Spatial Evolution of High Frequency Phonons in Superfluid 4He from a Pulse-Heated Metal Film

1573-7357

Springer Online Journal Archives 1860-2000

Physics

Abstract An almost monochromatic spectrum of high frequency (ħω/k B ∼ 10 K) phonons in superfluid 4 He 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 liquid 4 He 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/k B ∼ 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 liquid 4 He 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).

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