NEW PHASE OF DISORDERED FERMI SYSTEMS

For systems without impurity spin-flip scattering we show that the metal-insulator transition can be preceded on the metallic side by a phase transition involving only the spin degrees of freedom. This new phase transition is characterized by a vanishing spin diffusion coefficient, a diverging spin susceptibility, a logarithmic anomaly in the specific heat, and an uncritical electrical conductivity. The phase transition can be obtained both by a renormalization-group analysis, and by solving integral equations that resum perturbation theory. We argue that the resulting critical exponents can be calculated exactly for all dimensions. The results of the theory are in good agreement with experiments on Si:P.