We have studied the transport properties of disordered WSi films near the metal/insulator transition (MIT) and we have also reviewed the data for several other disordered materials near their MIT. In all cases, we found the presence of enhanced superconductivity. We constructed a superconductivity “phase diagram” (i.e., $T_c$ versus $\sigma$) for each system, which reveals a striking correlation: In all cases, $T_c$ values are significantly enhanced only for samples whose conductivities lie within a narrow range on the metallic side of, and moderately near, the MIT. We present a heuristic model to explain this phenomenon.

• Received 26 March 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.197004