Multiple Andreev reflections in diffusive SNS structures

We report new measurements on subgap energy structures originating from multiple Andreev reflections in mesoscopic SNS junctions. The junctions were fabricated in a planar geometry with high-transparency superconducting contacts of Al deposited on highly diffusive and surface delta-doped n(++)-GaAs. For samples with a normal GaAs region of active length 0.3 mu m, the Josephson effect with a maximal supercunent I-C = 3 mu A at T = 237 mK was observed. The subgap structure was observed as a series of local minima in the differential resistance at de bias voltages V = +/-2 Delta/(ne) with n = 1, 2, 4, i.e, only the even subgap positions. While at V = +/-2 Delta/e(n = 1) only one dip is observed, the n = 2 and the n = 4 subgap structures each consists of two separate dips in the differential resistance. The mutual spacing of these two dips is independent of temperature, and the mutual spacing of the n = 4 dips is half the spacing of the n = 2 dips. The voltage bias positions of the subgap differential resistance minima coincide with the maxima in the. oscillation amplitude when a magnetic field is applied in an interferometer configuration, where one of the superconducting electrodes has been replaced by a flux-sensitive open loop. (C) 1999 Academic Press.