Double layer structures have been used to measure the compressibility of both two-dimensional electron (2DEG) and two-dimensional hole gases (2DHG). This has been achieved by making independent contacts to the lower layer and using it to detect the compressibility of the upper layer. Both positive and negative compressibilities are observed, with the negative compressibility enhanced for hole gases, where the changes of the interaction energy are larger than those of the kinetic energy. The negative compressibility of the top layer leads to a charge transfer between the layers; this has been modelled and gives good agreement with the experiments when exchange and correlation is included. Due to disorder in the layers, however, the model deviates from the experimental results close to depletion. The dependence of the charge transfer upon distance between the electron gases has also been studied, with the charge transfer decreasing linearly with increasing spacing between the 2DEGs. This suggests that inter-layer interactions are insignificant. (C) 1996 American Institute of Physics.
