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

Anisotropic pair breaking close to surfaces favors the chiral A phase of the superfluid ^{3}He over the time-reversal invariant B phase. Confining the superfluid ^{3}He into a cavity of height D of the order of the Cooper pair size characterized by the coherence length ξ_{0}-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 ^{4}He 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/ξ_{0}=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 p_{x}+ip_{y} superfluid under more extreme confinement.