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 w ere 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 wit h delta S-z = 0 to an energy close to the Zeeman splitting suggests th e existence of unstable spin-flip intersubband excitations with delta S-z = 1.