| We present continuous-wave NMR measurements of superfluid 3He confined to two low-density silica aerogels, with porosities of 99.3% and 98.6%. we find a first-order transition on cooling between two superfluid phases, from an A-like equal-spin pairing phase to a B phase. Careful measurements of the NMR spectra, integrated susceptibility, and average frequency shift demonstrate a broadened first-order transition on warming, from the B phase to possibly another A-like equal-spin pairing phase. This A-B transition on warming occurs over a broad range of temperatures, and in both samples, it is unable to convert the entire sample from the B to A-like phase before reaching Tc,aero By performing "trajectory" measurements (warming from the B phase until the A-like phase is seen, then subsequently cooling), we are able to show that the A-like phases on warming and cooling are identical. Measurements of the gap suppression in the A-like phase show that the aerogel has a single value of Tc, so that the width of the A-B transition is caused not by inhomogeneous broadening of the superfluid transition, but rather by pinning of the A-B interface.; Having thus established the initial temperatures of the A-B transition on warming and cooling, TAB,w and TAB,c respectively, we consider the value of the equilibrium thermodynamic transition temperature, TAB. We develop a method for determining this temperature from precise measurements of the maximum frequency shifts and susceptibilities of the A-like and B phases. We find that TAB is equal to TAB,w within experimental error for both samples at all pressures where this measurement is possible. On this basis, we present the first thermodynamic A-B phase diagram of superfluid 3He in aerogel. We find that this phase diagram is substantially modified from the bulk, in a manner that is density-independent. We show that for any equal spin pairing state with transverse frequency shifts that agree with the measured value of our A-like phases, the order parameter is uniquely determined from the slope of the equilibrium A-B phase diagram. We tentatively identify the A-like phase as being the planar (two-dimensional) state, which has never before been reported to exist. |
