New nonlinear phenomena in focused acoustic beam in pressurized superfluid 4He

We have studied the propagation properties of a focused acoustic beam in superfluid 4He at 300 mK under several pressures and observed new nonlinear phenomena under pressures higher than 18 atm. Under these high pressures(normal dispersion region), the received power is increased in proportion to the power input for the low-power region, but it is depleted rapidly after the saturation for higher power input. It exhibits several sharp dips for further increased power. This nonlinear phenomenon is a completely new one, so there is as yet no explanatory theory. We propose a possible mechanism to explain it which consists of two nonlinear processes. (author)