On Josephson effects in insulating spin systems

TL;DR

This paper explores the theoretical possibility of observing Josephson effects in coupled magnetic insulators interpreted as Bose-Einstein condensates of magnetic quasiparticles, highlighting conditions for experimental detection.

Contribution

It introduces a theoretical framework for Josephson effects in magnetic insulators with different critical fields and discusses experimental feasibility.

Findings

Spontaneous alternating quasiparticle current with oscillation frequency determined by critical field difference.

Additional spectral sidebands appear due to phase coherence in coupled systems.

Unobservable direct Josephson current in identical systems but detectable effects in non-identical systems.

Abstract

We discuss an experiment in which two magnetic insulators that both show a field-induced magnetic-ordering transition are weakly coupled to one another and are placed into an ex- ternal magnetic field. If the respective magnetic states can be interpreted as phase coherent Bose-Einstein condensates of magnetic bosonic quasiparticles, one expects the occurrence of Josephson effects. For two identical systems, the resulting d.c. Josephson effect formally represents a constant quasiparticle Josephson current across the weak link, which turns out to be unobservable in an experiment. For magnetic insulators with different critical fields, a spontaneous alternating quasiparticle current develops with a leading oscillation frequency {\omega}a.c. that is determined by the difference between the critical fields. As a result of the coupling, additional sidebands appear in the energy spectrum of the coupled device that would be absent without phase coherence. We discuss the primary conditions for such an effect to take place and conclude that its detection can be feasible for a proper choice of compounds with suitable and realistic material parameters.