Discontinuities in acoustic parametric processes in a degenerate electron gas

Calculations and experimental results are presented on the nonlinear part of the interaction of free electrons with acoustic phonons which gives rise to acoustic sum-frequency generation. For a degenerate electron gas, the nonlinearity increases discontinuously at |q→3|=2kF, where q→3 is the acoustic wave vector at the sum frequency and kF is the electron Fermi wave vector. This abrupt increase was observed in GaAs using x-ray scattering. © 1972 The American Physical Society.