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.

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The room-temperature dynamics of a magnon gas driven by short microwave pumping pulses is studied. An overpopulation of the lowest energy level of the system following the pumping is observed. Using the sensitivity of the Brillouin light scattering technique to the coherence degree of the scattering magnons we demonstrate the spontaneous emergence of coherence of the magnons at the lowest level, if their density exceeds a critical value. This finding is clear proof of the quantum nature of the observed phenomenon and direct evidence of Bose-Einstein condensation of magnons at room temperature.

Bose-Einstein condensation of quasi-particles such as excitons, polaritons, magnons and photons is a fascinating quantum mechanical phenomenon. Unlike the Bose-Einstein condensation of real particles (like atoms), these processes do not require low temperatures, since the high densities of low-energy quasi-particles needed for the condensate to form can be produced via external pumping. Here we demonstrate that such a pumping can create remarkably high effective temperatures in a narrow spectral region of the lowest energy states in a magnon gas, resulting in strikingly unexpected transitional dynamics of BoseEinstein magnon condensate: the density of the condensate increases immediately after the external magnon flow is switched off and initially decreases if it is switched on again. This behaviour finds explanation in a nonlinear 'evaporative supercooling' mechanism that couples the low-energy magnons overheated by pumping with all the other thermal magnons, removing the excess heat, and allowing Bose-Einstein condensate formation.

The term supercurrent relates to a macroscopic dissipation-free collective motion of a quantum condensate and is commonly associated with such famous low-temperature phenomena as superconductivity and superfluidity. Another type of motion of quantum condensates is second sound-a wave of the density of a condensate. Recently, we reported on an enhanced decay of a parametrically induced Bose-Einstein condensate (BEC) of magnons caused by a supercurrent outflow of the BEC phase from the locally heated area of a room temperature magnetic film. Here, we present the direct experimental observation of a long-distance spin transport in such a system. The condensed magnons being pushed out from the potential well within the heated area form a density wave, which propagates through the BEC many hundreds of micrometers in the form of a specific second sound pulse-Bogoliubov waves-and is reflected from the sample edge. The discovery of the long distance supercurrent transport in the magnon BEC further advances the frontier of the physics of quasiparticles and allows for the application of related transport phenomena for low-loss data transfer in perspective magnon spintronics devices.Supercurrent is a macroscopic quantum phenomenon when many bosons (real-or quasiparticles) being selfassembled in one quantum state with minimum energy and zero velocity-a Bose-Einstein condensate (BEC)[1-11]-move as a whole due to a phase gradient imposed on their joint wave function. This phenomenon being mostly associated with resistant-free electric currents of Cooper pairs [12] in superconductors and superfluidity of liquid Helium [13-17] is, however, much more widespread [18][19][20]. It is experimentally confirmed in the quantum condensates of diluted ultracold gases [21,22], of nuclear magnons in liquid 3 He [23-25], of polaritons in semiconductor microcavities [26] and, recently, of electron magnons in room-temperature ferrimagnetic films [27]. Supercurrents being topologically confined often manifest themselves in a form of quantum vortices [21,28,29].The quantum condensate supports another form of motion-second sound [15,30]. Second sound can be considered as elementary excitations of various types, which can propagate in continuous media with an almost linear dispersion law in the long-wavelength limit. The term second sound stems from an analogy with the ordinary sound waves or first sound-the wave oscillations of media density and mechanical momentum. The most well-known example of second sound is anti-phase oscillations of the densities ρ n and ρ s of the normal-fluid and superfluid components of superfluid 4 He, in which the total density ρ = ρ n + ρ s does not oscillate [15]. These oscillations can be associated with temperature waves, because the ratio ρ n /ρ s strongly depends on the local temperature, while ρ in 4 He is practically temperature independent. Some solid dielectrics represent another system type which supports the propagation of temperature waves at low temperatures [31][32][33][34]. In this case, the second soun...