Non-linear ultrasonic absorption in normal and superfluid³He

Abstract

Sound absorption of³He 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 experiments with different pulse lengths (4 to 15 μs) and pulse powers (0.005 to 20 mW). In the first sound regime of the normal fluid, we found no significant power dependence. However, in zero sound attenuation an additional absorption mechanism was found which does not depend on temperature. This attenuation increased with increasing sound intensity and appeared with a frequency dependence similar to that of Landau’s quantum attenuation. Above a certain sound energy level in superfluid³He−B, the squashing-mode was shifted towards lower temperatures and a new mode was detected near T_c. At high pulse powers we also found frequency tripling in the whole temperature range below T_c.