Collective modes and sound propagation in a magnetic field in superfluid 3He-B

A high-resolution, ultrasonic (12-89 MHz) acoustic impedance technique has been used to investigate the order parameter collective modes in superfluid 3He-B over a pressure range of 0-15 bar and in magnetic fields up to 180 mT. In agreement with earlier experiments, the J = 2 real squashing mode has been observed to split into five components in small magnetic fields. However, contrary to earlier theoretical estimates, the Zeeman shifts have been found to become extremely nonlinear as the magnetic field is increased. The extent of this nonlinearity is largest at low pressures and at temperatures close to T/sub c/. In comparison with recent theoretical work, the nonlinear Zeeman shifts may be explained as a result of two effects. First, there is a significant distortion of the B-phase energy gap in large magnetic fields. Second, there is an important coupling between the J/sub z/ substates of the different J modes. In this sense the nonlinear evolution of the real squashing mode constitutes the observation of the Paschen-Back effect in 3He-B