Observation of room-temperature magnon supercurrents*

Finding new ways for fast and efficient processing and transfer of data is one the most challenging tasks nowadays. Elementary spin excitations - magnons (spin wave quanta) - open up a very promising direction. Magnons are bosons, and thus they are able to form spontaneously a spatially extended, coherent ground state, a Bose-Einstein condensate (BEC). The BEC can be conveniently created in a single-crystal film of yttrium iron garnet even at room temperature using parametric pumping. An extraordinary challenge is the use of this macroscopic quantum state for information transfer and processing. Recently we have succeeded to create a magnon supercurrent in such a macroscopic quantum state by introducing a spatial phase gradient to the wave function created by local heating. The temporal evolution of the magnon BEC and the supercurrent was studied by means of time- and wavevector-resolved Brillouin light scattering spectroscopy. We have found that local heating in the focal point of a probing laser beam leads to a decay of the BEC, which is a fingerprint of the outflow of condensed magnons driven by a thermally induced phase gradient. I will demonstrate non-local probing of the magnon supercurrent, which provides direct evidence of the condensate propagation. By utilizing a separate pulsed blue laser for heating purpose, we are able to control the phase gradient, while a low-power green laser is used for local probing of the condensate area. The supercurrent pulse is detected on an undisturbed background of the slowly decaying magnon BEC. The occurrence of the supercurrent directly confirms the phase coherency of the magnon condensate and opens door to studies in the general field of magnonic macroscopic quantum transport phenomena at room temperature as a novel approach in the field of information processing.