NONLINEAR ULTRASONIC-ABSORPTION IN NORMAL AND SUPERFLUID HE-3

Sound absorption of He-3 at pressures of 0 to 11 bar, temperatures of 0.2 to 200 mK and frequencies of 10 to 70 MHz was studied in pulsed ex periments with different pulse lengths (4 to 15 mu s) and pulse powers (0.005 to 20 mW). In the first sound regime of the normal fluid, we f ound no significant power dependence. However, in zero sound attenuati on an additional absorption mechanism was found which does not depend on temperature. This attenuation increased with increasing sound inten sity and appeared with a frequency dependence similar to that of Landa u's quantum attenuation. Above a certain sound energy level in superfl uid He-3-B, the squashing-mode was shifted towards lower temperatures and a new mode was detected near T-c. At high pulse powers we also fou nd frequency tripling in the whole temperature range below T-c.