Phonon excitation of a two-dimensional electron system around ν=1

We have measured the absorption of ballistic non-equilibrium phonons by a two-dimensional electron system in the quantum Hall effect around filling factor ν=1. Directly at ν=1, the system is insensitive to phonons. However, moving slightly away from the resistance minimum, a strong ballistic phonon absorption starts to show up. We assign this to a drastic decrease of the excitation gap which in turn increases the absorption probability for ballistic phonons.