Abstract
A dc technique based on the measurement of the magnetic-field dependence of the critical current of bicrystal grain boundary Josephson junctions (GBJs) is used to precisely determine the temperature change Δλab(T) = λab(T) − λab(0) of the in-plane London penetration depth of ybco (YBCO) and lsco (LSCO) thin films. The resolution of the applied measuring technique is better than 0.2 Å and the measured dependences are not sensitive to extrinsic influences. Over a wide temperature range the data obtained for different high-temperature superconductors confirm with high accuracy the theoretical prediction for a dx2 − y2-symmetry of the superconducting order parameter. The same temperature dependence is measured for materials with single and double CuO layers.