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

34 Chapter 3. Photoemission Study of YbB12 with Variable Photon Energies
3.3 Results and Discussion
Figure 3.2 shows the valence-band spectrum taken with the photon energy of 125 eV,
for which the Yb 4f contribution is dominant [3.14]. The spectral features in the range
from 4 eV to 13 eV are assigned to the 4f 13 → 4f 12 multiplet structure [3.15].
Intensity (arb. units)
Yb 3+
4f 13 → 4f 12
Yb 2+
4f 14 → 4f 13
YbB12 hν = 125 eV
surface
bulk
16 12 8 4 0
Binding Energy (eV)
Figure 3.2: Valence-band spectrum of YbB12 in a wide energy range. For the 4f 13 →
4f 12 part, the calculated multiplet structure [3.15] is also shown.
Between EF and 4 eV are observed 4f 14 → 4f 13 transitions. As shown in Fig. 3.3,
they consist of two sets of 4f 13 spin-orbit doublets. The sharper doublet closer to EF
originates from Yb atoms in the bulk and the broader one away from EF from divalent
Yb atoms on the surface [3.10, 11]. The surface and bulk signals are represented by
Gaussians and Mahan’s asymmetric line shapes [3.16], respectively, as shown in the figure.
The Yb valence in the bulk estimated from the intensity ratio of the 4f 13 → 4f 121
1 13 12 We have evaluated the intensity of the f → f signal not by integrating the signal from −4
to −12 eV but by comparing the measured line shape with an appropriately broadened f 13 → f 12
multiplet structure. The extra intensity can be attributed to non-negligible B 2s contribution, on the
basis of the atomic photoionization cross sections of the B 2s and Yb 4f orbitals at hν = 125 eV
[3.14] and the calculated B s partial density of states [3.20]. Due to the ambiguity in decomposing