Origin of deviation from perfect specularity at He-3 film boundaries

Determine the microscopic physical origin of the observed deviation from perfect specular quasiparticle scattering at the boundaries of a slab-shaped cavity of superfluid helium-3 preplated with a superfluid helium-4 film, as evidenced by surface specularity S > 0.97 but less than unity, and quantify the contributions from nonuniformity of the silicon substrate’s adsorption potential and finite solubility of helium-3 in helium-4.

Background

To achieve nearly specular boundary conditions in a nanofabricated 80 nm cavity, the authors preplated the silicon surfaces with superfluid helium-4. Quasiclassical theory parameterizes boundary scattering in terms of the specularity S, and the minimal suppression of the superfluid transition temperature Tc observed in the experiment indicates S > 0.97 across pressures 0.2–23.0 bar.

Despite the near-specular behavior, the measured S is not exactly unity, and the authors state that the physical origin of this small deviation is not understood. They suggest possible causes: nonuniformity of the adsorption potential of the silicon substrate and finite solubility of helium-3 in helium-4. Resolving this uncertainty is important for controlling pair-breaking effects and optimizing boundary conditions for deeper confinement regimes aimed at realizing a fully gapped two-dimensional chiral He-3 A phase.