Magnon BEC and Spin Superfluidity: a 3He primer

TL;DR

This paper discusses magnon Bose-Einstein condensation in superfluid helium-3, highlighting its manifestation as a spin superfluid state with properties akin to other macroscopic quantum phenomena.

Contribution

It provides a detailed analysis of magnon BEC in superfluid helium-3, emphasizing its role as a spin superfluid and its relation to coherent quantum states.

Findings

Magnon BEC occurs as a state of coherent spin precession.

The magnon BEC exhibits properties of spin superfluidity.

The phenomenon is exemplified in superfluid phases of helium-3.

Abstract

Bose-Einstein condensation (BEC) is a quantum phenomenon of formation of the collective quantum state, in which the macroscopic number of particles occupies the lowest energy state and thus is governed by a single wave function. Here we highlight the BEC in a magnetic subsystem -- the BEC of magnons, elementary magnetic excitations. Magnon BEC is manifested as the spontaneously emerging state of the precessing spins, in which all spins precess with the same frequency and phase even in the inhomogeneous magnetic field. We consider this phenomenon on example of spin precession in superfluid phases of $^3$He. The magnon BEC in these phases has all the properties of spin superfluidity. The states of the phase-coherent precession belong to the class of the coherent quantum states, which manifest themselves by superfluidity, superconductivity, quantum Hall effect, Josephson effect and many other macroscopic quantum phenomena.