Engineering superfluid $^3$He phase stability with disorder

We report our NMR measurements on $^3$He superfluid in well-characterized aerogel samples, with anisotropy induced by uniaxial compression of $\approx 20\%$. By comparing with our previous work on the same sample without compression, we show that a critical field appears in the $T$-$H^2$ phase diagram induced by the aerogel anisotropy, providing clear evidence that anisotropic impurity scattering modifies the relative phase stability of different $p$-wave superfluid state, making the isotropic B-phase energetically more favorable than the magnetic field induced A-phase. Furthermore, we demonstrate that the 3-dimensional glass phase of $^3$He-A observed in the isotropic aerogel is suppressed by the anisotropic disorder, and the remaining 2-D continuous symmetry in the plane perpendicular to the strain axis gives rise to a 2-D glass phase of $^3$He-A, contrary to the expected 1-D alignment of the A-phase texture along the strain axis.