ABSENCE OF SPIN-DENSITY EXCITATIONS IN QUASI-2-DIMENSIONAL ELECTRON-SYSTEMS

Inelastic light scattering spectra reveal a new behavior of electron-e lectron interactions in two-dimensional double quantum well semiconduc tor structures. It manifests in intersubband excitations when both sub bands are densely populated. In such high density limit, spin excitati ons, well defined when only one subband is populated, cannot be distin guished from single-particle transitions. A quantitative analysis show s a collapse of vertex corrections (exciton shift), which we attribute to the cancellation between contributions from electrons in the two s ubbands.