SPECULAR AND DIFFUSE-SCATTERING OF QUASI-PARTICLES BY A MACROSCOPIC OBJECT MOVING THROUGH SUPERFLUID HE-3-B

When a wire resonator is moving in superfluid He-3-B at low temperatur e, there is a drag force due to scattering of thermally excited quasip articles. The form of the gl nph of the drag force against the wile ve locity depends strongly on the type of scattering af the wire surface. The force always rises linearly at zero velocity, and it tends to a l imiting value at high wire velocity. However the graph is much more sh arply curved if the scattering is diffuse than if it is specular. Here we present an analytic study of the form of the graph, by making esti mates of the effect of Andreev reflection of scattered quasiparticles iii the superflow round the wire. We show that the differences between diffuse and specular scattering can be understood intuitively, by con sidering the geometrical restrictions imposed by Andreev reflection on the scattering at the wire surface.