Observation of the coherent state in a system of nuclear spin-wave pairs excited by microwave noise pumping

The parametric excitation of nuclear magnons by a microwave noise field was observed in an antiferromagnet. Two critical pumping amplitudes were found to exist. The first one corresponds to the onset of nonlinear microwave absorption. Above the second amplitude, strong phase correlations appear in a system of excited magnon pairs to form a nonequilibrium Bose condensate, which produces intense coherent electromagnetic radiation from the sample and gives rise to the coherent response of parametric magnons to the modulation of their spectrum (modulation response). It was found that, for the noise pumping, the contribution from the processes of elastic magnon relaxation to the threshold pumping amplitudes becomes nonadditive.