PRECISION-MEASUREMENT OF THE AB-PLANE LONDON PENETRATION DEPTH IN HIGH-TEMPERATURE SUPERCONDUCTING THIN-FILMS

The temperature change of the in-plane penetration depth Delta lambda( ab)(T) = lambda(ab)(T) - lambda(ab)(O) of HTS thin films was determine d with an experimental resolution of better than 0.2 Angstrom using a novel dc-technique based on the measurement of the magnetic field depe ndence of the critical current of bicrystal grain boundary Josephson j unctions (GBJs). Over a wide temperature range the data obtained for d ifferent high temperature superconductors confirm with high accuracy t he theoretical prediction for a d(x2-y2)-symmetry of the superconducti ng order parameter in the limit of weak coupling.