Electrical properties of grain boundaries in Ba1−xKxBiO3 polycrystalline thin films

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Abstract
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 Tc 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. 1 0, 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.