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TT: Fachverband Tiefe Temperaturen

TT 15: Topology: Majorana Physics

TT 15.10: Vortrag

Montag, 9. März 2026, 17:30–17:45, HSZ/0105

Phase dynamics and parity effects in quantum spin Hall Josephson junctions with coupled edges — •Cajetan Heinz¹, Patrik Recher^1,2, and Fernando Dominguez³ — ¹Technische Universität Braunschweig, D-38106 Braunschweig, Germany — ²Laboratory for Emerging Nanometrology, D-38106 Braunschweig, Germany — ³Universität Würzburg, D-97074 Würzburg, Germany

We investigate a novel backscattering mechanism in 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 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 of the SNS junction, opening gaps whenever both types of ABS are at resonance. In this scenario, the 4π-periodic ABS decouples from the rest of the 2π-periodic spectrum. Using the parameters extracted from the microscopic calculation, we construct an RSJ model that incorporates the 4π-periodic ABS, the remaining 2π-periodic part of the spectrum and the quasi-particle continuum. Here, two types of parity changes can arise: transitions between the 4π- and 2π-periodic ABS, and parity leakage from the 2π-periodic ABS into the continuum. Our results show different dynamical regimes depending on the rate of non-adiabatic transitions, ranging from Shapiro steps at integer multiples of ℏ ω_ac/2e, over non-integer steps, to steps only at even integers, with ω_ac as the external ac driving frequency.

Keywords: Quantum spin Hall insulator; fractional Josephson effect; lattice Green's functions; RSJ model; superconductivity