Superconducting Ba1−xKxBiO3 thin films and junctions

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Abstract
Superconducting Ba1−xKxBiO3 (BKBO) thin films and heteroepitaxial multilayers were deposited by in situ off‐axis sputtering on various substrate at 400 °C. A typical superconducting BKBO thin film on the SrTiO3 and MgO substrates had a very sharp zero resistance transition at a temperature of 22–24 K as measured by transport four point and mutual inductance probes. The normal state resistivity exhibited metallic to semiconductive characteristics depending on film composition with the lowest room temperature resistivity of about 0.25 mΩ cm. The film orientation was highly epitaxial to both SrTiO3 and MgO substrates. Normal metal‐insulator‐superconductor (NIS) type tunnel functions with native insulator layer and deposited MgO layer were reproducibly fabricated. The superconducting energy gap of about 3.6 meV was obtained from those NIS junctions. Heteroepitaxial superconductor‐ insulator‐superconductor (SIS) trilayer junctions were also fabricated using MgO insulator layers. For a 5 nm thick insulator layer, quasiparticle SIS tunnel junctions with nonhysteric I‐V characteristics were obtained. By reducing MgO insulator layer thickness down to 3 nm, SIS tunnel junctions with Josephson supercurrent were obtained with hysteric I‐V characteristics.