Mode mixing in antiferromagnetically correlated double quantum wells

We examine the robustness of a recently predicted exchange-induced zero-field magnetic phase in semiconductor double quantum wells in which each well is spin polarized and the polarization vectors are antiparallel. Magnetic instabilities are a general feature of Coulombic double-quantum-well systems at low densities. We argue that this antiferromagnetic phase is stabilized relative to ferromagnetic ones by an effective superexchange interaction between the wells. Detailed self-consistent Hartree-Fock calculations using a point-contact model for the interaction show that the antiferromagnetic phase survives intrasubband repulsion matrix elements neglected in earlier work in a large portion of the model’s parameter space. We also examine the role of asymmetry due to biasing or to differences in the widths of the two quantum wells. The asymmetry creates a mode coupling between the intra- and intersubband collective spin-density excitations (SDE’s) that changes the Raman spectroscopy signature of the phase transition from a complete softening of the intersubband SDE to a cusp as the density is tuned through the transition. This cusp may be detectable in inelastic light scattering experiments in samples of sufficient quality at low enough temperatures and densities.