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

Chapter 7
Temperature and Co-Substitution
Dependence of the Photoemission
Spectra of FeSi
We have studied the effect of varying temperature and Co substitution on the lowenergy
electronic structure of FeSi by photoemission spectroscopy. We have observed
that the density of states (DOS) of FeSi is reduced at low temperatures over a wide
energy range of ∼ 50 meV below the Fermi level (EF ), which is the energy scale observed
by the magnetic, transport and optical measurements. The reduced DOS is completely
recovered with increasing temperature but only partly, i.e., predominantly near EF ,
with Co substitution, again consistent with the transport properties.
7.1 Introduction
Among various transition-metal compounds, FeSi shows unique magnetic and transport
properties which are sometimes discussed in connection with the characteristics of
narrow-gap 4f-electron systems or Kondo insulators [7.1]. At low temperatures FeSi is
a nonmagnetic semiconductor. With increasing temperature the magnetic susceptibility
rises exponentially to reach a broad maximum at ∼ 500 K and follows a Curie-Weiss
law at higher temperatures [7.2]. The electrical resistivity falls more than three orders
of magnitude with increasing temperature up to ∼ 300 K [7.3] but rises slowly above
∼ 300 K like a metal. In recent years, the temperature-dependent electronic structure
of FeSi has been studied extensively by spectroscopic methods [7.4–14]. Schlesinger
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