Localized magnetic moments in Si:P near the metal-insulator transition

We report systematic specific-heat measurements of crystalline Si doped with P in the concentration range enclosing the metal-insulator transition, in the temperature range from 40 mK to 3 K and in magnetic fields up to 7 T. The data can be interpreted in terms of a phonon contribution, a term linear in T due to itinerant electrons, which persists even on the insulating side, and an excess contribution {Δ}C varying sublinearly with temperature. The strong magnetic field dependence of {Δ}C suggests an interpretation in terms of local magnetic moments. We discuss a theoretical model for the metallic phase, which provides a quantitative description of the concentration of local moments. We show that the temperature power law in zero field can be explained by the Kondo effect.