Bose-Einstein condensation of magnons in superfluid 3He

TL;DR

This paper demonstrates that various observed states of coherent magnetization precession in superfluid 3He are examples of Bose-Einstein condensation of magnons, driven by different spin-orbit interactions.

Contribution

It unifies multiple observed phenomena in superfluid 3He as manifestations of magnon Bose-Einstein condensation based on spin-orbit coupling effects.

Findings

Identification of all coherent precession states as magnon BEC

Demonstration of spin-orbit coupling as the interaction mechanism

Observation of BEC in both 3He-B and 3He-A phases

Abstract

The possibility of Bose-Einstein condensation of excitations has been discussed for a long time. The phenomenon of the phase-coherent precession of magnetization in superfluid 3He and the related effects of spin superfluidity are based on the true Bose-Einstein condensation of magnons. Several different states of coherent precession has been observed in 3He-B: homogeneously precessing domain (HPD); persistent signal formed by Q-balls at very low temperatures; coherent precession with fractional magnetization; and two new modes of the coherent precession in compressed aerogel. In compressed aerogel the coherent precession has been also found in 3He-A. Here we demonstrate that all these cases are examples of a Bose-Einstein condensation of magnons, with the magnon interaction term in the Gross-Pitaevskii equation being provided by different types of spin-orbit coupling in the background of the coherent precession.