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

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.