The capacitance of grain boundaries in high-temperature superconductor thin films

We have studied the capacitance of grain boundary (GB) Josephson junctions in YBa2Cu3O7-delta thin films on 24 degrees SrTiO3 bicrystals. In order to reveal the contribution of the substrate to the capacitance of the junction s, we have varied the width of the junctions hence tuning the Fiske resonan t frequency. The results clearly indicate that the capacitance of the Josep hson junctions is dominated by the stray capacitance of the subtrate below a frequency of 130 GHz at 4.2 K. At this frequency a sharp change of the Sr TiO3 dielectric constant occurs which is associated with the paraelectric a nd the ferroelastic properties of this material. (C) 2000 Published by Else vier Science B.V. All rights reserved.