ENERGY SPECTROSCOPY OF ANDREEV LEVELS BETWEEN 2 SUPERCONDUCTORS

We perform energy spectroscopy of Andreev reflection processes occurri ng at two superconducting electrodes connected in series via a ballist ic two-dimensional channel, by measuring the voltage dependence of the part of the conductance which is modulated by the macroscopic phase d ifference. The modulation amplitude oscillates as a function of energy and the phase exhibits an abrupt shift close to pi at the energy for which the amplitude is minimum. We discuss how our findings are relate d to the properties of bound states formed between the two superconduc tors. (C) 1998 Elsevier Science B.V. All rights reserved.