Doppler shift in Andreev reflection from a moving superconducting condensate in Nb/InAs Josephson junctions

We present experimental and theoretical studies of narrow ballistic Josephson weak links in InAs quantum wells contacted by Nb electrodes and subject to an external magnetic field. We find a dramatic magnetic-field suppression of the Andreev reflection amplitude, which occurs even for in-plane field orientation with essentially no magnetic flux through the junction. Our observations demonstrate the presence of a Doppler shift in the energy of the Andreev levels, which results from diamagnetic screening currents in the hybrid Nb/InAs banks. The data for conductance, excess and critical currents can be consistently explained by taking into account the field- and geometry-dependent phase gradient of the superconducting order parameter and the McMillan energy, characterizing the proximity effect in the Nb/InAs banks. Our analysis suggests that a similar Doppler shift in Andreev reflection should, generally, be expected in other 2D hybrid systems such as, e.g., graphene-superconductor Josephson links.