Superconductivity in a two-dimensional electron gas

In a series of recent experiments, Kravchenko and colleagues(1,2) observed unexpectedly that a two-dimensional electron gas in zero magnetic field can become conducting at low temperatures: the two-dimensionality was imposed by confining the electron gas to the interface between two semiconductors. The observation of this conducting phase is surprising, as the conventional theory of metals precludes the existences of a metallic state at zero temperature in two dimensions(3). Nevertheless, there are now several experiments confirming the existence of the new conducting phase in dilute two-dimensional electron gases in zero magnetic field(4-7). Here we argue, on the basis of an analysis of these experiments and general theoretical grounds, that this phase is in fact a superconductor with an inhomogeneous charge density.