Kinetics of Bose-Einstein condensation of magnons in Yttrium Iron Garnet films

In this article, we explain the reason of the apparent contradiction between recent experiments [1] and [2] and earlier theoretical predictions [3] of strongly asymmetric condensate resulting in attractive interaction between the condensate magnons. We show that the relaxation time for equilibrium between two condensates at two minima of energy exceeds the time of experiment. Therefore, it should be described by Boltzmann kinetic equation. We develop the proper kinetic theory and find the relation between the critical pumping power and the effective temperature of over-condensate magnons.

💡 Research Summary

This paper addresses the apparent discrepancy between recent experimental observations of magnon Bose‑Einstein condensation (BEC) in thick Yttrium Iron Garnet (YIG) films and earlier theoretical predictions that assumed a strongly asymmetric population of the two energy minima at wave vectors ±Q, leading to an effective attractive interaction among condensate magnons. The authors argue that the root of the contradiction lies in the relaxation dynamics of the inter‑minimum degree of freedom.

First, the authors review the magnon spectrum in YIG films with thickness d ≫ ℓ_dip (≈ 40 nm). The dipolar interaction dominates at short distances, producing two degenerate minima at ±Q in the plane of the film. Earlier theory