Experimental tests for macroscopic phase coherence in magnetic-quasiparticle condensates of insulating spin systems

TL;DR

This paper explores experimental methods to detect macroscopic phase coherence in magnetic quasiparticle condensates within insulating spin systems, focusing on the potential observation of Josephson effects in TlCuCl3.

Contribution

It proposes a novel experimental approach to observe superfluidity and the a.c. Josephson effect in magnetic insulators, which has not been demonstrated before.

Findings

Detection of superfluid behaviour in TlCuCl3 is feasible.

A proposed device could observe the a.c. Josephson effect in magnetic insulators.

Experimental evidence for macroscopic phase coherence in such systems remains to be obtained.

Abstract

For most kinds of already known Bose-Einstein condensates experimental evidence for the existence of a macroscopic coherent quantum state has been provided, e.g., from the observation of interference phenomena, the formation vortices, the detection of Josephson effects, or even by the manifestation of superfluid behaviour. However, none of these hallmarks for a true, macroscopic phase-coherent state has ever been reported for any insulating spin system in a solid in which magnetic bosonic quasiparticles are thought to condense close to a quantum critical point. We describe an attempt to detect superfluid behaviour in TlCuCl3, and discuss in more detail a proposal for an experiment in which the a.c. Josephson effect can be probed by using a device composed of two weakly coupled magnetic insulators with different critical fields. We conclude that the detection of the a.c. Josephson effect is feasible for a proper choice of compounds with realistic material parameters.