Contribution of Andreev reflection to the mobility of surface state electrons on superfluid ${}^3$He-B

The mobility of the Wigner solid on the superfluid ${}^3$He is determined by the momentum transfer from the scattered ${}^3$He quasiparticles at the free surface. The scattering process of the quasiparticles is classified into the normal reflection and the Andreev retroreflection. Since the quasiparticles nearly conserve the momentum in the process of the Andreev retroreflection at the free surface, the Andreev reflected quasiparticles do not produce a resistive force to the Wigner solid. In this report, we have analytically calculated the contribution of the Andreev retroreflection to the mobility of the Wigner solid on superfluid ${}^3$He-B by employing a realistic model order parameter with the free surface. The Andreev retroreflection is lacked for quasiparticles with energy above the bulk energy gap under the model order parameter. Then, the Andreev retroreflection does not contribute to a rise in the mobility of the Wigner solid on the superfluid ${}^3$He-B. The present model calculation is in good agreement with the previous experimental observation. We have also discussed the Andreev retroreflection under a self-consistently calculated order parameter.