X-ray Absorption Study of Amorphous Metal Semiconductor Alloys $M_x Si_{1-x} $ (M: Gd,Y) Near the Metal Insulator Transition

X-ray absorption structure (XAS) at both Si K edges and Gd M edges were measured at compositions close to the metal insulator transition (MIT) for amorphous $Gd_x Si_{1-x} $ (x = 0.11 - 0.21) and $Y_x Si_{1-x} $ (x=0.13) from 10-300K. Spectral lineshape is unchanged as a function of composition, despite the presence of the MIT at x = 0.14. Comparison with calculations indicates that Gd is in the 3+ state for all compositions and temperatures measured. An anomalous temperature dependent absorption is seen below approximately 70K; the energies of the absorption peaks are unaffected, indicating no change in valence, but the absolute magnitude of absorption is temperature dependent for both K and M edges, up to 40 eV from the edges. This temperature dependence is related to changes in the nature of the conduction band states, specifically a transfer of weight from Si p-states to more localized Gd p-states. However similar shifts in the magnitude of the Si K edge are found in the non-magnetic analog system $Y_x Si_{1-x} $. Thus this transfer cannot be solely related to the magnetically-dependent localization phenomena previously observed in $Gd_x Si_{1-x} $, and we argue that it is related to electronic correlation effects present in both systems.