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

Nature volume 443, pages 430–433 (2006)Cite this article

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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Authors and Affiliations

  1. Institute for Applied Physics, University of Münster, 48149, Münster, Germany

    S. O. Demokritov, V. E. Demidov & O. Dzyapko

  2. Department of Radiophysics, National Taras Schevchenko University of Kiev, 01033, Kiev, Ukraine

    G. A. Melkov

  3. Fachbereich Physik, Technische Universität Kaiserslautern, 67663, Kaiserslautern, Germany

    A. A. Serga & B. Hillebrands

  4. Department of Physics, Oakland University, Rochester, 48309, Michigan, USA

    A. N. Slavin

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  1. S. O. Demokritov
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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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