Pair breaking at the surface of the unconventional superfluid*

In unconventional p-wave superfluid ³He, Cooper pairs are subject to anisotropic pair breaking in the vicinity of surfaces and interfaces. This effect dominates when the superfluid is confined in a cavity of height comparable to the coherence length. The surface pair-breaking depends on the boundary condition for surface scattering of quasiparticles. We study superfluid ³He confined in a single nanofabricated 200 nm high cavity, using SQUID-NMR as a probe of superfluid order parameter, and pressure as a tuning parameter. The surface scattering is also tuned in situ, by adjustment of the surface boundary layer. We make an accurate determination of the suppression of the superfluid transition temperature, T_c. We also measure the gap suppression for different boundary conditions. With a solid ⁴He surface layer we find close to diffuse scattering, while by coating the surface with a superfluid ⁴He film we find close to perfect specular scattering, with the almost complete elimination of T_c and gap suppression. With a solid ³He layer on the surface, the observed T_c suppression is significantly stronger than that predicted by quasiclassical theory with diffuse scattering boundary conditions.