Direct evidence of spatial stability of Bose-Einstein condensate of magnons

TL;DR

This paper provides direct experimental evidence that magnons in a Bose-Einstein condensate exhibit repulsive interactions, leading to spatial stability, and proposes a mechanism for interaction inversion, advancing understanding of magnon condensates at room temperature.

Contribution

It demonstrates the spatial stability of magnon condensates due to repulsive interactions and introduces a mechanism for interaction inversion supported by theoretical modeling.

Findings

Magnons in a condensate exhibit repulsive interactions.

The magnon condensate is spatially stable contrary to previous expectations.

A theoretical model based on the Gross-Pitaevskii equation supports the experimental results.

Abstract

Bose-Einstein condensation of quasi-equilibrium magnons is one of few macroscopic quantum phenomena observed at room temperature. Since its discovery, it became an object of intense research, which led to the observation of many exciting phenomena such as quantized vortices, second sound, and Bogolyubov waves. However, for a long time it remained unclear, what physical mechanisms can be responsible for the spatial stability of the magnon condensate. Indeed, since magnons are believed to exhibit attractive interaction, it is generally expected that the magnon condensate should be unstable with respect to the real-space collapse, which contradicts all the experimental findings. Here, we provide direct experimental evidence that magnons in a condensate exhibit repulsive interaction resulting in the condensate stabilization and propose a mechanism, which is responsible for the interaction inversion. Our experimental conclusions are additionally supported by the theoretical model based on the Gross-Pitaevskii equation. Our findings solve a long-standing problem and provide a new insight into the physics of magnon Bose-Einstein condensates.