Exchange instability of the two-dimensional electron gas in semiconductor quantum wells
A.R. Goñi, U. Haboeck, C. Thomsen, K. Eberl, F.A. Reboredo, C. R. Proetto, F. Guinea
Abstract
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 E_{01} 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 T_c = 35\pm 5 K is inferred.