Josephson effect in junctions of conventional and topological superconductors

TL;DR

This paper provides a theoretical analysis of the Josephson effect in hybrid devices combining conventional and topological superconductors, revealing unique behaviors such as a finite supercurrent at zero phase difference and the influence of Majorana states.

Contribution

It introduces a detailed theoretical framework for understanding the Josephson effect in S-TS hybrid junctions, including the impact of quantum dots and multi-terminal geometries.

Findings

Critical current is smaller and shows a kink dependence on Zeeman field in topological regime.

Quantum dots enable finite Josephson current even in deep topological phase.

The setup can support $$-junction behavior with supercurrent at zero phase difference.

Abstract

We present a theoretical analysis of the equilibrium Josephson current-phase relation in hybrid devices made of conventional s-wave spin-singlet superconductors (S) and topological superconductor (TS) wires featuring Majorana end states. Using Green's function techniques, the topological superconductor is alternatively described by the low-energy continuum limit of a Kitaev chain or by a more microscopic spinful nanowire model. We show that for the simplest S-TS tunnel junction, only the s-wave pairing correlations in a spinful TS nanowire model can generate a Josephson effect. The critical current is much smaller in the topological regime and exhibits a kink-like dependence on the Zeeman field along the wire. When a correlated quantum dot (QD) in the magnetic regime is present in the junction region, however, the Josephson current becomes finite also in the deep topological phase as shown for the cotunneling regime and by a mean-field analysis. Remarkably, we find that the S-QD-TS setup can support $\varphi_0$-junction behavior, where a finite supercurrent flows at vanishing phase difference. Finally, we also address a multi-terminal S-TS-S geometry, where the TS wire acts as tunable parity switch on the Andreev bound states in a superconducting atomic contact.