Spin Instabilities in Coupled Semiconductor Quantum Wells

We study the magnetic phases of two coupled two-dimensional electron gases in order to determine under what circumstances these phases may occur in real semiconductor quantum wells and what the experimental properties of the broken-symmetry ground states may be. Within the local-density-approximation to time-dependent density functional theory (DFT), we find a phase transition signaled by the vanishing of the intersubband spin-density excitations at low but accessible (\sim 10^10-10^11 cm^{-2}) electron densities. Through a self-consistent Hartree-Fock calculation, we associate this transition with an antiferromagnetic phase and study the phase diagram, thermodynamics, and collective modes in it. The collective modes are in principle observable in inelastic light scattering experiments, and we discuss the implications of our calculations for these measurements. We also examine the ferromagnetic transition in both single and double quantum wells within the local-spin-density approximation to DFT and obtain a critical density which depends on the well width and which is far below that of the antiferromagnetic transition.