He in the Limit of Small Anisotropy*

The polar phase of superfluid ³He is characterized by a single orbital component of the p-wave order parameter, and is favored by the breaking of the orbital rotational symmetry. This can be achieved by introducing anisotropic impurities into pure superfluid. This method has been used experimentally to stabilize the polar phase in the limit of extremely large impurity anisotropy produced by aligned Al₂O₃ strands.¹ However, it has been predicted that even small anisotropy is enough to break the rotational symmetry and stabilize the polar phase close to the transition temperature.² Here, we report evidence for the polar phase in the limit of small impurity anisotropy. We produce this anisotropic impurity by compressing 98% porous silica aerogel by ~20%. At low pressure, P < 7.5 bar, we observe a change in the behavior of the NMR spectra in the superfluid. The high temperature phase at these pressures has a larger NMR frequency shift than the A-phase, while remaining an equal spin pairing state. This is consistent with the polar phase being the equilibrium state near T_c at low pressure.

1. V.V. Dmitriev, et al, Phys. Rev. Lett., 115, 165304, (2015).
2. K. Aoyama and R. Ikeda, Phys. Rev. B. 73, 060504(R) (2006).