Bose–einstein condensation of magnons

Abstract

In this chapter we study the phenomenon of quasi-equilibrium Bose–Einstein condensation (BEC) of magnons at room temperature, experimentally observed in films of yttrium iron garnet (YIG) parametrically excited by microwave radiation. In the first section we briefly present the concept of BEC, as observed in systems of boson particles in ultralow temperatures, and describe some of the experimental results obtained with magnons excited by microwave parallel pumping in YIG films at room temperature. Then, in three sections, we present a theoretical model, based on the formalism used in previous chapters, for the dynamics of the magnon gas driven by a microwave field far out of equilibrium, that provides rigorous support for the formation of a BEC of magnons as observed experimentally. The theory demonstrates that if the microwave-driving power exceeds a threshold value, the nonlinear magnetic interactions create cooperative mechanisms for the onset of a phase transition, leading to spontaneous quantum coherence. The theory also leads to an equation for the wave function of the condensed magnons that is similar to the one established for other BEC systems.