Demonstration of the phase coherence of the superconducting wavefunctions between conventional and high- T_c superconductors

Abstract

IF phase coherence of the quantum-mechanical wavefunctions between conventional (BCS) and high-transition-temperature (high-T_c) superconductors were shown to exist, this would place powerful constraints on the as yet unknown mechanism in the latter materials. Here we demonstrate the quantum coherence of the macroscopic superconducting wavefunctions of a conventional and a high-T_c superconductor by the observation of persistent supercurrents and the quantization of magnetic flux in units of h/2e within a composite ring of niobium and YBa₂Cu3O7 (YBCO). This shows unambiguously that the macroscopic wavefunctions are coupled at the interface between the two superconductors, regardless of the mechanism and nature of electronic pairing in high-T_c superconductors. The experiment is a variant of an earlier measurement in which the quantization of flux was measured in an YBCO superconductor ring¹. In our experiment, an adjustable niobium point was used to bridge the gap in a superconducting circuit of YBCO. In addition to observing persistent supercurrents around the ring with discrete flux states separated by the flux quantum, h/2e, the critical current across the junctions also exhibits the expected flux-quantum periodicity. This provides the first demonstration of a d.c. superconducting quantum interference device (SQUID) formed from a composite high-T_c/BCS superconducting circuit.