Termodynamic behaviour near a metal-insulator transition.

We have measured the low temperature specific heat of phosphorus- doped silicon for densities near the metal-insulator (MI) transition. We find that the specific heat is enhanced over the conduction band itinerant electron value. The enhancement increases toward lower temperatures but is less than the corresponding susceptibility enhancement. The data are compared with various theoretical models, and the large susceptibility to specific heat ratio points to the emergence of localized spin excitations as the MI transition is approached from the metallic side.