Coherent magnon optics in a ferromagnetic spinor Bose-Einstein condensate

TL;DR

This paper reports precise measurements of magnon properties in a ferromagnetic spinor Bose-Einstein condensate, revealing an unusually heavy magnon mass and a nonzero energy gap caused by magnetic dipole interactions.

Contribution

The study provides the first experimental measurement of magnon mass, gap, and magnetic moment in a ferromagnetic spinor BEC, highlighting deviations from theoretical estimates and the role of dipole interactions.

Findings

Magnon mass is approximately 1.038 times the atomic mass.

Magnon energy gap is about 2.5 Hz, consistent with mean-field predictions.

Magnetic dipole-dipole interactions induce a nonzero magnon energy gap.

Abstract

We measure the mass, gap, and magnetic moment of a magnon in the ferromagnetic $F=1$ spinor Bose-Einstein condensate of $^{87}$Rb. We find an unusually heavy magnon mass of $1.038(2)_\mathrm{stat}(8)_\mathrm{sys}$ times the atomic mass, as determined by interfering standing and running coherent magnon waves within the dense and trapped condensed gas. This measurement is shifted significantly from theoretical estimates. The magnon energy gap of $h\times 2.5(1)_\mathrm{stat}(2)_\mathrm{sys}\;\mathrm{Hz}$ and the effective magnetic moment of $-1.04(2)_\mathrm{stat}(8)\,\mu_\textrm{bare}$ times the atomic magnetic moment are consistent with mean-field predictions. The nonzero energy gap arises from magnetic dipole-dipole interactions.