Features of interaction of microwaves with HeII

TL;DR

This paper explains how microwave interactions with superfluid helium (HeII) involve complex resonant and scattering processes, revealing the influence of streams and identifying a critical velocity where absorption transitions to radiation.

Contribution

It provides a theoretical framework linking thermodynamic and electromechanical phenomena to microwave interactions in HeII, including the effects of streams and critical velocity determination.

Findings

Resonant excitation corresponds to minimal roton energy relaxation.

Presence of a narrow resonant peak on a wide pedestal due to scattering processes.

Identification of a critical stream velocity where microwave absorption switches to radiation.

Abstract

Based on interrelation between the thermodynamic and electromechanical phenomena in superfluid helium, the explanation of experimentally found features of microwave interaction in the frequency range of 40-200 GHz is given. Due to fast roton-roton and roton-phonon interactions resonant excitation on frequency correspond to minimal roton energy relaxes and forms a wide pedestal. Alongside these fast processes, there are also slower processes of rotons' scattering by microwave photons taking place, which lead to additional absorption of energy of resonant microwaves and to the appearance of a narrow resonant peak on the background of a wide pedestal. The theoretical explanation of the influence which streams exert on resonant absorption of microwaves is given. The critical velocity of stream at which absorption of microwaves was replaced by their radiation is found.