He Stabilized in Anisotropic Silica Aerogel*

Silica aerogel can be used to introduce disorder into superfluid ³He, stablizing new phases and order parameter textures that do not occur in the pure superfluid^1,2,3. In particular, aerogel can be processed to have anisotropic disorder by either stretching or compressing the samples. Using NMR spectroscopy, we observe evidence for a new superfluid phase in ~20% compressed aerogel at low pressure. The magnetic susceptibility is consistent with a state where the atoms forming the Cooper pairs have aligned spin, an equal-spin pairing state. In addition, the frequency shift of this new phase is greater than the A-phase frequency shift in isotropic aerogel. Finally, there is a second-order phase transition into the B-phase at low temperature. These three observations indicate that the polar phase of superfluid ³He has been stabilized at low pressure by the anisotropic disorder of the aerogel.

[1] J. Pollanen et al, Nature Physics 8, 317-320 (2012).
[2] J.I.A. Li et al, Phys. Rev. Lett. 114, 105302 (2015).
[3] V.V. Dmitriev et al, Phys. Rev. Lett. 115, 165304 (2015).