Effects of the exchange instability on collective spin and charge excitations of the two-dimensional electron gas

Abstract

We report inelastic light scattering measurements of the elementary excitations of a two-dimensional (2D) electron gas formed in a modulation-doped GaAs single quantum well, when the first excited conduction subband begins to be populated and the sudden renormalization of its energy occurs due to an exchange-driven instability of the 2D system. The Hartree and exchange-correlation terms of the Coulomb interaction, which can be precisely determined from the energies of the charge and spin-density excitations in light scattering spectra, exhibit abrupt changes upon filling of the excited subband. Density-functional calculations of the collective excitations within the time-dependent local spin-density approximation provide a simple explanation for the strong enhancement of the Hartree term observed in the experiment. In contrast, the sudden reduction of exchange-correlation vertex corrections at the instability are not well accounted for by the theory. We discuss this discrepancy and present its peculiar temperature dependence.