Compressibility studies of two-dimensional electron gases

Double quantum well structures have been used to study the compressibility of two-dimensional electron gases. By using independent contacts to the two layers we are able to measure accurately the carrier density changes of the lower layer (which acts as a detector) in response to altering the top layer density (whose compressibility we are probing). The carrier density changes are related to the chemical potential changes in the top layer which is proportional to the inverse compressibility. Comparison with a model which solves the Poisson and Schrodinger equations in a self-consistent manner provides good qualitative agreement with the experimental results, with the compressibility changing from positive at high carrier densities to negative at low densities when the energy changes due to interactions dominate the chemical potential changes. Samples with different well widths have also been studied, these show that the compressibility varies with well width. When analysing this data we find the Hartree contribution is significant and cannot be ignored. (C) 1997 Academic Press Limited