Bose–Einstein condensation of quasi-equilibrium magnons at room temperature under pumping

Abstract

Bose–Einstein condensation1,2 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 predicted3,4 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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Figure 1: The set-up for magnon excitation and detection.
Figure 2: BLS spectrum of thermal magnons recorded without pumping.
Figure 3: BLS spectra from pumped magnons at different delay times, τ.
Figure 4: Evolution of the magnon population after the pumping is switched off at τ = 1,000 ns.

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Acknowledgements

Support by the Deutsche Forschungsgemeinschaft, the US Army Research Office, and the Science & Technology Center of Ukraine is acknowledged. We are also indebted to D. Mills, V. Safonov and M. Katsnelson for discussions.

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Correspondence to S. O. Demokritov.

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Demokritov, S., Demidov, V., Dzyapko, O. et al. Bose–Einstein condensation of quasi-equilibrium magnons at room temperature under pumping. Nature 443, 430–433 (2006). https://doi.org/10.1038/nature05117

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