Chiral Superfluid Helium-3 in the Quasi-Two-Dimensional Limit

Anisotropic pair breaking close to surfaces favors the chiral 𝐴 phase of the superfluid 3He over the time-reversal invariant 𝐵 phase. Confining the superfluid
3He into a cavity of height 𝐷 of the order of the Cooper pair size characterized by the coherence length 𝜉0—ranging between 16 nm (34 bar) and 77 nm (0 bar)—extends the surface effects over the whole sample volume, thus allowing stabilization of the 𝐴 phase at pressures 𝑃 and temperatures 𝑇 where otherwise the 𝐵 phase would be stable. In this Letter, the surfaces of such a confined sample are covered with a superfluid 4He film to create specular quasiparticle scattering boundary conditions, preventing the suppression of the superfluid order parameter. We show that the chiral 𝐴 phase is the stable superfluid phase under strong confinement over the full 𝑃−𝑇 phase diagram down to a quasi-two-dimensional limit 𝐷/𝜉0 =1, where 𝐷 =80  nm. The planar phase, which is degenerate with the chiral 𝐴 phase in the weak-coupling limit, is not observed. The gap inferred from measurements over the wide pressure range from 0.2 to 21.0 bar leads to an empirical ansatz for temperature-dependent strong-coupling effects. We discuss how these results pave the way for the realization of the fully gapped two-dimensional 𝑝𝑥 +𝑖⁢𝑝𝑦 superfluid under more extreme confinement.