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

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