Spin Dynamics of Nearly Localized Electrons

The electron-spin-resonance signal of metallic Si:P near the metal-insulator transition has been measured down to a temperature of 30 mK. The paramagnetic spin susceptibility and the resonance linewidth are found to increase sharply with decreasing temperature. We argue that these effects are due to the enhancement of spin fluctuations, and the accompanying slowing down of spin diffusion near the metal-insulator transition. The results are compared with the predictions of recent theories for the susceptibility and linewidth of disordered interacting-electron systems.