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
and theoretical 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 formof a complete gap 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, but the properties of these insulators are not well
understood. Here we have investigated the electronic properties of
disordered beryllium flms, 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 a strongly insulating beryllium film into a low-temperature
"quantum metal" phase with resistance near the quantum resistance
Rq = h/e^2, where h is Planck's constant and e is the electron
charge. This behavior is shown on the left in a 1.7 nm thick Be film. The film was of too high of a resistance to superconduct.
The low-temperature magnetotransport characteristics of the Be films are most effectively summarized using the schematic phase
diagram on the left. The x-axis measures the amount of disorder,
which we parametrize as 1/t where t is the film thickness. The y-axis
is the applied magnetic field. The weakly temperature-
dependent, high-field state is depicted in yellow and essentially
behaves as a weak metal. Thus the yellow region in Fig. 5 describes films of varying disorder but with resistance near Rq. The almost
ideal morphological characteristics of Be films, along with the fact
that Be is intrinsically non-magnetic, lead us to believe that the
general features of the phase diagram are generic to high-density, disordered, 2D
electron gases.
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