Multiple Andreev reflection and critical current in topological superconducting nanowire junctions

We study transport in a voltage-biased superconductor-normal-superconductor (SNS) junction made of semiconducting nanowires with strong spin-orbit coupling, as it transitions into a topological superconducting phase for increasing Zeeman field. Despite the absence of a fractional steady-state ac Josephson current in the topological phase, the dissipative multiple Andreev reflection current I-dc at different junction transparencies is particularly revealing. It exhibits unique features related to topology, such as gap inversion, the formation of Majorana bound states and fermion-parity conservation. Moreover, the critical current I-c, which remarkably does not vanish at the critical point where the system becomes gapless, provides direct evidence of the topological transition.