Effect of Spin-Orbit Interaction on the Metal-Insulator Transition in Doped Silicon

We show that in disordered metallic systems, spin-orbit interactions lead to the existence of two metallic phases separated in temperature. The low-temperature metallic phase arises from spin-orbit interactions. The crossover temperature is proportional to (Z — Z')2 (Z is the atomic number of the donor and Z' that of the matrix) and is expected to be observable for donors with large atomic numbers. The presence of two metallic phases implies two mobility edges, existing at different temperatures, and affecting the conductivity when the Fermi energy E_Flies below the mobility edge Ec. We thus predict anomalies in the conductivity in this range of concentration, consistent with the measurements of Long and Pepper on Si: Sb.