Bose-Einstein Condensation of Magnons by Instant Cooling
Abstract: Bose Einstein condensation (BEC) is an ensemble of (quasi-)particles occupying a single state in momentum space. BECs have not only been observed in clouds of ultracold atoms, but also the BEC of different bosonic quasi-particles such as exciton polartions and magnons, the quanta of spin wave excitation, have been experimentally reported. This has opened up a manifold of research directions and allows for the realization of coherent macroscopic states in a solid state at room temperature. Recent advances such as the observation of room-temperature magnonic supercurrents are just one example for the rich physics of quasi-particle BEC.
Here, we report on a fundamentally new way to create such a quasi-particle BEC by using magnons in the ferromagnetic insulator yttrium iron garnet (YIG) as a model system. In previous realizations of quasi-particle BECs, a large number of "hot" quasi-particles have been injected into the respective quasi-particle system either by microwave pumping (magnons) or, for instance, by optical pumping (polaritons). In our experiment, we demonstrate that the interaction of magnons with the phonon bath can provide a very simple way to achieve a magnon BEC at room temperature: By cooling down a previously heated nanostructure made of YIG in contact with platinum and gold heat sinks on a time scale that lies below the lifetime of dipolar magnons, a nonequilibrium is induced in the magnon subsystem. Using time-resolved Brillouin light scattering spectroscopy, we demonstrate that this results in the formation of a magnon BEC manifesting itself by a large spin wave intensity at the bottom of the spectrum during the cooling of the magnetic structure. In my presentation, I will reveal the influence of the heating time and the achieved maximum temperature. I will demonstrate that a high cooling rate on the order of 20 K/ns is crucial for the realization of the quasi-particle BEC.