Electron-phonon interaction in the quantum Hall effect regime

The absorption of ballistic phonons with frequencies in the 100-GHz range by a two-dimensional electron gas (2DEG) has been studied in a quantizing magnetic field. The 2DEG was formed at the interface of a GaAs/${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterojunction. Acoustic phonons were created by heating the substrate locally with a focused laser beam. The phonons traveled ballistically through the crystal and were partially absorbed by the 2DEG. This led to a transfer of momentum into the 2DEG (phonon-drag effect) resulting in phonon-induced voltages and currents. These quantities gave detailed information about the interaction between acoustic phonons and the 2DEG as a function of both the incident angle of the absorbed phonons and the magnetic field. We observed that the dependence of the phonon-drag signal on the angles of incidence was neither affected by the magnetic field nor by the phonon spectrum. The absolute intensity of the phonon-drag signal, however, oscillated in phase with the Shubnikov–de Haas oscillations. These results could be explained with a simple microscopic theory of the electron-phonon interaction together with a macroscopic model for the response of the 2DEG on the absorption of ballistic phonons.