Stimulated Thermalization of a Parametrically Driven Magnon Gas as a Prerequisite for Bose-Einstein Magnon Condensation

TL;DR

This study investigates how a driven magnon gas thermalizes through stimulated scattering, leading to Bose-Einstein condensation, by using time- and wavevector-resolved spectroscopy to observe the process.

Contribution

It reveals the mechanism of stimulated thermalization involving secondary magnons that facilitate condensation at the spectrum's bottom, a novel insight into magnon BEC formation.

Findings

Secondary magnons enable effective scattering to the lowest energy states.

Condensation is preceded by parametric stimulated scattering of initial magnons.

Time- and wavevector-resolved spectroscopy captures the thermalization process.

Abstract

Thermalization of a parametrically driven magnon gas leading to the formation of a Bose-Einstein condensate at the bottom of a spin-wave spectrum was studied by time- and wavevector-resolved Brillouin light scattering spectroscopy. It has been found that the condensation is preceded by the conversion of initially pumped magnons into a second group of frequency degenerated magnons, which appear due to parametrically stimulated scattering of the initial magnons to a short-wavelength spectral region. In contrast to the first magnon group, which wavevectors are orthogonal to the wavevectors of the magnons at the lowest energy states, the secondary magnons can effectively scatter to the bottom of the spectrum and condense there.