RESONANT ANDREEV REFLECTION IN HYBRID SUPERCONDUCTING-NORMAL NANOSTRUTURES

After a brief description of hybrid superconducting-normal nanostructures and a brief introduction of the Andreev reflection, we present a theoretical investigation of the electron tunneling through a normal-metal/quantum-dot/superconductor (N-QD-S) system where multiple discrete levels of the QD is considered. By using the nonequilibrium-Green-function (NGF) method, the current I is obtained and studied in detail. We find that the Andreev tunneling shows clear resonant behavior, as obtained in previous works. Moreover, the current I versus the gate voltage V_g exhibits different kinds of peaks, depending on the bias voltage, the level-spacing of the QD, and the energy gap of the superconducting electrode. Besides, in I-V characteristics extra peaks superimposed on the conventional current plateaus emerge, which stem from the resonant Andreev reflections.