Double pulse phonon injection into liquid 4He

Abstract

A thin metal film immersed in liquid 4He injects phonons into the liquid 4He when it is pulse heated. The continuous spectrum of phonons that is created at the heater forms into two spectral groups after a few mm of propagation. The high energy group has energy ℏωk B∼10K and the low energy group ∼ 1K due to 3 phonon decays. However the high and low energy phonons can interact via the 4 phonon process and this limits the injection of high energy phonons. The scattering is strongest near the heater where the densities are highest. To investigate the attenuation of high energy phonons by low energy phonons, the heater has been double pulsed. One pulse creates the high energy phonons which are detected by quantum evaporation and the second creates low energy phonons. By moving one pulse through the other in time and measuring the attenuation, the position and type of scattering can be inferred. The results are consistent with the saturation of the high energy phonon signal at pulse lengths longer than 250ns.