Zeyu Hao1,Andrew Zimmerman1,Yuval Ronen1,Danial Haei Najafabadi1,Kenji Watanabe2,Takashi Taniguchi2,Philip Kim1
Harvard Universiy1,National Institute for Materials Science (NIMS)2
Zeyu Hao1,Andrew Zimmerman1,Yuval Ronen1,Danial Haei Najafabadi1,Kenji Watanabe2,Takashi Taniguchi2,Philip Kim1
Harvard Universiy1,National Institute for Materials Science (NIMS)2
The realization of magic angle twisted bilayer graphene (TBG) reshaped our understanding of how complex many body physics can be generated in relatively simple materials with straightforward single particle physics. In particular, the superconducting states in TBG hold the possibility of being entirely interaction driven, although many works also point to a phonon-based origin. This difficulty in understanding the nature of the superconductivity extends to newly discovered superconducting graphene systems such as multilayer twisted graphene, normal Bernal-bilayer and rhombohedral-trilayer graphene. These systems all show some features that are suggestive of an unconventional nature, but the interpretations, thus far, are not conclusive. Getting a full picture of one system may help bring us closer to a unified understanding of superconductivity in all graphene systems. Motivated by this, I will discuss our recent efforts in developing a Josephson interferometry technique for moiré superlattices. Fabricated Josephson junctions using a s-wave superconductor have been crucial in identifying the d-wave pairing in cuprate superconductors because they are phase sensitive and can be used to controllably interfere the superconducting order parameter along different spatial directions. Here we report for the first time the realization of Josephson junctions between the TTG superconductors and a bulk 3D s-wave superconductor niobium nitride (NbN). We fabricate NbN contacts that are highly transparent as evidenced by conduction enhancement when TTG is tuned to the normal state, indicating Andreev reflection. When the TTG is tuned to be superconducting, we observe a supercurrent and a Fraunhofer pattern that arises from the junction with a magnetic field period that is distinct from that of the intrinsic Josephson junctions in TTG. More interestingly, the Fraunhofer pattern is significantly modulated when the TTG superconducting phase is tuned. With this successful realization, we are equipped to create corner junctions or SQUIDs that can interfere tunneling in different spatial directions and reveal the pairing symmetry of the TTG superconducting order parameter.