Superconducting diode effect in quantum spin Hall insulator based Josephson junctions

The superconducting diode effect (SDE) is a magnetoelectric phenomenon where an external magnetic field imparts a nonzero center-of-mass momentum to Cooper pairs, either facilitating or hindering the flow of supercurrent depending on its direction. We propose that quantum spin Hall insulator (QSHI)-based Josephson junctions can serve as versatile platforms for nondissipative electronics exhibiting the SDE when triggered by a phase bias and an out-of-plane magnetic field. By computing the contributions from Andreev bound states and the continuum of quasiparticle states, we provide both numerical and analytical results scrutinizing various aspects of the SDE, including its quality $Q$ factor. The maximum value of the $Q$ factor is found to be universal at low (zero) temperatures, which ties its origin to underlying topological properties that are independent of the junction's specific details. As the magnetic field increases, the SDE diminishes due to the closing of the induced superconducting gap caused by orbital effects. To observe the SDE, the QSHI-based Josephson junction must be designed so that its edges are transportwise nonequivalent. Additionally, we explore the SDE in a more exotic yet realistic scenario, where the fermionic ground-state parity of the Josephson junction remains conserved while driving a current. In this $4\ensuremath{\pi}$-periodic situation, we predict an enhancement of the SDE compared to its $2\ensuremath{\pi}$-periodic, parity-unconstrained counterpart.