Superconductivity in YbGa$_x$Si$_{2-x}$ with the AlB$_2$-type structure

The discovery of superconductivity in MgB$_2$ has triggered the search for novel superconductors with the AlB$_2$-type structure. Very recently, we discovered superconductivity in the AlB$_2$- type compound YbGa$_{1.15}$Si$_{0.85}$ below $T_{\rm C}$ = 2.5K $^{*}$. We report here on the structural and electronic properties of the YbGa$_x$Si$_{2-x}$ series. XRD and SEM/EDS analysis suggested that the AlB$_2$-type phase can be sustained for $1.0 \leq x \leq 1.4 $. $T_{\rm C}$ is found to decrease from 2.5K for $x$ = 1.0 to 1.9K for $x$ = 1.3 and eventually vanish for $x$ = 1.4. High-resolution x-ray absorption spectra across the Yb-L$_{III} $ edge were measured at SPring-8 on the beamline BL15XU. The valence of Yb was estimated to be 2.3+, suggesting a predominant Yb$^{2+}$ character with minor importance of 4f electrons in the superconductivity.\newline $^{*}$ M. Imai et al., submitted.