Thermalization of magnons in yttrium-iron garnet: nonequilibrium functional renormalization group approach

TL;DR

This paper develops a nonequilibrium functional renormalization group method to study magnon thermalization in yttrium-iron garnet, capturing feedback effects and going beyond traditional approximations, with results aligning well with experiments.

Contribution

The paper introduces a novel FRG-based approach for modeling magnon thermalization that includes feedback effects and surpasses the Born approximation.

Findings

Accurately models magnon thermalization in yttrium-iron garnet.

Achieves good agreement with recent experimental data.

Provides a new framework for nonequilibrium magnon dynamics.

Abstract

Using a nonequilibrium functional renormalization group (FRG) approach we calculate the time evolution of the momentum distribution of a magnon gas in contact with a thermal phonon bath. As a cutoff for the FRG procedure we use a hybridization parameter {\Lambda} giving rise to an artificial damping of the phonons. Within our truncation of the FRG flow equations the time evolution of the magnon distribution is obtained from a rate equation involving cutoff-dependent nonequilibrium self-energies, which in turn satisfy FRG flow equations depending on cutoff-dependent transition rates. Our approach goes beyond the Born collision approximation and takes the feedback of the magnons on the phonons into account. We use our method to calculate the thermalization of a quasi two-dimensional magnon gas in the magnetic insulator yttrium-iron garnet after a highly excited initial state has been generated by an external microwave field. We obtain good agreement with recent experiments.