Zero-energy pinning of topologically trivial bound states in multiband semiconductor-superconductor nanowires

Abstract

Recent tunneling conductance measurements on semiconductor-superconductor nanowires find zero-bias peaks to be ubiquitous across wide ranges of chemical potential and Zeeman energy J. Chen et al., Phys. Rev. Lett. 123, 107703 (2019). Motivated by this, we demonstrate that topologically trivial Andreev abound states (ABSs) pinned near zero energy are produced rather generically in inhomogeneous systems with multiband occupancy in the presence of interband coupling. We first investigate the interband coupling mechanism responsible for the pinning within a multiband one-dimensional toy model, then we confirm the findings using a three-dimensional Schrödinger-Poisson approach that incorporates the geometric and electrostatic details of the actual device. Our analysis shows that level repulsion generated by interband coupling can lead to a rather spectacular pinning of the lowest-energy mode near zero energy in systems (or regions) characterized by very short length scales (∼100nm). We show that level repulsion between the lowest-energy levels can mimic the gap opening feature (simultaneous with the emergence of a near-zero-energy mode) predicted to occur in Majorana hybrid systems. We also demonstrate that nearly zero-bias differential conductance features exhibiting particle-hole asymmetry are due to the presence of (topologically trivial) ABSs pinned near zero energy by level repulsion, not to Majorana zero modes, quasi-Majoranas, or any other low-energy mode that involves (partially) separated Majorana bound states. Our findings demonstrate the importance of understanding in detail multiband physics and electrostatic effects in the context of the ongoing search for Majorana modes in semiconductor-superconductor heterostructures.

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