Measurements of the conductivity ${\sigma }_{\mathrm{xx}}$ on GaAs-${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As heterostructures have been performed in the quantum Hall regime at integer filling factors ν using shunted bridge geometry. The dependences on 1/ν (2<ν<16) as well as on temperature (1.4 K<T<4.2 K) have been investigated. Hydrostatic pressure has been used as a means to vary the two-dimensional electron density. Our results show evidence for the nearest-neighbor hopping processes, never before observed in two-dimensional structures in the presence of a magnetic field. The observations are attributed to thermal electron excitations from the Fermi energy in the middle of the Landau gap to a band of localized states, whose energy is proportional to magnetic field. This interpretation is closely related to recent cyclotron-resonance measurements on similar heterostructures. The experimental dependences are qualitatively explained by a model assuming Coulomb character of the localized centers.

• Received 3 August 1988

DOI:https://doi.org/10.1103/PhysRevB.39.1832