Magnetoresistance oscillations of quasi-ballistic two-dimensional electron gas coupled with a superconducting ring

We have investigated normal transport in a quasi-ballistic two-dimensional electron gas (2DEG) coupled with a superconducting Nb loop. The oscillations in the resistance are observed as a function of the applied magnetic field which tunes the macroscopic phase difference of the loop. Around zero bias voltage, the resistance maxima occur at even multiples of pi. For energies higher than 0.17 meV the sign of the oscillations becomes opposite, and for the middle energies both types of oscillations are observed. We interpret this crossover to be a manifestation of the two types of contributions from Andreev interfered quasi-particles. The temperature dependence of the oscillation amplitude exhibits reentrant behavior which is similar to that shown in phase-dependent conductance of a diffusive S/N system. (C) 1998 Elsevier Science B.V. All rights reserved.