Stimulated thermalization of a parametrically driven magnon gas as a prerequisite for Bose-Einstein magnon condensation

Thermalization of a parametrically driven magnon gas leading to the formation of a Bose-Einstein condensate at the bottom of a spin-wave spectrum was studied by time- and wave-vector-resolved Brillouin light scattering spectroscopy. It has been found that the condensation is preceded by the conversion of initially pumped magnons into a second group of frequency-degenerated magnons, which appear due to parametrically stimulated scattering of the initial magnons to a short-wavelength spectral region. In contrast to the first magnon group, in which wave vectors are orthogonal to the wave vectors of the magnons at the lowest energy states, the secondary magnons can effectively scatter to the bottom of the spectrum and condense there.