Pair density wave and loop current promoted by Van Hove singularities in moire systems

We theoretically show that in the presence of conventional or higher order Van Hove singularities (VHS), the bare finite momentum pairing, also known as the pair density wave (PDW), susceptibility can be promoted to the same order of the most divergent bare BCS susceptibility through a valley-contrasting flux 3 phi in each triangular plaquette at phi = ir/3 and ir/6 in moire systems. This makes the PDW order a possible leading instability for an electronic system with repulsive interactions. We confirm that it indeed wins over all other instabilities and becomes the ground state under certain conditions through the renormalization group calculation and a flux insertion argument. Moreover, we also find that a topological nontrivial loop current order becomes the leading instability if the Fermi surface with conventional VHS is perfectly nested at phi = ir/3. Similar to the Haldane model, this loop current state has the quantum anomalous Hall effect. If we dope this loop current state or introduce a finite next-nearest-neighbor hopping t', the chiral d-wave PDW becomes the dominant instability. Experimentally, the flux can be effectively tuned by an out-of-plane electric field in moire systems based on graphene and transition metal dichalcogenides.

Automated summary

We show that the pair density wave (PDW) susceptibility can be enhanced to the same level as the BCS susceptibility through a valley-contrasting flux in moire systems. The PDW order becomes the leading instability under certain conditions, while a topological loop current order emerges if the Fermi surface with conventional Van Hove singularities is perfectly nested. The flux can be controlled experimentally in moire systems based on graphene and transition metal dichalcogenides.