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

We use Linearized time-dependent (current) density-functional theory to stu dy collective transitions between the two lowest subbands in GaAs/AlxGa1-xA s quantum wells. We focus on two particular systems, for both of which expe rimental results are available: a wide single square well and a narrow asym metric double quantum well. The aim is to calculate the frequency and linew idth of collective electronic modes damped through electron-electron intera ction only. Since Landau damping, i.e., creation of single electron-hole pa irs, is not effective here, the dominant damping mechanism involves dynamic al exchange-correlation effects such as multipair production. To capture th ese effects, one has to go beyond the widely used adiabatic local-density a pproximation (ALDA) and include retardation effects. We perform a comparati ve study of two approaches which fall in this category. The first one is th e dynamical extension of the ALDA by Gross and Kohn. The second one is a mo re recent approach which treats exchange and correlation beyond the ALDA as viscoelastic stresses in the electron liquid. It is found that the former method is more robust: it performs similarly for strongly different degrees of collectivity of the electronic motion. Results for single and double qu antum wells compare reasonably to experiment, with a tendency towards overd amping. The viscoelastic approach, on the other hand, is superior for syste ms where the electron dynamics is predominantly collective, but breaks down if the local velocity field is too rapidly varying, which is the case for single-electron-like behavior such as tunneling through a potential barrier , [S0163-1829(98)01748-2].