MODE MIXING IN ANTIFERROMAGNETICALLY CORRELATED DOUBLE-QUANTUM WELLS

We examine the robustness of a recently predicted exchange-induced zer o-field magnetic phase in semiconductor double quantum wells in which each well is spin polarized and the polarization vectors are antiparal lel. Magnetic instabilities are a general feature of Coulombic double- quantum-well systems at low densities. We argue that this antiferromag netic phase is stabilized relative to ferromagnetic ones by an effecti ve superexchange interaction between the wells. Detailed self-consiste nt Hartree-Fock calculations using a point-contact model for the inter action show that the antiferromagnetic phase survives intrasubband rep ulsion matrix elements neglected in earlier work in a large portion of the model's parameter space. We also examine the role of asymmetry du e to biasing or to differences in the widths of the two quantum wells. The asymmetry creates a mode coupling between the intra-and intersubb and collective spin-density excitations (SDE's) that changes the Raman spectroscopy signature of the phase transition from a complete soften ing of the intersubband SDE to a cusp as the density is tuned through the transition. This cusp may be detectable in inelastic light scatter ing experiments in samples of sufficient quality at low enough tempera tures and densities.