Excess current of a point contact on a normal-metal single crystal backed by a superconductor

Electrons that are injected into a Ag single-crystal slab at a point contact are focused back onto the contact as holes after Andreev reflection at a Ag-Pb interface. A small applied magnetic field shifts the position of the focusing spot on the scale of the point-contact diameter. The excess current due to the holes is measured both as a function of magnetic field and as a functions of voltage. The results do not completely agree with a simple ballistic model. The deviation for large magnetic field can be understood qualitatively by assuming that the electrons and holes are deflected at random over small angles, for instance by the long-range strain field of dislocations. The observed structure of the excess current for small magnetic field and voltage is probably related to the presence of scattering centres in the point-contact area, although not all features can be explained this way. The corresponding length scales imply that the wave character of the quasi-particles has to be taken into account.