A-B Transition of Superfluid ³He with a Film Geometry

We have performed cw NMR experiments on superfluid ³He confined to a parallel-plate geometry with a μm scale spacing for a wide pressure range. A static field was applied parallel or perpendicular to the plate surface. The spectra of two absorption signals, a main and a satellite, have been observed below the superfluid transition temperature in a parallel field. As the temperature decreased, the main signal decreased with shifts to higher frequencies, and the satellite grew with shifts to much higher frequencies. From the temperature dependence of these signals and the result in the perpendicular field, it is confirmed that the main signal and the satellite correspond to the A phase signal (ABM state) and the B phase signal (BW state), respectively. The temperature dependence of the two signals indicates that a phase transition from the A phase to the B phase occurs with decreasing temperature. By analyzing these signals, we determine A-B transition temperatures experimentaly. The A-B transition temperature normalized by the superfluid transition temperature is 0.95 at 20 bar, and decreased further to 0.70 at 0 bar for a thickness of 0.88 μm for pure ³He. The values of T_AB/T_c were slightly elevated when covering the surface with 4.5 layers of ⁴He film, which suggests that this transition is also influenced by the surface condition.