Josephson and persistent spin currents in Bose-Einstein condensates of magnons

Kouki Nakata; Kevin A. van Hoogdalem; Pascal Simon; Daniel Loss

doi:10.1103/PhysRevB.90.144419

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Josephson and persistent spin currents in Bose-Einstein condensates of magnons

Kouki Nakata, Kevin A. van Hoogdalem, Pascal Simon, Daniel Loss

DOI 10.1103/PhysRevB.90.144419 · Physical Review B

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Abstract

Using the Aharonov-Casher (A-C) phase, we present a microscopic theory of the Josephson and persistent spin currents in quasiequilibrium 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.