Thesis High-Resolution Photoemission Study of Kondo Insulators ...
Thesis High-Resolution Photoemission Study of Kondo Insulators ...
Thesis High-Resolution Photoemission Study of Kondo Insulators ...
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
low-temperature and low-energy electronic structure <strong>of</strong> FeSi and recovers the DOS in<br />
the vicinity <strong>of</strong> EF , consistent with the optical conductivity and the dc conductivity.<br />
A systematic substitution effect on FeSi has been studied thoroughly in FeSi1−xAlx.<br />
The depression in the vicinity <strong>of</strong> the Fermi level (∼ −20 meV) is filled with a small<br />
amount <strong>of</strong> Al substitution (Fig. 9.2 (b)) and temperature increase. The pseudogap<br />
region (∼ −50 meV) depends on the temperature and the Al substitution (Fig. 9.2<br />
(c)). The shoulder around ∼−100 meV, which may correspond to the <strong>Kondo</strong> peak in<br />
4f-electron systems, depends on the Al substitution up to x = 0.30 (Fig. 9.2 (b)) and<br />
such substitution dependence <strong>of</strong> the shoulder is enhanced at higher temperature. The<br />
broad peak at ∼−400 meV is stable against both temperature and substitution.<br />
A clear difference between YbB12 and FeSi appears in the substitution dependence.<br />
The disappearance <strong>of</strong> the gap has been observed at x ∼ 0.25-0.5 in Yb1−xLuxB12 and<br />
x ∼ 0.1 in FeSi1−xAlx. These critical concentrations (xc) are consistent with the transport<br />
properties. The rapid collapse <strong>of</strong> the gap with substitution indicates the importance<br />
<strong>of</strong> the intersite interaction for the gap opening in FeSi. While the gap in<br />
Yb1−xLuxB12, which survives up to x ∼ 0.25-0.50, reflects the localized character <strong>of</strong><br />
the 4f states, the x-nonlinearity <strong>of</strong> the substitution dependence <strong>of</strong> the photoemission<br />
spectra indicates an effect <strong>of</strong> the intersite interaction in the gap formation for YbB12.<br />
The difference in xc might be one <strong>of</strong> the origin <strong>of</strong> the relatively high residual DOS for<br />
FeSi: At and around the scraped surface, where the atomic coordination is not perfect,<br />
the gap character would be more robust for YbB12.<br />
The relation between the peak position in the photoemission spectra and Tmax, where<br />
the magnetic susceptibility shows a maximum, is different between YbB12 and FeSi.<br />
The sharp <strong>Kondo</strong> peak, which is observed in the Yb <strong>Kondo</strong> systems, does not appear in<br />
FeSi. However, the substitution dependence <strong>of</strong> the spectra <strong>of</strong> FeSi at low temperatures<br />
resembles that <strong>of</strong> YbB12 and is reminiscent <strong>of</strong> the hybridization gap model.<br />
129