Borromean Triads of Phonons in a Magnet

The results of experimental observation of coupled excitations of three pairwise noninteracting phonons, two of which belong to continuous and the third to discrete acoustic spectra, are reported. The coupling is detected by generation of a reversed acoustic wave under transversal electromagnetic pumping in an antiferromagnetic crystal α-Fe 2 O 3 . It is shown that the condition for the coupling of the forward and reversed phase conjugate waves is excitation of an additional acoustic mode of the discrete spectrum, similarly to the Feshbach resonance. The mechanism of the coupling is modulation of the nonlinear acoustic parameter of the crystal by an alternating magnetic field.