  Metallic phases have been observed in several disordered two dimensional (2d)
systems, including thin films near superconductor-insulator transitions and
quantum Hall systems near plateau transitions. The existence of 2d metallic
phases at zero temperature generally requires an interplay of disorder and
interaction effects. Consequently, experimental observations of 2d metallic
behavior have largely defied explanation. We formulate a general stability
criterion for strongly interacting, massless Dirac fermions against disorder,
which describe metallic ground states with vanishing density of states. We show
that (2+1)-dimensional quantum electrodynamics (QED$_3$) with a large, even
number of fermion flavors remains metallic in the presence of weak scalar
potential disorder due to the dynamic screening of disorder by gauge
fluctuations. We also show that QED$_3$ with weak mass disorder exhibits a
stable, dirty metallic phase in which both interactions and disorder play
important roles.
