Spin-Excitation-Instability-Induced Quantum Phase Transitions in Double-Layer Quantum Hall Systems

We study the intersubband spin density collective modes in double-layer quantum Hall systems at $\nu=2$ within the time-dependent Hartree-Fock approximation. We find that these intersubband spin density excitations may soften under experimentally accessible conditions, signaling a phase transition to a new quantum Hall state with interlayer antiferromagnetic spin correlations. We show that this antiferromagnetic phase is energetically stable and that the phase transition is continuous. Our findings are consistent with recent experiments.