Direct observation of Bose–Einstein condensation in a parametrically driven gas of magnons

A gas of thermalized quasi-equilibrium magnons with a non-zero chemical potential created in a magnetic film using parametric pumping is studied. The value of the chemical potential of the gas is determined directly from the measured distribution of magnons over the spectrum. With increasing pumping power the value of the chemical potential increases. At high enough pumping powers it reaches the energy corresponding to the lowest magnon frequency. Under these conditions a very narrow peak of magnon population at the lowest magnon frequency appears. The measured width of the peak is five orders of magnitude smaller than that expected for the thermal distribution. We associate this effect with Bose–Einstein condensation of magnons.