Transport spectroscopy of N S nanowire junctions with Majorana fermions

We investigate transport through normal-superconductor nanowire junctions in the presence of spin-orbit coupling and magnetic field. As the Zeeman field crosses the critical bulk value B-c of the topological transition, a Majorana bound state (MBS) is formed, giving rise to a sharp zero-bias anomaly (ZBA) in the tunneling differential conductance. We identify novel features beyond this picture in wires with inhomogeneous depletion, such as the appearance of two MBSs inside a long depleted region for B < B-c. The resulting ZBA is in most cases weakly split and may coexist with Andreev bound states near zero energy. The ZBA may appear without evidence of a topological gap closing. This latter aspect is more evident in the multiband case and stems from a smooth pinch-off barrier. Most of these features are in qualitative agreement with recent experiments [V. Mourik et al., Science 336, 1003 (2012)]. We also discuss the rich phenomenology of the problem in other regimes which remain experimentally unexplored.