Condensing magnons in a degenerate ferromagnetic spinor Bose gas

TL;DR

This paper reports the experimental observation of magnon condensation in a ferromagnetic spinor Bose-Einstein condensate of rubidium-87, demonstrating spontaneous symmetry breaking and complex spin textures.

Contribution

It provides the first quantitative evidence of magnon condensation as free particles in an effective potential within a spinor BEC, including direct imaging of spin textures.

Findings

Magnons condense at a critical point as free particles in an effective potential.

Spontaneous inhomogeneous symmetry breaking occurs in the magnon condensate.

Spin textures such as Mermin-Ho structures are observed in the condensate wavefunction.

Abstract

We observe the condensation of magnon excitations within an $F=1$ $^{87}$Rb spinor Bose-Einstein condensed gas. Magnons are pumped into a longitudinally spin-polarized gas, allowed to equilibrate to a non-degenerate distribution, and then cooled evaporatively at near-constant net longitudinal magnetization whereupon they condense. We find magnon condensation to be described quantitatively as the condensation of free particles in an effective potential that is uniform within the ferromagnetic condensate volume, evidenced by the number and distribution of magnons at the condensation transition. Transverse magnetization images reveal directly the spontaneous, inhomogeneous symmetry breaking by the magnon quasi-condensate, including signatures of Mermin-Ho spin textures that appear as phase singularities in the magnon condensate wavefunction.