Precision measurement of the ab-plane London penetration depth in high temperature superconducting thin films

The temperature change of the in-plane penetration depth Δλ ab (T)=λ ab (T)-λ ab (0) of HTS thin films was determined with an experimental resolution of better than 0.2A using a novel dc-technique based on the measurement of the magnetic field dependence of the critical current of bicrystal grain boundary Josephson junctions (GBJs). Over a wide temperature range the data obtained for different high temperature superconductors confirm with high accuracy the theoretical prediction for ad x 2 −y 2-symmetry of the superconducting order parameter in the limit of weak coupling.