Angle-resolved ballistic phonon absorption spectroscopy in the lowest Landau level

We report the results of a study of the absorption of non-equilibrium balli stic phonons by a two-dimensional electron system, in strong magnetic field s, by measuring its change in resistance. At the even denominator Landau le vel filling factors, v, of 1/2 and 3/2 the electron-phonon interaction is o bserved to be cut-off above a particular value of the component of the wave vector perpendicular to the low dimensional system. This cut-off is consist ent with that due to the finite thickness of the 2DES. At fractional quantu m Hall effect minima (1/3, 2/5, 2/3) we observe that the cut-off to the int eraction depends on the filling factor. At v=1/3, the energy gap determined from these experiments is consistent with the theoretical prediction of th e magnetoroton energy gap. (C) 1998 Elsevier Science B.V. All rights reserv ed.