Magnon BEC at Room Temperature and Its Spatio-Temporal Dynamics
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agnon BEC at Room Temperature and Its Spatio-Temporal Dynamics S. O. Demokritov Institute for Applied Physics and Center for NanoTechnology, University of Muenster, Muenster, 48149 Germany e-mail: [email protected] Received January 30, 2020; revised January 30, 2020; accepted March 13, 2020
Abstract—Recent advances in the studies of magnon gases have opened new horizons for the investigation of room-temperature macroscopic coherent states and discovery of Bose–Einstein condensation of magnons. Although the phenomenon has been discovered almost 15 years ago, a lot of important issues connected with magnon Bose–Einstein condensation remain unclear. Here I review the recent experimental achievements in investigations of this phenomenon. I show that the magnon condensate possesses high degrees of temporal and spatial coherency, the latter leading to observation of interference of two condensate. Discovery of second sound in magnon condensate is also discussed. Finally, I demonstrate a practical way to realize magnon laser, which create a freely propagating cloud of magnons. DOI: 10.1134/S1063776120070158
1. INTRODUCTION Bose–Einstein condensation (BEC) is one of the most striking manifestations of quantum nature of matter on the macroscopic scale. It represents a formation of a collective quantum state of particles with integer spin—bosons. In 1925 Einstein, using the method proposed by Bose, has shown that in a gas of noninteracting bosons at the thermal equilibrium the density of particles is described by the occupation function:
If the density of the particles in the system is larger than Nc, BEC takes place: the gas is spontaneously divided into two fractions: particles with the density Nc distributed according to Eq. (1) with μ = εmin and particles accumulated in the ground state with ε = εmin. The latter fraction represents a Bose-Einstein condensate [1]. I took 70 years to realize the idea of Einstein experimentally: BEC was observed in diluted atomic alkali gases at ultra-low temperatures of 10–7 K [2, 3].
(1) n(ε) = (exp((ε − μ)/kBT ) − 1)−1, where ε is the energy of the particle, T is the temperature, and kB is the Boltzmann constant [1]. The chemical potential μ is determined from the condition for the total density of the particles, N:
Gases of quasi-particles are very attractive objects for the observation of BEC, since such extreme experimental conditions are not needed for the transition: (i) because of the effective mass of quasi-particles can be essentially smaller than that of atoms, the BEC transition should occur at smaller densities or higher temperatures; (ii) a large number of quasi-particles exists at non-zero temperatures due to thermal fluctuations; (iii) if necessary, the density of quasi-particles can be increased using different methods of external excitation such as microwave pumping [4] or illumination with laser light [5]. At the same time a possibility of BEC in quasi-particle gases is not evident from the point of view of thermodynamics, since quasi-particles are characterize
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