Tricrystal Tests of Pairing Symmetry in Cuprate Superconductors

A definitive determination of the order parameter symmetry in cuprate superconductors is very important to understanding the origin of high-temperature superconductivity. By using a scanning Superconducting Quantum Interference Device (SQUID), we made the first direct observations of half integer flux quanta in controlled-orientation tricrystal rings of high-Tc cuprates such as YBa2Cu3O7-δ with three grain boundary Josephson junctions. By varying the grain misorientation and the grain boundary angles in the tricrystals, we have proven that the pair state in YBa2Cu3O7 is consistent with d-wave, but not with the g-wave pairing symmetry.