Effect ofHe4on the surface scattering ofHe3

We describe and compare the results of two investigations in which the boundary condition for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ was altered by the addition of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, which is preferentially confined to the surfaces by virtue of its smaller zero-point motion. In the first experiment, carried out in normal ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, we observed an increase of specularity in the slip between ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and the polished silicon substrate at a coverage consistent with the onset of superfluidity in the surface ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ layer. The second experiment was performed on superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ confined within the pores of sintered silver. By coating the surfaces with ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, we observed a partial restoration of the superfluid fraction and transition temperature, which had been reduced below bulk by finite-size effects. This effect is due to a change from diffuse to specular boundary scattering. We found that the specularity induced by the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ was destroyed upon the application of a pressure sufficient to solidify the surface ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, confirming the role of surface superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ on specular scattering.