Supercurrent in a room-temperature Bose–Einstein magnon condensate

Abstract

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...

Spin superfluidity and spin waves in YIG films

“…This produced a total phase variation δϕ = δωt across the BEC cloud growing linearly with time t and generating spin currents. For the maximal δω = 2π ×550 rad/sec and the maximal life time t = 0.5 µsec of the condensate in the experiment of Bozhko et al 8 (see their Fig. 5) one can conclude that the total phase variation δϕ never exceeded about 1/3 of the full 2π rotation.…”

“…6) that spin superfluidity is possible in a coherent magnon condensate created in yttrium-iron-garnet (YIG) magnetic films by strong parametric pumping 7 , and Bozhko et al 8 declared experimental detection of spin supercurrent in a decay of this condensate. Although the experimental evidence of spin superfluidity was challenged 9,10 (see discussion in the end of the paper) the very idea of spin superfluidity in YIG films deserves a further analysis.…”

Towards properties on demand in quantum materials

“…This was first observed using light-scattering spectroscopy in thin films of yttrium iron garnet 93 (YIG), a ferrimagnet that can be efficiently pumped and has a long spin-lattice relaxation time, allowing for magnon condensation at ambient temperature. More recently, experiments on YIG films uncovered a room-temperature magnon supercurrent 94 : a macroscopic collective motion of magnon condensate subjected to a thermal gradient. This result is appealing in the context of spintronic devices operational at ambient temperature.…”

Spin currents and magnon dynamics in insulating magnets

“…Experimentally observing these phenomena would undoubtedly constitute milestones in this research area. We believe these goals are within reach using the present experimental techniques 32,92,150,151,156,168,190,191 .…”