Probing quantum turbulence in $^4$He by quantum evaporation measurements

TL;DR

This paper predicts that vortex lines in superfluid helium-4 create density modulations mainly involving rotons, which can be experimentally detected through quantum evaporation measurements, providing insights into quantum turbulence.

Contribution

It introduces a theoretical model linking vortex density modulations to virtual rotons and proposes experimental detection via quantum evaporation to study quantum turbulence.

Findings

Vortex density modulations are dominated by rotons with positive group velocity.

Quantum evaporation rates due to vortex-induced rotons are detectable with current technology.

Detection of high-energy phonon evaporation could further elucidate turbulence decay processes.

Abstract

Theory of superfluid $^4$He shows that, due to strong correlations and backflow effects, the density profile of a vortex line has the character of a density modulation and it is not a simple rarefaction region as found in clouds of cold bosonic atoms. We find that the basic features of this density modulation are represented by a wave--packet of cylindrical symmetry in which rotons with positive group velocity have a dominant role: The vortex density modulation can be viewed as a cloud of virtual excitations, mainly rotons, sustained by the phase of the vortex wave function. This suggests that in a vortex reconnection some of these rotons become real so that a vortex tangle is predicted to be a source of non-thermal rotons. The presence of such vorticity induced rotons can be verified by measurements at low temperature of quantum evaporation of $^4$He atoms. We estimate the rate of evaporation and this turns out to be detectable by current instrumentation. Additional information on the microscopic processes in the decay of quantum turbulence will be obtained if quantum evaporation by high energy phonons should be detected.