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

12 Chapter 1. Introduction
activation gap. For YbB12 that ratio is as large as ∼ 2 [1.30] while ∆opt is just the
same as 2∆act for FeSi. Within the hybridization gap model, a larger difference in the
dispersion slope between the two hybridizing bands would lead to a large optical gap
of ∆opt > 2∆act as shown in Fig. 1.2 (a). The difference in xc reflects the importance
of intersite interaction for FeSi and the localized character of the Yb 4f states.
Table 1.2: Characteristic temperature or energy scales for YbB12 and FeSi: Peak position
in the magnetic susceptibility (Tmax), activation energy determined by resistivity
measurements (∆act), the gap in the optical conductivity (∆opt), the temperature at
which the gap appears in the optical conductivity (Tgap). xc is the critical substi-
tution content at the magnetic-ion site to eliminate the semiconducting behaviors in
Yb1−xLuxB12 and in Fe1−xCoxSi.
Tmax (K) 2∆act (meV) ∆opt (meV) Tgap (K) xc
YbB12 75 [1.13] 12 [1.13] 25 [1.30] 70 0.25-0.50 [1.14]
FeSi 500 [1.26] 60 [1.18] 60 [1.18] 200-250 ∼ 0.1 [1.31]
To discuss the physics of FeSi connected with the 4f Kondo insulator, it should
be noted that the Kondo effect appears mostly in dilute magnetic-impurity systems
for transition-metal compounds3 . The role of Kondo effect in the paramagneticnonmagnetic
transition is not clear in FeSi. On the contrary, such a single-site effect
appears in the photoemission spectra of many Ce- and Yb-based Kondo metals.
The following Chapters are organized as follows: We describe the principles and
techniques of PES in Chap. 2 and then present the results of the photoemission study
of YbB12 and FeSi in Chaps. 3-8. Chapters 3 and 4 deal with the electronic structures
of YbB12 and Yb1−xLuxB12 at low temperature (30 K). There we have observed both
the Yb 4f and B 2sp-Yb 5d-derived conduction-band states by using various excitation
energies of photons. Chapters 5 and 6 focus on the temperature dependence of the
conduction-band states. We present the results of Fe1−xCoxSi in Chap. 7 and those of
FeSi1−xAlx in Chap. 8. The results of YbB12 and FeSi are compared in the last Chapter.
3 Recently, S. Kondo et al. has reported a heavy Fermi liquid behavior in LiV2O4 [1.32].