Superconducting "metals" and "insulators"

We propose a characterization of zero-temperature phases in disordered supe rconductors on the basis of the nature of quasiparticle transport. In three -dimensional systems, there are two distinct phases in close analogy to the distinction between normal metals and insulators: the superconducting "met al'' with delocalized quasiparticle excitations and the superconducting "in sulator" with localized quasiparticles. We describe experimental realizatio ns of either phase, and study their general properties theoretically. We su ggest experiments where it should be possible to tune from one superconduct ing phase to the other, thereby probing a novel ''metal-insulator'' transit ion inside a superconductor. We point out various implications of our resul ts for the phase transitions where the superconductor is destroyed at zero temperature to form either a normal metal or a normal insulator.