Energy Spectroscopy of Andreev Levels between Two Superconductors

We perform energy spectroscopy of Andreev reflection processes occurring at two superconducting electrodes connected in series via a ballistic two dimensional channel, by measuring the voltage dependence of that part of the conductance modulated by the macroscopic phase difference. The amplitude of the modulation 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. A microscopic theoretical description ascribes the phenomenon to the precursor of a bound state formed between the two superconductors.