Two-phonon absorption by the real squashing collective mode in superfluid He-3

Ultrasound is a powerful probe of collective modes of superfluid He-3 which are temporal oscillations of its order parameter. The nonlinear interaction of zero sound with certain collective modes allows a two-phonon absorption: in a pulsed sound transmission experiment the real squashing collective mode in He-3-B is excited with two coincident pulses of frequencies f(sub s) and f(sub p) at the temperature T(+), h(f(sub s) + f( sub p)) = (square root of (8/5))Delta(T(+)), where Delta(T(+)) is the energy gap of superfluid He-3. Two T(+) peaks are seen: the main peak is accompanied by a smaller one on the low temperature side. The heights of the attenuation peaks at the frequency f(sub s) scale almost linearly with the energy of the pump wave at frequency f(sub p).