Bottleneck Accumulation of Hybrid Magnetoelastic Bosons

It is known that an ensemble of magnons, quanta of spin waves, can be prepared as a Bose gas of weakly interacting quasiparticles with conservation of the particle number. Furthermore, the thermalization of the overpopulated magnon gas can lead to the formation of a Bose-Einstein condensate at the bottom of a spin-wave spectrum. However, magnon-phonon scattering processes can significantly modify this scenario and new quasiparticles are formed - magneto-elastic bosons. Our observations of a parametrically populated magnon gas in a single-crystal film of Yttrium Iron Garnet by means of wavevector-resolved Brillouin light scattering spectroscopy including magneto-elastic coupling resulted in the discovery of a novel condensation phenomenon: A spontaneous accumulation of hybrid magneto-elastic bosonic quasiparticles at the intersection of the lowest magnon mode and a transversal acoustic wave.