  The fundamental phenomenon of Bose-Einstein Condensation (BEC) has been
observed in different systems of real and quasi-particles. The condensation of
real particles is achieved through a major reduction in temperature while for
quasi-particles a mechanism of external injection of bosons by irradiation is
required. Here, we present a novel and universal approach to enable BEC of
quasi-particles and to corroborate it experimentally by using magnons as the
Bose-particle model system. The critical point to this approach is the
introduction of a disequilibrium of magnons with the phonon bath. After heating
to an elevated temperature, a sudden decrease in the temperature of the
phonons, which is approximately instant on the time scales of the magnon
system, results in a large excess of incoherent magnons. The consequent
spectral redistribution of these magnons triggers the Bose-Einstein
condensation.
