Accumulation of magnetoelastic bosons in yttrium iron garnet: kinetic theory and wave vector resolved Brillouin light scattering

TL;DR

This paper develops a quantum kinetic theory to describe how magnetoelastic bosons accumulate in yttrium iron garnet, matching experimental Brillouin light scattering data and revealing non-equilibrium steady states in magnon gases.

Contribution

It introduces a novel kinetic framework for magnetoelastic boson accumulation in magnon gases, aligning theoretical predictions with experimental observations.

Findings

Magnetoelastic bosons accumulate in a narrow momentum region below the magnon spectrum.

A non-equilibrium steady state exists with incoherent boson accumulation.

Theoretical results agree well with Brillouin light scattering measurements.

Abstract

We derive and solve quantum kinetic equations describing the accumulation of magnetoelastic bosons in an overpopulated magnon gas realized in a thin film of the magnetic insulator yttrium iron garnet. We show that in the presence of a magnon condensate, there is a non-equilibrium steady state in which incoherent magnetoelastic bosons accumulate in a narrow region in momentum space for energies slightly below the bottom of the magnon spectrum. The results of our calculations agree quite well with Brillouin light scattering measurements of the stationary non-equilibrium state of magnons and magnetoelastic bosons in yttrium iron garnet.