Nonlinear Spin Dynamics and Magnetic Field Distortion of the Superfluid ${}^3$He-B Order Parameter

We have measured magnetic field distortion of the order parameter and the longitudinal resonance frequency of superfluid ${}^3$He-B using pulsed nuclear magnetic resonance. The experiments were guided by numerical solutions of Leggett's equations which we have generalized to include gap distortion. We have found stable regions for the nonlinear spin dynamics and taken advantage of this to obtain the first measurements of the temperature dependence of the distortion of the energy gap in the B phase. At low temperatures and pressures the results agree with theoretical predictions of Fishman and Sauls.