Finite-wavevector studies of two-dimensional systems

Abstract

The results of ballistic phonon and microwave absorption experiments are presented in the fractional quantum Hall regime of two-dimensional electron and hole systems respectively. Time-resolved ballistic phonon results suggest that acoustic phonons can be absorbed by the magnetoroton excitations of a fractional quantum Hall fluid. The technique allows the determination of the magnetoroton gap and holds out the promise of being able to measure the magnetoroton dispersion curve in angle-resolved experiments. At lower Landau level filling factors, high-mobility two-dimensional systems become insulating. Microwave absorption experiments at finite wavevector find a series of sharp absorption lines in the insulating phase that become more pronounced as the temperature is lowered and the magnetic field increased. The results strongly suggest that the two-dimensional system forms a pinned Wigner solid at these low filling factors. Further analysis allows the determination of the pinning frequency and its variation as a function of magnetic field.