Josephson effect in symmetry-mismatched van der Waals hybrid junctions of NbSe2-Bi2Sr2CaCu2O8+x and NbSe2-Bi2Sr2-xLaxCuO6+y

Abstract

Coupling superconductors of different symmetries may lead to emergent quantum phenomena but experimental studies on heterojunctions made of lattice-mismatched crystals are scarce. Here, we fabricate Josephson junction by van der Waals stacking NbSe2 flakes on freshly cleaved surfaces of Bi2Sr2CaCu2O8+x or Bi2Sr2-xLaxCuO6+y. We obtain pronounced Josephson currents across the heterojunctions at temperatures below 7 K. The Josephson coupling strength, characterized by the product of the critical Josephson current I_c and the normal state resistance R_n, can reach about 0.3 mV, much exceeding the values previously reported in Pb-Bi2Sr2CaCu2O8+x or Nb-Bi2Sr2CaCu2O8+x junctions. Microwave irradiation induces integer Shapiro steps. We also observe the Fraunhofer diffraction pattern by applying an in-plane magnetic field. We compare our results with the theoretical calculations that take into account the coherent tunneling and the order parameter mixing. Our work demonstrates the hybrid junctions as a distinctive platform for addressing pairing symmetry and emergent quantum phenomena.