Modification of the Superfluid ${}^3\mathrm{He}$ Phase Diagram by Impurity Scattering

The effect of impurity scattering on the phase diagram of pure superfluid ${}^3\mathrm{He}$ has been investigated by acoustic techniques near the bulk polycritical pressure. Impurities were introduced with a $98\%$ porous silica aerogel. In zero applied field, the equilibrium phase is a $B$ phase, consistent with predictions from isotropic scattering. The superfluid transition in a magnetic field at 25 bars is from normal to $A$ phase and is independent of the magnetic field up to 2.9 kG. A first-order phase transition between $A$ and $B$ superfluids is observed with surprisingly strong supercooling in both zero and nonzero applied field.