Abstract
Bose–Einstein condensation (BEC) of magnons is one of the few macroscopic quantum phenomena observable at room temperature. Due to the competition of the exchange and the magnetic dipole interactions, the minimum-energy magnon state is doubly degenerate and corresponds to two antiparallel non-zero wavevectors. Correspondingly, the room-temperature magnon BEC differs essentially from other condensates, since it takes place simultaneously at ± kmin. The degeneracy of BEC and interaction between its two components have significant impact on condensate properties. Phase locking of the two condensates causes formation of a standing wave of the condensate density and quantized vortices. Additionally, interaction between the two components is believed to be important for stabilization of the condensate with respect to a real-space collapse. Thus, the possibility to create a non-degenerate, single-component condensate is decisive for understanding of underlying physics of magnon BEC. Here, we experimentally demonstrate an approach, which allows one to accomplish this challenging task. We show that this can be achieved by using a separation of the two components of the degenerate condensate in the real space by applying a local pulsed magnetic field, which causes their motion in the opposite directions. Thus, after a certain delay, the two clouds corresponding to different components become well separated in the real space. We find that motion of the clouds can be described well based on the peculiarities of magnon dispersion characteristics. Additionally, we show that, during the motion, the condensate cloud harvests non-condensed magnons, which results in a partial compensation of condensate depletion.
Highlights
Bose–Einstein condensation (BEC) is one of the most striking manifestations of quantum nature of matter on the macroscopic scale
Since magnon dispersion in yttrium iron garnet (YIG) films is governed by the competition of two interactions—the dipole interaction dominating at small wavevectors and the exchange interaction dominating at large wavevectors,the spectrum of magnons in magnetic films possess two energy minima corresponding to non-zero wavevectors ± kmin[23]
The magnon condensate is created at room temperature in the yttrium iron garnet film with the thickness of 5.1 μm and the lateral dimensions of 2 × 10 m m2 by using microwave parametric pumping
Summary
Bose–Einstein condensation (BEC) is one of the most striking manifestations of quantum nature of matter on the macroscopic scale. Since its discovery[6], significant efforts have been put into studies of the nature of this phenomenon and understanding of characteristics of magnon c ondensates[10,11,12,13,14,15,16,17] Very recently, such important issues as spatial and temporal coherence of the magnon condensate, its stability. Another unique feature of the room-temperature magnon BEC in YIG, which makes its study interesting and challenging, is its double degeneracy. In24, it was experimentally shown that the two components of the condensate can be phase-locked to each other, resulting in formation of a standing wave of the total condensate density in the real space. We find experimentally that the observed separation process conserves the total number of magnons in the split clouds
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