Resonant tunneling spectroscopy of interacting localized states: Observation of the correlated current through two impurities

We study the effects of Coulomb interactions between localized states in a potential barrier by measuring resonant-tunneling spectra with a small bias applied along the barrier. In the Ohmic regime the conductance of 0.2-$\mu$m-gate lateral GaAs microstructures shows distinct peaks associated with individual localized states. However, when an electric field is applied new states start contributing to the current, which becomes a correlated electron flow through two interacting localized states. Several situations of such correlations have been observed, and the conclusions are confirmed by calculations.