LIGHT-SCATTERING BY COLLECTIVE EXCITATIONS IN THE FRACTIONAL QUANTUM HALL REGIME

Resonant inelastic scattering methods give access to measurements of l ow-lying collective excitations of the 2D where electron gas at Landau level filling v = 1/3. In experiments at finite light scattering wave vectors \k\ = k less than or equal to 0.15 l(0)(-1), where I-0 is the magnetic length, we find strong light scattering by the collective ga p mode of the fractional quantum Hall liquid. Long wavelength spin wav es also become allowed in the tilted-field configuration of the presen t experiments. Such measurements enable the exploration of dispersive charge- and spin-excitations. The energies of the long wavelength coll ective gap mode at v = 1/3 display a square-root dependence on perpend icular magnetic field in agreement with current theories of the incomp ressible quantum fluid. We also consider weaker spectra assigned to li ght scattering by large wave vector spin wave and collective gap excit ation modes. These features are associated with breakdown of wave vect or conservation in resonant inelastic light scattering. (C) 1998 Elsev ier Science B.V. All rights reserved.