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

10 Chapter 1. Introduction
direction. Among the 3d transition-metal monosilicides, CrSi, MnSi, FeSi, CoSi, and
NiSi form the B20-type structure [1.16]. They show a variety of magnetism: MnSi
shows a helical magnetic order below ∼ 30 K, CoSi is a diamagnet, and FeSi is a narrow
gap insulator which shows a gradual transition into a paramagnetic metal with
increasing temperature. The semiconducting behavior of FeSi appears in the electrical
resistivity and various spectroscopies (tunneling [1.17], IR [1.18, 19], photoemission
[1.20–23], and Raman [1.24]). From these measurements, the activation energy of FeSi
has been estimated to be 30-50 meV [1.16]. Angle-integrated PES measurements have
already been performed [1.20, 21], but the interpretation of spectra has not been established.
Therefore, we have studied not only the temperature dependence but also the
substitution dependence of the photoemission spectra of Fe1−xCoxSi and FeSi1−xAlx
to understand the electronic structure of FeSi. We have used the He I resonance line,
which mainly probes the Fe 3d orbitals [1.7], as an excitation light source. The effect of
Co substitution at the Fe site and Al substitution at the Si site would appear different
in photoemission spectra and response of the gap to such substitution is expected to
reveal the character of the gap in FeSi.
Figure 1.8: Crystal structure of FeSi. The position coordinates are (u, u, u), ( 1 1 + u, 2 2 −
u, ū), (ū, 1 1
1
1
+ u, − u), and ( − u, ū, + u) with u(Fe) = 0.1358 and u(Si) = 0.844
2 2 2 2
[1.25]. The parameters u(Fe) = 0.25 and u(Si) = 0.75 give the rocksalt structure.
Figure 1.9 shows the magnetic susceptibility of FeSi. With increasing temperature,
the magnetic susceptibility rises exponentially to show a broad maximum at ∼ 500
K and then follows a Curie-Weiss law at higher temperatures. Band-structure calculation
gives an almost correct gap size, but the bands at the both side of the gap