In-plane penetration depth of high-temperature superconductors with single and double CuO layers

A de 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 Δλ_ab(T) = λ_ab(T) - λ_ab(O) of the in-plane London penetration depth of YBa₂Cu₃O_7-δ (YBCO) and La_1.85Sr_0.15CuO_4-δ (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 d_{x2 - y2}-symmetry of the superconducting order parameter. The same temperature dependence is measured for materials with single and double CuO layers.