Transport spectroscopy of NS nanowire junctions with Majorana fermions

TL;DR

This paper explores transport phenomena in NS nanowire junctions with spin-orbit coupling and magnetic fields, revealing complex Majorana-related features and inhomogeneity effects that align with recent experimental observations.

Contribution

It uncovers new transport signatures of Majorana bound states in inhomogeneous nanowires, including weakly split zero-bias anomalies and coexistence with Andreev states, expanding understanding beyond idealized models.

Findings

Zero-bias anomalies can occur without topological gap closing.

Multiple Majorana states can form in inhomogeneous regions.

Features align qualitatively with recent experimental results.

Abstract

We investigate transport though 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, like 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 [Mourik et al, Science 336, 1003 (2012)]. We also discuss the rich phenomenology of the problem in other regimes which remain experimentally unexplored.