Flux quantization in tricrystal cuprate rings - a new probe of pairing symmetry

Based on macroscopic quantum coherence effects, flux quantization and pair tunneling, a tricrystal experiment has been designed and carried out to study the microscopic phase of the pair wavefunction in high Tc 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. By varying the grain misorientation and the grain boundary angles in the tricrystal, we have proved that the order parameter in YBCO has nodes and lobes consistent with d-wave, but not with the g-wave pairing symmetry. The results of this work demonstrate that the half-integer flux quantum effect in superconducting systems containing multi-grains with deliberately designed grain orientation can be used as a general technique for probing the pairing symmetry. © 1995.