Electronic structure of YbGa_1.15Si_0.85 and YbGa _xGe_2-x probed by resonant x-ray emission and photoelectron spectroscopies

We performed an x-ray spectroscopic study combining resonant x-ray emission spectroscopy (RXES) and photoelectron spectroscopy on the superconducting ternary silicide YbGa_1.15Si_0.85 and nonsuperconducting ternary germanide YbGa_xGe_2-x (x=1.0 and 1.1). The Yb valence for all three compounds is found to be about 2.3. In YbGa _1.15Si_0.85 no temperature dependence of the Yb valence is observed in the RXES spectra in the temperature range of 7-300 K, while the valence shows a drastic increase under pressure from the Yb2⁺ state partially including itinerant electrons to the localized Yb3⁺ state. Differences are observed in the valence-band spectra of the photoelectron spectroscopy between YbGa_1.15Si_0.85 and YbGa _xGe_2-x, which may be attributed to the difference of crystal structure. We conclude that both the crystal structure of the planar GaSi layer in YbGa_1.15Si_0.85 and the resultant electronic structure may have a crucial role in the occurrence of superconductivity.