ELECTRICAL-PROPERTIES OF GRAIN-BOUNDARIES IN BA1-XKXBIO3 POLYCRYSTALLINE THIN-FILMS

Ba1-xKxBiO3 polycrystalline thin films are prepared on sapphire substrates by evaporation. In these films, grain boundaries affect the film resistivity in the normal state and I-V characteristics in the superconducting state. The temperature dependence of the resistivity is metallic for low-resistivity samples. On the other hand, that for high-resistivity is semiconductive and a bend is observed near 100 K. The I-V characteristics show series-connected tunneling Josephson junctions formed along a grain boundary. The ratio between the superconducting energy gap and T c is estimated to be 3.5, equal to that of BaPb1 -xBixO3. Using the resistively shunted junction model and Ambegaokar and Baratoff's theory [Phys. Rev. Lett. 10, 486 (1963)], the tunneling resistance in a superconducting state is derived from the Josephson critical current densities and the hysteresis of the I-V curves. It increases with decreasing temperature at low temperatures and changes the junctions from weak link to tunneling.