Synthesis and properties of artificially layered (Ba,K)BiO$_3$ and Ba(Bi,Pb)O$_3$ structures*

The possibility of superconductivity in a negative U material due to the proximity effect with a normal metal (Berg, Phys.Rev.B 78, 094509), and of doping via charge transfer at interfaces (Ohtomo, Nature 427, 423-426) has led us to investigate the properties of artificially-layered films of BaBiO$_3$ (a negative-U insulator) and BaPbO$_3$ (a normal metal) and of their doped variants (Ba,K)BiO$_3$ and Ba(Bi,Pb)O$_3$, both of which are isotropic (3D) and are superconducting over a specific range of doping. Samples were either Ruddlessden-Popper phases of (Ba,K)BiO$_3$ synthesized by pulsed laser deposition or bi-layers of BaPbO$_3$ and BaBiO$_3$ and their variants by e-beam evaporation. We studied the transport properties of these samples to investigate the presence of superconductivity and the influence of anisotropy.