ELECTRON CORRELATIONS AND COULOMB GAP IN A 2-DIMENSIONAL ELECTRON-GAS IN HIGH MAGNETIC-FIELDS

We apply tunnel spectroscopy to investigate the two-dimensional electron gas (2DEG) in high-mobility GaAs/AlxGa1-xAs heterostructures at very low temperatures. Vertical transport establishes equilibrium between the high-mobility electron system and an n+-type-doped back electrode separated from the 2DEG by a shallow tunnel barrier. We find that at magnetic fields B7 T and temperatures T<0.6 K the tunnel resistance as function of Landau-level filling factor exhibits a camel-back structure centered around filling factor =1. We argue that the structure reflects the formation of a classical Coulomb gap at the phase transition from a correlated electron liquid to an insulating phase. © 1995 The American Physical Society.