Physics Tomorrow Letters

Introduction

Similar to a traditional semiconductor diode, a superconductor with non-reciprocal current flow, an SC diode, may form the building block for, e.g., dissipationless SC digital logic. The recent observation of such an SC diode effect in a complex thin-film superconductor heterostructure subjected to an external magnetic field has stimulated vigorous activity towards understanding and replicating it1. Supercurrent rectification has also been demonstrated in multiple Josephson junction systems including Al-InGaAs/InAs-Al2, NbSe2/Nb3Br8/NbSe23 and Nb-NiTe2-Nb4, where the largest nonreciprocity is observed at large in-plane magnetic fields2,4. Furthermore, an intrinsic SC diode effect has been observed in few-layer NbSe25 and twisted trilayer graphene/WTe2 heterostructures6 in an out-of-plane magnetic field. Several theoretical mechanisms have been proposed to explain the SC diode effects in superconductors7-9 and in Josephson junctions10, with special emphasis on a potential role of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state7,9,10. The breaking of time-reversal and inversion symmetries in a superconductor with Rashba spin-orbit coupling (SOC) supports a helical superconducting phase, with the order parameter modulated in a direction transverse to the Zeeman field: Δ(𝑟)=Δ𝑒𝑖𝑞0𝑟. In such an FFLO-like phase, Cooper pairs gain a finite momentum 𝑞0, and the depairing effect for supercurrents flowing parallel and anti-parallel to 𝑞0 is different, leading to a critical current nonreciprocity. On general grounds, interfaces cause inversion symmetry-breaking and Rashba SOC along the normal z-direction thereby admitting a polar and odd-under-time-reversal vector 𝑇 along 𝑧×ℎ, where ℎ is an applied magnetic field or an exchange field induced by an adjacent ferromagnet (FM)11. Thus, the application of an in-plane magnetic field perpendicular to the current flow direction should activate 𝑇, resulting in peculiar superfluid condensate properties and a critical current nonreciprocity. To quantify the diode effect, it is common to introduce an asymmetry parameter, called the diode efficiency, 𝜂=𝐼𝑐+−𝐼𝑐−𝐼𝑐++𝐼𝑐− where Ic+ and Ic- are the critical currents in the two directions. The value denotes the magnitude of the diode effect, while the sign defines the polarity. Up to now, reported values of 𝜂 range from a few percent to 30%1-6.