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

4.3. Results and Discussion 53
Intensity
LuB 12
Yb 0.5 Lu 0.5 B 12
YbB 12
hν = 21.2 eV
80 40 0 -40
Binding Energy (meV)
Figure 4.7: Upper panel: He I spectra of Yb1−xLuxB12 and YbB12. The solid curves
have been obtained by convoluting the curves in the lower panel with a Gaussian. Lower
panel: the assumed DOS multiplied by the Fermi distribution function.
by ∆ = π � B
B 0 ρ(ε)|V (ε)|2dε, where ρ(ε) is the conduction-band DOS and V (ε) is
the hybridization matrix element between the f and conduction electrons [4.21]. In
Yb1−xLuxB12, Lu-substitution may change two parameters of the model: (i) electron
doping raises the Fermi level and thus reduces ε0 f; (ii) the recovery of the B sp band
DOS ρ(ε) around EF increases ∆. With these changes in ε0 f and ∆, δ increases and
hence ¯nf decreases according to Eq.(4.1), in qualitative agreement with the PES results.
However, the calculations to lowest order in 1/Nf deal with only the unoccupied side
of the conduction band for Yb compounds and are not influenced by the change in the
conduction-band DOS below EF as observed in the He I spectra. Also, the line shape
of the Kondo peak cannot be analyzed with lowest-order calculations, which necessarily
give a single and hence symmetric peak below EF .
In order to consider the effect of changes in the B sp band near EF on the f-electron
spectra, we have calculated the 4f photoemission spectra at T = 0 K to second order in
1/Nf [4.23]. A flat DOS of 0.3 eV width is considered for the conduction band with EF