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

4.2. Experimental 47
Figure 4.2: Magnetic susceptibility per mole rare earth (Yb1−xLux) ofYb1−xLuxB12
[4.11].
showing that the gap disappears only for a large amount of Lu-substitution. The Lu 4f
level forms the closed-shell 4f 14 configuration and is located well below EF [4.12]. Thus
in the first approximation, the extra electrons added by Lu-substitution are trapped
in the Lu 4f level and do not contribute to the transport properties. Nevertheless the
Lu-substitution leads to the doping of a small number of conduction electrons because
the valence of Lu (3+) is somewhat larger than that of Yb (∼ 2.86+) [4.4].
4.2 Experimental
Polycrystalline samples of Yb0.5Lu0.5B12 and LuB12 were prepared by reducing Yb2O3
and Lu2O3 at 2200 ◦ C. The LuB12 samples contained a trace amount (∼3%) of Yb.
PES measurements were performed using He resonance lines (He I: hν = 21.2 eV and
He II: 40.8 eV) and synchrotron radiation (hν = 125 eV). The latter measurements
were made at beam line BL-3B of the Photon Factory, National Laboratory for High
Energy Physics. The Fermi edge of Au film evaporated on the sample surface after
each series of measurements was used to determine the Fermi level (EF ) position and
the instrumental resolution. The resolution was 21-25, ∼28 and ∼55 meV for He I,
He II and hν = 125 eV, respectively. All the measurements were done at ∼ 30 K.
The base pressure of the spectrometer was ∼ 5 × 10−11 Torr for the He I and He II