ABSENCE OF UNSTABLE ZERO-FIELD INTERSUBBAND SPIN EXCITATIONS OF DILUTE ELECTRON BILAYERS

Low-energy spin-density intersubband excitations of electron bilayers in GaAs double quantum wells have been investigated by inelastic light scattering in the search for unstable modes. Excitonic vertex correct ions due to exchange Coulomb interactions were thus determined down to very low electron densities. The experiments are interpreted within t he time-dependent local-density approximation (TDLDA) and with a nonlo cal theory. The pronounced softening at zero magnetic field (a vertex- correction driven instability) predicted by the TDLDA for densities n< 10(11) cm(-2) was not observed. At these lower densities we achieve be tter agreement with experiment using the nonlocal theory.