Coherent multiple Andreev reflections and current resonances in SNS quantum point contacts

We study coherent multiple Andreev reflections in ballistic superconductor-normal conductor-superconductor junctions with a quantum point contact in the normal region of the junction (superconductor-normal region-quantum point contact-normal region-superconductor) with arbitrary transparency. The presence of superconducting bound states in these junctions gives rise to great enhancement of the subgap current. The effect is most pronounced in low-transparency junctions, D≪1, and in the interval of applied voltage Δ/2<eV<2Δ, where the amplitude of the current structures is proportional to the first power of the junction transparency D. The resonant current structures consist of steps and oscillations of the two-particle current and also of multiparticle resonance peaks. The positions of the two-particle current structures have a pronounced temperature dependence, which scales with Δ(T), while the positions of the multiparticle resonances have a weak temperature dependence, being mostly determined by the junction geometry. Despite the large, resonant two-particle current, the excess current at large voltage is small and proportional to D2.