1. Institute for Applied Physics, University of Münster, 48149 Münster, Germany
2. Department of Radiophysics, National Taras Schevchenko University of Kiev, 01033 Kiev, Ukraine
3. Fachbereich Physik, Technische Universität Kaiserslautern, 67663 Kaiserslautern, Germany
4. Department of Physics, Oakland University, Rochester, Michigan 48309, USA

Correspondence to: S. O. Demokritov¹ Correspondence and requests for materials should be addressed to S.O.D. (Email: demokrit@uni-muenster.de).

Abstract

Bose–Einstein condensation^{1, 2} 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^{3, 4} 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.

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