SPATIAL EVOLUTION OF HIGH FREQUENCY PHONONS IN SUPERFLUID 4HE FROM A PULSE-HEATED METAL FILM

Abstract

An almost monochromatic spectrum of high frequency (ħω/kB∼ 10 K) phonons in superfluid4He is created by a short (∼ 0.1μs) pulse of Joule-heating in a metal film submerged in the liquid at saturated vapour pressure (svp). These phonons have lifetimes that tend to infinity as T → 0, and are the ones effective in quantum evaporation experiments. Most of these high frequency (hf) phonons are not injected into the liquid4He across the metal—liquid interface, but are created in the liquid by energy-increasing interactions which begin with the injected phonons of much lower energy (ħω/kB∼ 1 K). These hf phonons are created up to ∼ 5 millimetres in front of the heater, hence the time of flight from a heater to a detector only gives an approximate value (lower bound) of their energies. Here we present measurements at svp of phonon energy fluxes in liquid4He at various distances from a pulse-heated metal film. Analysis of these signals gives an improved determination of the hf phonon spectrum (peaked at 10.20 ± 0.05 K with HWHM ≃ 0.2 K on the high energy side).