Theory of zero-field superconducting diode effect in twisted trilayer graphene

Abstract

In a recent experiment [Lin et al., arXiv:2112.07841], the superconducting phase hosted by a heterostructure of mirror-symmetric twisted trilayer graphene and WSe2 was shown to exhibit significantly different critical currents in opposite directions in the absence of external magnetic fields. We here develop a microscopic theory and analyze necessary conditions for this zero-field superconducting diode effect. Taking into account the spin-orbit coupling induced in trilayer graphene via the proximity effect, we classify the pairing instabilities and normal-state orders and derive which combinations are consistent with the observed diode effect, in particular, its field trainability. We perform explicit calculations of the diode effect in several different models, including the full continuum model for the system, and illuminate the relation between the diode effect and finite-momentum pairing. Our theory also provides a natural explanation of the observed sign change of the current asymmetry with doping, which can be related to an approximate chiral symmetry of the system, and of the enhanced transverse resistance above the superconducting transition. Our findings not only elucidate the rich physics of trilayer graphene on WSe2, but also establish a means to distinguish between various candidate interaction-induced orders in spin-orbit-coupled graphene moiré systems, and could therefore serve as a guide for future experiments as well.

Josephson diode effect from Cooper pair momentum in a topological semimetal

“…Recent theoretical work [16][17][18] has proposed that in two-dimensional (2D) superconductors with strong spin-orbit coupling under an in-plane magnetic field, Cooper pairs can acquire a finite momentum and give rise to a diode effect, where the direction of the Cooper pair momentum determines the polarity of the effect. At the same time, although there exist prior theoretical proposals [19][20][21][22] , none of them have been experimentally realized, and a theoretical description for a field-induced diode effect in JJs (the Josephson diode effect (JDE)) that would match experimental observations has not yet been formulated.…”

Non-Reciprocal Supercurrents in a Field-Free Graphene Josephson Triode

“…The utility of diodes resides in their ability to offer nonreciprocity -a low resistance to current flowing in one direction and a high resistance to current flowing in the opposite direction. While traditional diodes exploit P-N interfaces in semiconducting materials, a flurry of theoretical interest has focused on developing their superconducting analogues [1][2][3][4][5][6][7][8].…”

Josephson Diode Effect in High Mobility InSb Nanoflags

“…The first experimental report on the SDE, based on electrically polar materials, 10 has appeared very recently, demonstrating supercurrent rectification. Soon after, other systems [11][12][13][14][15][16][17][18][19][20][21][22] have been inspected looking at supercurrent non-reciprocal transport, complemented by theoretical efforts, 9,[23][24][25][26][27][28][29][30][31][32] in order to shed light on the microscopic mechanisms responsible for the SDE. However, both from an experimental and a theoretical point of view, this field is still in its infancy with several open questions.…”