Collapse of Spin Excitations in Quantum Hall States of Coupled Electron Double Layers

Remarkable softenings of long wavelength intersubband spin excitations of dilute electron double layers are observed at even integer quantum Hall states. These excitations in coupled GaAs double quantum wells were probed by resonant inelastic light scattering. Their softening is attributed to enhanced exchange vertex corrections (excitonic binding) in the quantum Hall states. The collapse of the spin-density mode with $\delta S_z=0\mathrm{}$ to an energy close to the Zeeman splitting suggests the existence of unstable spin-flip intersubband excitations with $\delta S_z=1\mathrm{}$.