Spin superfluidity and magnon BEC

TL;DR

This paper explores spin superfluidity as a form of Bose-Einstein condensation of magnons, highlighting its phases, signatures like Josephson effects and vortices, and its relation to other superfluid systems.

Contribution

It provides a comprehensive overview of magnon superfluidity, including phases, experimental signatures, and connections to broader superfluid phenomena in condensed matter and particle physics.

Findings

Magnon Bose condensation leads to spin superfluidity.

Signatures include spin supercurrent, Josephson effect, and topological vortices.

Spin superfluidity relates to phenomena like spin Hall effects.

Abstract

The spin superfluidity -- superfluidity in the magnetic subsystem of a condensed matter - is manifested as the spontaneous phase-coherent precession of spins first discovered in 1984 in 3He-B. This superfluid current of spins - spin supercurrent - is one more representative of superfluid currents known or discussed in other systems, such as the superfluid current of mass and atoms in superfluid 4He; superfluid current of electric charge in superconductors; superfluid current of hypercharge in Standard Model of particle physics; superfluid baryonic current and current of chiral charge in quark matter; etc. Spin superfluidity can be described in terms of the Bose condensation of spin waves - magnons. We discuss different phases of magnon superfluidity, including those in magnetic trap; and signatures of magnons superfluidity: (i) spin supercurrent, which transports the magnetization on a macroscopic distance more than 1 cm long; (ii) spin current Josephson effect which shows interference between two condensates; (iii) spin current vortex - a topological defect which is an analog of a quantized vortex in superfluids, of an Abrikosov vortex in superconductors, and cosmic strings in relativistic theories; (iv) Goldstone modes related to the broken U(1) symmetry - phonons in the spin-superfluid magnon gas; etc. We also touch the topic of spin supercurrent in general including spin Hall and intrinsic quantum spin Hall effects.