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

In this article, we resolve the apparent contradiction between recent experiments and earlier theoretical studies predicting strongly asymmetric condensates resulting in an attractive interaction between condensate magnons. We show that the relaxation time required to achieve equilibrium of the two condensates at the two energy minima exceeds the experiment duration. Therefore, the system is inherently out of balance and must be described by Boltzmann’s kinetic equations. We develop an appropriate kinetic theory and derive the relation between the critical pumping power and the effective temperature of over-condensate magnons.