Half-integer flux quantum effect in tricrystal cuprate superconductors

We have designed and carried out a tricrystal experiment, based on macroscopic coherence effects, flux quantization, and pair tunneling, to study the microscopic phase of the pair wave function in high- T c cuprate superconductors. Using a high-resolution scanning SQUID microscope, we have made the first direct observation of spontaneously generated half-integer flux quanta in controlled-orientation tricrystal rings and blanket films. By varying the grain misorientation and the grain-boundary angles in the tricrystal, we have proved that the order parameter in YBa 2 Cu 3 O 7 and Tl 2 Ba 2 Cu 6 + δ has nodes and lobes consistent with d wave, but not with g wave pairing symmetry.