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

104 Chapter 7. Temperature and Co-Substitution Dependence of the ...
lowest panel of Fig. 7.6. 1 The spectral DOS thus obtained for FeSi and Fe1−xCoxSi
are shown in Fig. 7.6. One can see that the energy range where the DOS vary with
temperature is between EF and ∼ 50 meV below it, consistent with the data taken
at room temperature and ∼10 K reported by Breuer et al. [7.8] The size of this wide
dip or “pseudogap” is almost independent of the Co content. The pseudogap is wide
enough compared with the instrumental resolution and therefore the present approach
of dividing the spectra with the Fermi-Dirac distribution function is valid to discuss
this structure. From the present PES spectra, however, one cannot conclude where the
missing spectral weight in the pseudogap region is transferred. Here, it should also be
noted that spectral weight around EF remains substantially high at all temperatures,
contrasted with the optical conductivity data, in which almost all the spectral weight
is depleted below ∼−60 meV at low temperatures. Comparing the data from single
crystals and polycrystals, Breuer et al. [7.8] proposed that the surface might be metallic
and contribute the observed spectral weight at and around EF .
In Fig. 7.6 we note that the recovery of the DOS with temperature and with Co
substitution is dependent on the energy position: we show in Fig. 7.7 the temperature
dependence of the DOS at EF and 30 meV below it. Plotted are the intensities relative
to the values at room temperature. The DOS at EF of pure FeSi increases with temperature
more rapidly than those of Co-substituted samples. On the other hand, such
a difference between the pure and Co-substituted samples is absent at −30 meV. This
has already been suggested in Fig. 7.4, which shows that the Co substitution affects
mainly the vicinity of EF . Here, we note that the recovery of the spectral function both
at EF and 30 meV below it starts between 225 K and 150 K, low enough temperatures
compared with the gap size. This is consistent with the temperature dependence of
the optical conductivity, where the gap feature disappears above ∼ 200 K. The bottom
panel of Fig. 7.7 shows the dc conductivity of the samples studied here normalized to
the values at room temperature. (The room-temperature conductivity of pure FeSi is
∼ 75 % of those of the x = 0.05 and x = 0.10 samples.) Like the DOS at EF , the
dc conductivity depends on Co content mainly below ∼ 200 K. Thus we can state
1 On the other hand, one cannot discuss sharp structures compared with the energy resolution from
these DOS thus obtained although sharp structures sometimes appear as spurious features. We have
made extensive simulations for various DOS line shapes and temperatures in order to see to what
extent the present method is reliable in extracting the original DOS. Figure 7.6 indicates temperaturedependent
spectral changes in the wide energy range but from this figure alone one cannot exclude the
possibility that a sharp gap compared with the energy resolution is opened near EF .