Bose-Einstein condensation of magnons and spin superfluidity in the polar phase of $^3$He

TL;DR

This paper reports the observation of Bose-Einstein condensation of magnons and spin superfluidity in the polar phase of superfluid helium-3, revealing new insights into topological superfluidity and symmetry breaking.

Contribution

It demonstrates magnon BEC and spin superfluidity in the polar phase of $^3$He, highlighting the role of topological features and symmetry breaking in this quantum state.

Findings

Magnon BEC observed via coherent magnetization precession

Detection of Nambu-Goldstone boson associated with U(1) symmetry breaking

Measurement of the boson's mass under explicit symmetry violation

Abstract

The polar phase of $^3$He, which is topological spin-triplet superfluid with the Dirac nodal line in the spectrum of Bogolubov quasiparticles, has been recently stabilized in a nanoconfined geometry. We pump magnetic excitations (magnons) into the sample of polar phase and observe how they form a Bose-Einstein condensate, revealed by coherent precession of the magnetization of the sample. Spin superfluidity, which supports this coherence, is associated with the spontaneous breaking of U(1) symmetry by the phase of precession. We observe the corresponding Nambu-Goldstone boson and measure its mass emerging when applied rf field violates the U(1) symmetry explicitly. We suggest that the magnon BEC in the polar phase is a powerful probe for topological objects such as vortices and solitons and topological nodes in the fermionic spectrum.