Bose-Einstein condensation of magnons in atomic hydrogen gas

TL;DR

This paper reports the experimental observation of Bose-Einstein condensation-like behavior of magnons in a dense, spin-polarized atomic hydrogen gas, demonstrating spontaneous coherence and a sharp spectral feature.

Contribution

First experimental evidence of magnon BEC in atomic hydrogen, showing control and trapping of magnons with inhomogeneous magnetic fields and spectral signatures of condensation.

Findings

Observation of a sharp ESR spectral feature indicating magnon BEC

Spontaneous coherence of magnons at high hydrogen densities

Critical density dependence on trapping potential

Abstract

We report on experimental observation of BEC-like behaviour of quantized electron spin waves (magnons) in a dense gas of spin polarized atomic hydrogen. The magnons are trapped and controlled with inhomogeneous magnetic fields, and described by a Schr\"odinger-like wave equation, in analogy to the BEC experiments with neutral atoms. We have observed the appearance of a sharp feature in the ESR spectrum displaced from the normal spin wave spectrum. We believe that this observation corresponds to a sudden growth of the ground state population of the magnons and emergence of their spontaneous coherence for hydrogen gas densities exceeding a critical value, dependent on the trapping potential. We interpret the results as a BEC of non-equilibrium magnons which were formed by applying the rf power.