Electron correlations and Coulomb gap in a two-dimensional electron gas in high magnetic fields

We apply tunnel spectroscopy to investigate the two-dimensional electron gas (2DEG) in high-mobility GaAs/Alₓ{Ga}_{1{{-}}x}$As 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 B{}7 T and temperatures T0.6 K the tunnel resistance as function of Landau-level filling factor exhibits a camel-back structure centered around filling factor {ν}=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.