4-PHONON SCATTERING IN SUPERFLUID HE-4

The scattering of high-frequency (HBARomega/k(B) > 10 K) phonons injected into superfluid He-4 with low-frequency (HBARomegaBAR/k(B) < 1 K) thermal phonons in the liquid is studied both experimentally and theoretically. Quantum evaporation enables the selective study of only the high-frequency phonons. The attenuation of evaporation signals as the temperature is increased from 70 mK to 250 mK for various liquid path lengths is interpreted in terms of four-phonon scattering involving the high-frequency injected phonons and the low-frequency thermal phonons. Monte Carlo simulations of the signal variation with temperature show that the measured scattering is much weaker than the hydrodynamic theory has previously predicted. However, when this theory is extended to include diagrams representing further possible routes of the four-phonon scattering process, there are significant cancellations between these extra diagrams and those considered earlier. This leads to a weaker interaction and to a much improved agreement with the experimental results.