Collective intersubband transitions in quantum wells: a comparative density-functional study

Abstract

We use time-dependent (current) density functional theory to study collective transitions between the two lowest subbands in GaAs/AlGaAs quantum wells. We focus on two systems where experimental results are available: a wide single and a narrow asymmetric double well. The aim is to calculate frequency and linewidth of collective electronic modes damped via electron-electron interaction only. Since Landau damping is not effective here, the dominant damping mechanism involves dynamical exchange-correlation effects such as multipair production. To capture these effects, one has to go beyond the widely used adiabatic local density approximation (ALDA) and include retardation. We perform a comparative study of two approaches which fall in this category: the dynamical extension of the ALDA by Gross and Kohn, and a more recent method which treats exchange and correlation beyond the ALDA as viscoelastic stresses in the electron liquid. We find that the former method is more robust: it performs similarly for strongly different degrees of collectivity of the electronic motion. Results for quantum wells compare reasonably to experiment, with a tendency towards overdamping. By contrast, the viscoelastic approach is superior for systems where the electron dynamics is predominantly collective, but breaks down if the local velocity field is too rapidly varying, as in the case of a single-electron-like behavior such as tunneling through a potential barrier.