Bose–Einstein condensation is commonly observed in pulsed mode, when a cloud of particles (or quasiparticles) is cooled below the critical temperature and forms a coherent state for a limited time. In this work, a continuously existing magnon Bose–Einstein condensate, where the disappearance of magnons is compensated by the excitation of new magnons, is demonstrated. It is shown experimentally that new magnons are excited exclusively in the Bose–Einstein condensate state, in accordance with the Feynman theory. This result follows from the fact that the photon emission line of the magnon Bose–Einstein condensate is narrowed abruptly upon the transition to the Bose–Einstein condensate state and the spin–lattice broadening of this line vanishes. In other words, it is directly established that the relaxation of magnons and excitation of new magnons under these conditions do not change the quantum state of the magnon Bose–Einstein condensate. This result should be considered as proof that relaxation processes of individual magnons do not change the quantum state of the magnon Bose condensate. This is a prerequisite for the creation of multiparticle quantum devices.
Bunkov et al. (Thu,) studied this question.