On the effect of superfluid flows on the interaction of microwaves with He II

TL;DR

This paper investigates how superfluid flows influence microwave interactions with helium II, revealing a critical velocity where absorption shifts to emission, based on a proposed mechanism involving local equilibrium distributions.

Contribution

It introduces a new mechanism explaining microwave absorption peaks in superfluid helium and determines the critical flow velocity affecting microwave interactions.

Findings

Microwave absorption peak depends on local equilibrium distribution.

Critical flow velocity causes a transition from absorption to radiation.

The mechanism involves fast roton-roton and roton-phonon interactions.

Abstract

The paper proposes a possible mechanism of interaction of microwaves with superfluid helium that results in an experimentally observed narrow peak of microwave absorption on the frequencies by the order of the roton frequency. The obtained microwave photon absorption coefficient depends on the local equilibrium distribution function which is established due to fast roton-roton and roton-phonon interactions. With the availability of superfluid flows, the local equilibrium distribution function depends on their velocity. The critical velocity of the flows, at which the absorption of microwaves is replaced by their radiation, is found.