Spatial separation of degenerate components of magnon Bose-Einstein condensate by using a local acceleration potential

Abstract

Bose-Einstein condensation (BEC) of magnons is one of few macroscopic quantum phenomena observable at room temperature. Due to 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, magnon BEC differs essentially from other condensates, since it takes place simultaneously at +/-k_min. The degeneracy of BEC and interaction between its two components have significant impact on the 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 the 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 the motion of the clouds can be described well based on the peculiarities of the magnon dispersion characteristics. Additionally, we show that, during the motion, the condensate cloud harvests non-condensed magnons, which results in a partial compensation of the condensate depletion.