Superfluid ³He in strong magnetic fields: Anomalous sound attenuation in the B phase and evidence for splitting of the AB transition

Zero sound attenuation at 8.9 and 26.8 MHz in B3 shows an extremum at TAB in a field of 2 kG at low pressures, both in the stationary and in the rotating superfluid. This is in accordance with the prediction that a critical magnetic field separates two types of B phases at the AB transition line, with and without nodes in the energy gap, respectively, and with a change in the nature of the transition. The AB phase transition takes place via an intermediate state, possibly a new phase, characterized by excess sound attenuation.