Electronic and vibrational density of states through the metal-insulator transition in amorphous yttrium-silicon alloy thin films

Abstract

We present specific-heat measurements from 3–100 K of amorphous yttrium-silicon alloy films with compositions spanning the metal-insulator transition. In samples near or above the metal-insulator transition, we observe an electronic contribution to specific heat. Comparison to undoped amorphous silicon grown by the same technique allows a quantitative analysis of the vibrational modes associated with the heavy dopant atoms. The dopant atoms add nonpropagating vibrational modes to the amorphous-silicon matrix that can be modeled with Einstein modes. Near the metal-insulator transition, an additional contribution to specific heat appears that is most pronounced on the metallic side. We explore two possible explanations for this excess heat capacity: a significant change in the vibrational modes or an origin related to the correlated-electron physics of the metal-insulator transition.