Spins in Si: P Close to the Metal-Insulator Transition

The spin-lattice relaxation time $T_1$ of ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ nuclei is measured near the metal-insulator transition as a function of temperature and magnetic field. The observed relaxation rate in the metal is up to ${10}^3$ times larger than that for nuclei interacting with free, degenerate electrons. The enhanced relaxation rate and its magnetic field dependence suggest the existence of intrinsic, quasistatic spins with an ${\omega }^{-1\mathrm{}}$ spectrum. These spins may cause strong spin-flip scattering and may therefore be responsible for the anomalous critical exponent of the electrical conductivity.