Magnon supercurrent transport and interference effects

There is an enormous need for faster and more efficient information processing and data transfer. It is of fundamental importance that new physical phenomena are found, harvested and brought to a high level of understanding.
The utilization of magnon Bose-Einstein condensates (BECs) and magnon supercurrents driven by a phase gradient in a spatially extended coherent magnon BEC is a very promising approach for the transfer and processing of spin information. In our experiments, we have found a fingerprint of the supercurrent efflux of condensed magnons subjected to a phase gradient imposed by a local heating of the magnetic sample. Moreover, we revealed that the condensed magnons, being pushed out from the heated area, form compact density humps, which propagate over long distances through the thermally homogeneous magnetic medium. We refer to them as a superposition of Bogoliubov waves with oscillations of both the amplitude and the phase of the magnon BEC’s wave function. In the long-wavelength limit, these waves have a linear dispersion law and can be considered as a magnon second sound potentially featuring viscosity-free propagation.
A further consequence of the magnon BEC is the prediction of interference effects of the BEC wavefunctions. The well-known ac Josephson effect relies on the existence of two weakly coupled macroscopic quantum states. Recently, we discovered the ac Josephson effect in a magnon BEC carried by a room-temperature ferrimagnetic magnetic film. The BEC is formed in a parametrically populated magnon gas around a potential trench created by a dc electric current. The appearance of the Josephson effect is manifested by oscillations of the magnon BEC density in the trench, caused by a coherent phase shift between this BEC and the BEC in the nearby left and right zones.
All these findings advance the physics of room-temperature macroscopic quantum phenomena and will allow for their application for data processing in magnon spintronics devices.