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

6.1. Introduction 79
Figure 6.2: Magnetic susceptibility of Yb1−xLuxB12 (x = 0.00, 0.25, 0.50, 0.75, and
1.00) single crystals [6.3].
the single-site model or the itinerant picture. One can understand the mechanism of
the gap opening within the single-site model if no substantial changes in the spectra
appear by controlling the distances between the Kondo ions (Yb atoms) after having
normalized the spectral intensities to the Yb content. In that case the depression of
the spectral DOS of Yb1−xLuxB12 at the Fermi level (EF ) would be simply a linear
combination of the pseudogap of YbB12 and the Fermi edge of LuB12. On the other
hand, the effect of coherence in the YbB12 lattice would appears as sensitive changes
in the photoemission spectra with a small Lu substitution. Note that, in the case of
FeSi, only the spectral DOS in the vicinity of EF is sensitive to a small substitution,
indicating that the coherence in the periodic lattice plays an important role in the
low-energy properties of FeSi, as will be discussed in the following Chapters.
(ii) By comparing the spectra of YbB12 with those of the substituted samples, we
can discuss details of the spectral line shape of YbB12. Effects of the non-periodicity
and defects in the lattice at the surface prepared by scraping would be reflected in the
ultraviolet photoemission spectra. One can extract intrinsic signals of YbB12 using the
spectra of the Lu-substituted samples as references. It should be remembered that the
residual DOS is observed in the photoemission spectrum of YbB12 at low temperature
while a fully open gap is observed by tunneling spectroscopy [6.6] and optical reflectivity