Majorana bound states from exceptional points in non-topological superconductors

TL;DR

This paper proposes a novel method to realize Majorana bound states in non-topological superconductors by exploiting exceptional points in parameter space, leading to localized zero-energy states without the need for topological phases.

Contribution

It introduces a new mechanism for Majorana bound states via exceptional points in trivial superconductor-normal junctions, bypassing the need for topological superconductivity.

Findings

Majorana zero modes emerge without fine-tuning due to charge-conjugation symmetry.

Exceptional points cause bifurcation of quasibound levels into Majorana zero modes.

Majorana dark states exhibit properties similar to topological Majorana states.

Abstract

Recent experimental efforts towards the detection of Majorana bound states have focused on creating the conditions for topological superconductivity. Here we demonstrate an alternative route, which achieves fully localised zero-energy Majorana bound states when a topologically trivial superconductor is strongly coupled to a helical normal region. Such a junction can be experimentally realised by e.g. proximitizing a finite section of a nanowire with spin-orbit coupling, and combining electrostatic depletion and a Zeeman field to drive the non-proximitized (normal) portion into a helical phase. Majorana zero modes emerge in such an open system without fine-tuning as a result of charge-conjugation symmetry, and can be ultimately linked to the existence of `exceptional points' (EPs) in parameter space, where two quasibound Andreev levels bifurcate into two quasibound Majorana zero modes. After the EP, one of the latter becomes non-decaying as the junction approaches perfect Andreev reflection, thus resulting in a Majorana dark state (MDS) localised at the NS junction. We show that MDSs exhibit the full range of properties associated to conventional closed-system Majorana bound states (zero-energy, self-conjugation, $4\pi$-Josephson effect and non-Abelian braiding statistics), while not requiring topological superconductivity.