Intrinsic and tunable superconducting diode effect in quantum spin Hall systems

Nonreciprocal dissipationless transport has long been sought for applications in superconducting technologies. Recently, it has been implemented by the so-called superconducting diode effect. Such an effect arises from an imbalance in critical supercurrents flowing in opposite directions. In this work, we theoretically demonstrate how the superconducting diode effect emerges in the quantum spin Hall phase when brought into full proximity with a superconductor. We explore two regimes: large and narrow quantum wells. In the former geometry, we show that the superconducting diode effect can be externally controlled using both magnetic and electric fields, achieving unit efficiency. In the latter regime, where tunneling between opposite edges may occur, we propose a mechanism for an intrinsic superconducting diode effect driven by edge reconstruction, which does not require external magnetic fields. This phenomenon, which is related to the topological nature of the underlying system, represents one of the few examples of a superconducting diode effect in low dimensions enabled by spontaneous time-reversal symmetry breaking.