Chiral superfluidity of helium-3 in the quasi-two-dimensional limit

Anisotropic pair breaking close to surfaces favors chiral superfluid ³He-A over time-reversal invariant ³He-B. Confining 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 work 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. 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.