Adrian G. Sun

Josephson tunneling in high-{Tc} superconductors

Investigation of the overlap of the pair wavefunction of a conventional superconductor with that of a high Tc cuprate should reveal the symmetry of the superconducting order parameter of the cuprate. We have been studying this overlap by Josephson tunneling (tunneling as opposed to SNS or weak link) between conventional superconductors (Pb, Sn) and YBCO. The observation of Josephson coupling in the c direction implies that the order parameter in YBCO is not strictly of dx2-y2 symmetry. We have carried out studies on YBCO in the form of twinned and detwinned single crystals and thin films on various substrates. The magnetic field dependence of the Josephson current Ic(B) allows a direct, dc, and low field measurement of the low temperature penetration depth (lambda) ab. Measurements of (lambda) a and (lambda) b in detwinned single crystals of YBCO give the value of in-plane anisotropy ratio in this material.