Two phonon pair breaking in superfluid3He-B

We report on pulsed zero sound experiments in the B-phase of³He down to 0.6 mK. The investigated pressure and frequency range was 5 to 16 bar and 10 to 70 MHz, respectively. The dissipated energy during the sound pulses was between 0.4 to 90 nJ. The resolution in damping was about 0.1 cm^–1. Within the range of our experimental parameters we never observed two phonon absorption resulting from non-linear coupling of zero sound to the J =2⁺ mode. However, with pulses of single frequency a new peak structure was found at temperatures corresponding to (T) = . This behavior is attributed to pair breaking by two phonons of the same energy. The attenuation increased with increasing pulse power, but also depended on pulse duration. In addition to that, we found at all pressures and frequencies an unreproducible attenuation structure close to the position of the J=2⁺ mode.