Observation of quantum effects in the spectrum of piezoelectrically amplified acoustic flux in GaAs

Using x-ray scattering, we have made the first observations of the spectrum of acousto-electrically amplified phonons in the quantum limit, where the conduction-electron system obeys quantum statistics and the observed phonon wave vector q is comparable to the electron Fermi wave vector kF. In GaAs samples with degenerate carrier distributions, we observe a dramatic quantum anomaly consisting of an abrupt decrease in phonon gain for q larger than 2kF. © 1971 The American Physical Society.