A two-dimensional (2D) electron gas formed in a modulation-doped GaAs/AlGaAs single quantum well undergoes a first-order transition when the first excited subband is occupied with electrons, as the Fermi level is tuned into resonance with the excited subband by applying a dc voltage. Direct evidence for this effect is obtained from low-temperature photoluminescence spectra which display the sudden renormalization of the intersubband energy E01 upon the abrupt occupation of the first excited subband. Calculations within density-functional theory, which treat the 2D exchange potential {\it exactly}, show that this thermodynamical instability of the electron system is mainly driven by {\it intersubband} terms of the exchange Coulomb interaction. From temperature-dependent measurements the existence of a critical point at Tc=35±5 K is inferred.
@article{arxiv.cond-mat/0106472,
title = {Exchange instability of the two-dimensional electron gas in semiconductor quantum wells},
author = {A. R. Goñi and U. Haboeck and C. Thomsen and K. Eberl and F. A. Reboredo and C. R. Proetto and F. Guinea},
journal= {arXiv preprint arXiv:cond-mat/0106472},
year = {2009}
}