Approaching the metal-insulator transition

The problem of disordered interacting electrons is considered. We study a model for a generic disordered Fermi liquid without cooper pairs and in the absence of any spin-flip mechanisms. We prove to within logarithmic accuracy that there is a stable renormalization-group fixed point to all orders in a loop expansion. We prove that the conductivity exponent at this fixed point is identically equal to zero, while the spin-diffusion constant scales to zero with an exponent γ=4+O(d-2). By an explicit two-loop calculation we then show that this fixed point is suppressed by logarithmic terms, but a sizeable scaling region persists. The main conclusion is that the metal-insulator transition is preceded on the metallic side by a near instability in the spin system. There is a scaling region leading from a spin-diffusion phase to a region of very slow spin transport. Throughout this scaling region the charge transport is unaffected. Experiments are discussed in the light of these results, and further experiments to test the theory are proposed.