Light scattering by collective excitations in the fractional quantum hall regime

Resonant inelastic scattering methods give access to measurements of low-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 gap mode of the fractional quantum Hall liquid. Long wavelength spin waves also become allowed in the tilted-field configuration of the present experiments. Such measurements enable the exploration of dispersive charge- and spin-excitations. The energies of the long wavelength collective gap mode at v = 1/3 display a square-root dependence on perpendicular magnetic field in agreement with current theories of the incompressible quantum fluid. We also consider weaker spectra assigned to light scattering by large wave vector spin wave and collective gap excitation modes. These features are associated with breakdown of wave vector conservation in resonant inelastic light scattering. (C) 1998 Elsevier Science B.V. All rights reserved.