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

8 Chapter 1. Introduction
only the narrow gap at low temperature, but also the temperature dependence of the
magnetic susceptibility is reminiscent of the 4f Kondo insulator [1.1] although FeSi has
no f electrons. Photoemission spectra of FeSi may therefore resemble those of YbB12
in some respects but may not in others. The similarities and differences between YbB12
and FeSi would give an important information on the physics of these compounds,
including the contribution of single-site and intersite interactions.
1.2 YbB12 and FeSi
The semiconducting behavior of YbB12 was first reported by Kasaya et al. [1.4]. Its
properties have often been discussed together with another rare-earth-boride semicon-
ductor SmB6, but the simplicity of the 4f-level occupancy for Yb (4f 13 and 4f 14 ), which
is the same as that for Ce (4f 1 and 4f 0 ) considering the electron-hole symmetry, is a
remarkable advantage of studying YbB12. An Yb atom in YbB12 is almost a Kondo ion
(Yb 3+ ) while a Sm atom in SmB6 has an intermediate-valent character (Sm 2.6+ ). RB12
(R = late rare-earth element) forms a NaCl-type cubic structure which consists of the
rare-earth element and the B12 polyhedron (UB12 structure) as shown in Fig 1.6. In the
B12 polyhedron, 38 electrons fill the bonding orbitals [1.10]. Since three electrons (2s2 ,
2p1 ) per B atom contribute to the bonding, B12 becomes closed-shell for the divalent
anion (B 2−
12 ). Thus if R is a divalent cation, RB12 would be an insulator or a semimetal.
The magnetic susceptibility of YbB12 is shown in Fig. 1.7. It follows the Curie-
Weiss law above ∼ 170 K and Iga et al. has estimated the Yb valence to be ∼ 2.85
from the Curie constant [1.12]. On cooling, it decreases rapidly after showing a broad
maximum around 75 K. Since the bonding orbitals of the B12 polyhedron are filled by
two electrons per Yb atom, nearly one electron per Yb atom is expected to contribute
to the conductivity. However, the electrical resistivity of YbB12 rises sharply below 50
K with the activation energy of 6 meV [1.13]. Lu (4f 14 ) compounds are often good
references for Yb compounds just like La (4f 0 ) compounds for Ce compounds. LuB12,
as well as alloy system Yb1−xLuxB12, has been studied as a reference of YbB12 [1.14].
In LuB12, the Lu 4f level, which forms the closed-shell 4f 14 configuration, is located ∼
8 eV below EF [1.15].
On the other hand, the study of FeSi has a long history of more than a half century.
FeSi forms a B20-type cubic structure as shown in Fig. 1.8. The B20-type structure
is a modified NaCl structure, where both Fe and Si atoms are shifted along the (111)