Nonlinear transport of ions trapped below the free surface of superfluid

We have studied the driving-field dependent transport of ions trapped below the free surface of superfluid 3 He-B. For small driving fields, the velocity of ions is a linear function of the field. At high electric fields, the velocity is observed to be a nonlinear function of the field. The onset velocity of the nonlinearity is consistent in magnitude with the Landau critical velocity. This implies that the nonlinear behavior is caused by the pair-breaking effect of the moving ions.