Susceptibility of local magnetic moments in phosphorus-doped silicon near the metal-insulator transition

We report on a systematic investigation of the paramagnetic susceptibility $\chi_{\rm loc}(T)$ of local moments in crystalline P-doped silicon in a wide concentration range encompassing the metal-insulator transition (MIT) for temperatures between $30\,{\rm mK}$ and $1\,{\rm K}$. The Bhatt-Lee model of a hierarchy of antiferromagnetically coupled spin-pairs allows a consistent description of the thermodynamic properties far on the insulating side of the MIT as shown by comparison of $\chi_{\rm loc}(T)$ and specific heat measured on nearly identical samples, but fails on the metallic side, where presumably the Kondo effect is operative. Our data put strong constraints on the theory of localized moments on the metallic side of the MIT