Kavli Affiliate: Jeevak M. Parpia
| First 5 Authors: Petri J. Heikkinen, Lev V. Levitin, Xavier Rojas, Angadjit Singh, Nathan Eng
| Summary:
Anisotropic pair breaking close to surfaces favors chiral superfluid $^3$He-A
over time-reversal invariant $^3$He-B. Confining superfluid $^3$He into a
cavity of height $D$ of the order of the Cooper pair size characterized by the
coherence length $xi_0$ — 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 work 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 $D/xi_0 = 1$. 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.
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