Quantum metallicity in a two-dimensional insulator

One of the most far-reaching problems in condensed-matter physics is to understand how interactions between electrons, and the resulting correlations, affect the electronic properties of disordered two-dimensional systems. Extensive experimental (1-6) and theoretical (7-11) studies have shown that interaction effects are enhanced by disorder, and that this generally results in a depletion of the density of electronic states. In the limit of strong disorder, this depletion takes the form of a complete gap(12,13) in the density of states. It is known that this `Coulomb gap' can turn a pure metal film that is highly disordered into a poorly conducting insulator(14), but the properties of these insulators are not well understood. Here we investigate the electronic properties of disordered beryllium films, with the aim of disentangling the effects of the Coulomb gap and the underlying disorder. We show that the gap is suppressed by a magnetic field and that this drives the strongly insulating beryllium films into a low-temperature `quantum metal' phase with resistance near the quantum resistance R-Q = h/e(2), where h is Planck's constant and e is the electron charge.