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

6.3. Results and Discussion 89
of EF . Under this normalization, the DOS at − 50 meV and − 80 meV are almost
identical and both decrease slightly on cooling for all samples.
DOS(T) / DOS(300 K)
0
- 0 meV
- 20 meV
- 50 meV
- 80 meV
100
T (K)
Yb 1-x Lu x B 12
200
x = 0.00
x = 0.25
x = 0.50
x = 0.75
x = 1.00
Figure 6.10: Temperature-dependent DOS of Yb1−xLuxB12 at various energies. We
have re-normalized the DOS to the values at 300 K.
We subtracted the DOS at 7 K from those at 300 K for each x in Fig. 6.11. As
is visible in Fig. 6.9, the depressed DOS is always positive under the normalization
in Fig. 6.9. At − 80 meV, the depressed DOS decreases as Yb content increases corresponding
to the evolution of the broad peak. The x dependence at − 50 meV is
substantially the same as that at − 80 meV. The depression is much stronger at − 20
meV but its response against x resembles that at − 50 meV and − 80 meV. However,
the x dependence at EF is qualitatively different from those below − 20 meV: The
depression at EF is identical to that at − 20 meV for x ≥ 0.5 but the depression at
EF is remarkably enhanced for x ≤ 0.25. This indicates that the sharp (pseudo)gap,
which opens less than 20 meV from EF and governs the low-temperature transport and
thermodynamic properties of YbB12, disappears between x = 0.25 and 0.50. Such a
nonlinear response at EF against x would suggest that the effect beyond the single-site
mechanism plays an important role in the low-energy electronic properties in YbB12
300