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 factor, nu, of: the electron-phonon interaction is observed to be cut-off above a particular value of the component of the wavevector perp endicular to the low dimensional system. This cut-off isis consistent with that due to the finite thickness of the 2DES. At the fractional quantum Hal l effect minima of nu = 2/3 we observe that the cut-off occurs at a lower h eater temperature. The experimental results are consistent with phonon abso rption across a well-defined energy gap in a range of known wave vectors. ( C) 1999 Elsevier Science B.V. All rights reserved.