Thermalization of a Parametrically Driven Magnon Gas Leading to Bose-Einstein Condensation

Abstract

The thermalization of parametrically pumped magnons caused by nonlinear multimagnon scattering processes and leading to the magnon Bose-Einstein condensation is investigated experimentally with high temporal resolution. The threshold pumping power necessary for the thermalization is determined. For pumping powers above this threshold the thermalization time has been found to decrease rapidly with power reaching the value down to 50 ns, which is much smaller than the magnon lifetime.

Dynamics of a Bose-Einstein condensate of excited magnons

“…1 and were determined on the basis of what is shown by the experimental Brillouin spectra reported in Refs. [1] and [2]. Figure 1.…”

“…The kinetic of evolution of the system of spins in thin films of yttrium-irongarnets in the presence of a constant magnetic field, and being excited by a source of rf-radiation which drives the system towards far-removed from equilibrium conditions, has been reported in detailed experiments performed by Demokritov et al [1,2]. These experimental results have evidenced the occurrence of an unexpected large enhancement of the population of the magnons in the state lowest in energy in their energy dispersion relation.…”

“…A fourth one is the case of magnons already referred to [1,2], which we here analyze in depth. The thermal bath is constituted by the phonon system, with which a nonlinear interaction exists, and the magnons are driven arbitrarily out of equilibrium by a source of electromagnetic radio frequency [21].…”

Bose–Einstein condensation of spin wave quanta at room temperature

“…The experiments were performed on monocrystalline films of YIG with a thickness of 5 mm. YIG (Y 3 Fe 2 (FeO 4 ) 3 ) films are characterized by very small magnetic losses providing a long magnon lifetime, which appears to be much longer than the characteristic time of the magnon-magnon interaction [8]. This relation is a necessary precondition for the BEC in a gas of quasi-particles whose number is not exactly conserved [7].…”

“…The frequency resolution of the BLS set-up was good enough to detect changes in the magnon distribution over the spectrum. Both quasibackward [8] and quasi-forward scattering geometries [9] were used. Applying the quasi-backward geometry, one simultaneously detects magnons from the wavevector interval k = ±2 × 10 5 cm −1 determined by the wavevector of the incident light.…”