Superconducting and normal states of ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_3$ studied with use of high-quality thin films

An extensive study of ${\mathrm{Ba}}_{0.55}$ ${\mathrm{K}}_{0.45}$ ${\mathrm{BiO}}_3$ thin films is reported, including precise measurements of the transport coefficients, the reflectivity spectrum, the Raman spectra, the upper critical field, and the superconducting energy gap. From the results for the normal-state properties, it is suggested that the electronic state in the metallic phase of ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{BiO}}_3$ can be described by a simple band picture. The measurement of the superconducting-state properties shows good consistency with this picture within the BCS framework. Based on these experimental results, we firmly conclude that the density of states in the conduction band of ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{BiO}}_3$ is close to that expected from a band model in its metallic phase and the superconductivity is of BCS type. The coupling strength λ∼1 is also estimated from the gap measurements and the temperature dependence of the resistivity.