Pair breaking by two phonons in 3He-B

We report on pulsed zero-sound experiments in the B-phase of superfluid 3He 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 within the 1 to 5 μs long sound pulses was 0.4 to 90 nJ, yielding up to 2.5 kW/m2 sound intensities. We did not observe two-phonon absorption due to nonlinear coupling of zero sound to the J=2+ mode within a resolution in attenuation of about Δα∝0.1 cm-1. However, a new structure was found in the attenuation spectrum. With increasing pulse power the absorption of this structure increased up to values of Δα∝5 cm-1. This structure depends on pulse setting, but always occurred at temperature where pair breaking was induced by a two-phonon absorption process. (orig.)