Josephson and Persistent Spin Currents in Bose-Einstein Condensates of Magnons

TL;DR

This paper develops a microscopic theory for Josephson and persistent spin currents in magnon Bose-Einstein condensates, demonstrating control via the Aharonov-Casher phase and proposing experimental detection methods.

Contribution

It introduces a microscopic model linking magnon BECs to Josephson effects and persistent currents, utilizing the Aharonov-Casher phase for control and measurement.

Findings

Derivation of a two-state Josephson model for magnon BECs.

Proposal of electrical control of the dc Josephson effect via the A-C phase.

Introduction of a magnon-BEC ring with persistent currents due to the A-C phase.

Abstract

Using the Aharonov-Casher (A-C) phase, we present a microscopic theory of the Josephson and persistent spin currents in quasi-equilibrium Bose-Einstein condensates (BECs) of magnons in ferromagnetic insulators. Starting from a microscopic spin model that we map onto a Gross-Pitaevskii Hamiltonian, we derive a two-state model for the Josephson junction between the weakly coupled magnon-BECs. We then show how to obtain the alternating-current (ac) Josephson effect with magnons as well as macroscopic quantum self-trapping in a magnon-BEC. We next propose how to control the direct-current (dc) Josephson effect electrically using the A-C phase, which is the geometric phase acquired by magnons moving in an electric field. Finally, we introduce a magnon-BEC ring and show that persistent magnon-BEC currents flow due to the A-C phase. Focusing on the feature that the persistent magnon-BEC current is a steady flow of magnetic dipoles that produces an electric field, we propose a method to directly measure it experimentally.