES systems with a VG He lamp and an Omicron EA 125 HR analyzer. The advantage of this system compared with the previous one, which was used for the measurements
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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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Page 85 and 86: 6.3. Results and Discussion 81 6.3
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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
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Page 127: 8.4. Summary 123 temperature magnet
Page 130 and 131: 126 References [8.14] T. Tonogai, A
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Page 134 and 135: 130 Chapter 9. Conclusion (a) FeSi
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