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

6 Chapter 1. Introduction
peaks, which would rather reflect the atomic character, but also the Kondo peak is
mostly reproduced by the single-site model. However, Arko et al. reported dispersions
in angle-resolved photoemission spectra of the f levels in Ce and U compounds both
above and below TK [1.6] and showed the importance of the effect beyond the single-site
model.
13 12
4f 4f
1 2
(4f 4f )
14 13
4f 4f
0 1
(4f 4f )
E F
13 14
4f 4f
1 0
(4f 4f )
PES IPES
(IPES)
(PES)
Figure 1.3: Schematic illustration of the 4f spectral DOS for mixed-valent Yb (or
Ce) compounds, which is observed by photoemission (PES) and inverse photoemission
spectroscopy (IPES).
The initial motivation of our angle-integrated photoemission work was to study
how the intersite effect and the single-site effect, both of which are expected to play
important roles in the Kondo insulator, appear in photoemission spectra. Photoemission
spectroscopy (PES) is one of the most direct probes to study electronic structures
among various spectroscopies. A remarkable advantage of PES applied for rareearth
compounds, where the f and conduction electrons coexist, is that one can utilize
the photon-energy dependence of photoionization cross sections [1.7] to probe specific
orbitals with variable photon energies. Angle-integrated PES gives the momentumintegrated
spectral function, i.e., the spectral density of states (DOS), truncated by the
Fermi-Dirac distribution function. Since PES probes the occupied side of the Fermi
E