SURFACE IMPEDANCE OF YBA2CU3O(7-X) THIN-FILM GRAIN-BOUNDARY JOSEPHSON-JUNCTIONS - EVALUATION OF THE I(C)R(N) PRODUCT

The temperature and the dc magnetic field dependence of the effective surface impedance Z(s) of epitaxial YBa2Cu3O7-x thin films on [001] tilt SrTiO3 bicrystals with tilt angles of 36.8-degrees and 24-degrees have been investigated at 87 GHz. The effects of the grain boundaries become increasingly important with decreasing characteristic voltage I(c)R(n) and increasing unit areal normal resistance R(n)A. The boundaries can consistently be described with a resistively shunted Josephson transmission line model taking into account the effect of finite film thickness. Thermal or magnetic loading of the junction leads to additional losses, whereas the effective microwave penetration depth exhibits a specific extremal behavior. From the magnetic response of Z(s) at 4.2 K, the junction lower critical fields as well as the values of I(c)R(n) and R(n)A can be deduced. The data agree well with dc transport measurements.