title: Bose-Einstein condensation of quasi-equilibrium magnons at room temperature under pumping.
abstract: Bose-Einstein condensation is one of the most fascinating phenomena predicted by quantum mechanics. It involves the formation of a collective quantum state composed of identical particles with integer angular momentum (bosons), if the particle density exceeds a critical value. To achieve Bose-Einstein condensation, one can either decrease the temperature or increase the density of bosons. It has been predicted that a quasi-equilibrium system of bosons could undergo Bose-Einstein condensation even at relatively high temperatures, if the flow rate of energy pumped into the system exceeds a critical value. Here we report the observation of Bose-Einstein condensation in a gas of magnons at room temperature. Magnons are the quanta of magnetic excitations in a magnetically ordered ensemble of magnetic moments. In thermal equilibrium, they can be described by Bose-Einstein statistics with zero chemical potential and a temperature-dependent density. In the experiments presented here, we show that by using a technique of microwave pumping it is possible to excite additional magnons and to create a gas of quasi-equilibrium magnons with a non-zero chemical potential. With increasing pumping intensity, the chemical potential reaches the energy of the lowest magnon state, and a Bose condensate of magnons is formed.
authors.affiliation: Institute for Applied Physics, University of Münster, 48149 Münster, Germany. demokrit@uni-muenster.de
authors.forename: S O
authors.initials: SO
authors.lastname: Demokritov
authors.forename: V E
authors.initials: VE
authors.lastname: Demidov
authors.forename: O
authors.initials: O
authors.lastname: Dzyapko
authors.forename: G A
authors.initials: GA
authors.lastname: Melkov
authors.forename: A A
authors.initials: AA
authors.lastname: Serga
authors.forename: B
authors.initials: B
authors.lastname: Hillebrands
authors.forename: A N
authors.initials: AN
authors.lastname: Slavin