Spin superfluidity, coherent spin precession, and magnon BEC

TL;DR

This paper clarifies the distinctions between spin superfluidity, coherent spin precession, and magnon BEC, emphasizing the role of topology and spin stiffness in enabling dissipationless spin transport.

Contribution

It provides a clear definition of spin superfluidity, differentiates it from related phenomena, and discusses the topological conditions necessary for macroscopic spin transport.

Findings

Spin stiffness is necessary but not sufficient for spin superfluidity.

Spin superfluidity requires specific topological conditions in magnetic order.

Formal criteria for magnon BEC are not directly linked to dissipationless spin transport.

Abstract

Spin superfluidity, coherent spin precession, and magnon BEC are intensively investigated theoretically and experimentally nowadays. Meanwhile, clear definition and differentiation between these related phenomena is needed. It is argued that spin stiffness, which leads to existence of coherent spin precession and dissipationless spin supercurrents, is a necessary but not sufficient condition for spin superfluidity. The latter is defined as a possibility of spin transport on macroscopical distances with sufficiently large spin supercurrents. This possibility is realized at special topology of the magnetic-order-parameter space, such as, e.g., that in easy-plane antiferromagnets. It is argued that an arbitrarily chosen formal criterion for the existence of magnon BEC has no connection with conditions for observation of macroscopic dissipationless spin transport.