We measured the transverse acoustic impedance of superfluid ${}^3\mathrm{He}\mathrm{\text{−}}\mathrm{B}$ with a wall coated by several layers of ${}^4\mathrm{He}$. The coating is known to enhance the specularity in quasiparticle scattering by the wall. We found a new anomaly, a bump and a peak, in the temperature dependence of the transverse acoustic impedance. This agrees with a theoretical calculation using a partially specular wall boundary condition. The new anomaly is shown to arise from a change in the surface density of states by coating and the scattering of thermally occupied surface bound states to other states. The change is towards the density of states of Majorana cone in the specular limit.

• Received 5 March 2009

DOI:https://doi.org/10.1103/PhysRevLett.103.155301