Theory of time reversal topological superconductivity in double Rashba wires -- symmetries of Cooper pair and Andreev bound states

TL;DR

This paper investigates how double Rashba wires coupled with an s-wave superconductor can host time-reversal invariant topological superconductivity, characterized by zero-energy Andreev bound states and enhanced odd-frequency pairing.

Contribution

It derives the topological criterion for the system, classifies Cooper pair symmetries, and analyzes junction properties involving Majorana fermions and Andreev bound states.

Findings

Topological phase hosts Kramers pairs of Majorana fermions.

Odd-frequency pairing is significantly enhanced in the topological state.

Andreev bound state energy dispersion varies with phase difference, showing a sin(φ) dependence.

Abstract

We study the system of double Rashba wires brought into the proximity to an $s$-wave superconductor. The time reversal invariant topological superconductivity is realized if the interwire pairing corresponding to crossed Andreev reflection dominates over the standard intrawire pairing. We derive the topological criterion and show that the system hosts zero energy Andreev bound states such as a Kramers pair of Majorana fermions. We classify symmetry of the Cooper pairs focusing on the four degrees of freedom, $i.e.$, frequency, spin, spatial parity inside wires, and spatial parity between wires. The magnitude of the odd-frequency pairing is strongly enhanced in the topological state. We also explore properties of junctions occurring in such double wire systems. If one section of the junction is in the topological state and the other is in the trivial state, the energy dispersion of Andreev bound states is proportional to $\sim\pm\sin\varphi$, where $\varphi$ denotes the macroscopic phase difference between two sections. This behavior can be intuitively explained by the couplings of a Kramers pair of Majorana fermions and spin-singlet $s$-wave Cooper pair and can also be understood by analyzing an effective continuum model of the $s+p$/$s$-wave superconductor hybrid system.