Role of Ag doping in Ba8Si46 compounds -: art. no. 014511

The silicon clathrate compound Ba8Si46 shows superconductivity below the critical temperature (T-c) of 8 K, and the T-c decreases monotonically with doping Ag. In order to reveal effects of Ag doping on the electronic states, we have applied soft x-ray photoemission spectroscopy to Ag-doped silicon clathrate compounds Ba8AgxSi46-x (x=0,1,3,6). The valence band photoemission spectra show that a Ba 5d-derived state at the Fermi level (E-F), which is prominently observed in Ba8Si46, decreases with increasing Ag content. The reduction in the peak intensity at E-F with increasing Ag content is therefore in accord with the decrease of T-c in Ba8AgxSi46-x. Band structure calculation using local-density approximation reproduces the observed valence band spectra of x=0 and 6. The Si 2p and Ba 4d core-level photoemission spectra demonstrate that the valence electron of Si is attracted to the Ag site in x=1 and the 5d electron of Ba inside the Si-24 cage is further donated to Ag in x >= 3. Hence, Ag doping leads to the reduction of the peak at E-F.