Interedge backscattering in time-reversal symmetric quantum spin Hall Josephson junctions

Abstract: Using standard tight-binding methods, we investigate a novel backscattering mechanism taking place on quantum spin Hall N'SNSN' Josephson junctions in the presence of time-reversal symmetry. This extended geometry allows for the interplay between two types of Andreev bound states (ABS): the usual phase-dependent ABS localized at the edges of the central SNS junction \emph{and} phase-independent ABS localized at the edges of the N'S regions. Crucially, the latter arise at discrete energies $E_n$ and mediate a backscattering process between opposite edges on the SNS junction, yielding gap openings when both types of ABS are at resonance. In this scenario, a 4$\pi$-periodic ABS decouples from the rest of the spectrum, and thus, it can be probed preventing the emission to the quasicontinuum. Interestingly, this backscattering mechanism introduces a new length scale, determining the ratio between $4\pi$- and $2\pi$-periodic supercurrent contributions and distorts the superconducting quantum interference (SQI) pattern. Finally, to proof the participation of these ABS, we propose to use a magnetic flux to tune $E_n$ to zero, resulting in the selective lifting of the fractional Josephson effect.