Superconductivities in Twisted Interface of Atomically Thin Van der Waals Materials

Engineering moire superlattices by twisting and stacking two layers of van der Waals materials has proven to be an effective way to promote interaction effects and induce exotic phases of matter. After the discovery of superconductivity and correlated insulators in magic-angle twisted bilayer graphene, several different two-dimensional materials have been used to create twisted two-layer systems and various novel phases. In this talk, we will discuss the emergent electronic states observed in various twisted vdW materials. In the first part, we will discuss superconducting multilayer graphene, including twisted bilayer graphene, twisted trilayer graphene, and twisted quadrilayer graphene with alternative twist angles. In these twisted multilayer graphene systems, we also demonstrate a flat electron band tunable by perpendicular electric fields over a range of twist angles. Several correlated behaviors have been observed, including superconductivity and spontaneously broken symmetry states. In the second part of the talk, we will discuss twisted interfaces between stacked van der Waals cuprate crystals that enable tunable Josephson coupling. Using a novel cryogenic assembly technique, we fabricate high-temperature Josephson junctions with an atomically sharp twisted interface between Bi₂Sr₂CaCu₂O_8+x crystals. We find that near the $45^\circ$ twist angle, we observe two-period Fraunhofer interference patterns and fractional Shapiro steps at half integer values, a signature of co-tunneling Cooper pairs necessary for high-temperature topological superconductivity.