Experimental investigation of two-phonon absorption by the real squashing mode in superfluid ³He-B

We report on investigations of a nonlinear parametric excitation of the real squashing (RSQ) collective mode in superfluid ³He-B. Two-phonon absorption (TPA) at f_s + f_p = f_rsq has been observed by two coincident zero sound pulses, with the signal and the pump wave frequencies at f_s and f_p, respectively. We have also discovered and studied the Zeeman splitting of TPA in a magnetic field, which is a characteristic feature of the J = 2⁺ mode in the linear response regime. The dependence of the mode frequency on the magnitude of the wave vector has been investigated; this can be done easily in parametric measurements only. In our earlier, preliminary observations, the coupling of the RSQ mode by TPA was weaker than theory predicted; the discrepancy has now been systematically studied and clarified.