Synthesis and transport properties of superconducting thin films of K0.33WO3: Tc reduction due to disorder

Via a two-step deposition and post-annealing procedure, K-doped WO3 thin films with reproducible transport properties are obtained. We observe a larger critical field H-c2 along the c axis, consistent with the picture of the Fermi surface containing one-dimensional bands along this direction. Reducing the film thickness results in a superconductor to insulator transition. Scanning electron microscopy (SEM) images show that KWO3 crystallites become less connected as the deposition time is reduced, providing a microscopic explanation for the transport behavior. In the superconducting films, a resistive anomaly is observed similar to bulk crystals, with a characteristic temperature that shifts lower with decreasing film thickness. The competing electronic effects manifest as a suppression of the density of states at the Fermi level, observed using point contact tunneling spectroscopy, demonstrating that disorder-induced increased Coulomb interactions are present. Using the theory of Belitz for the reduction of T-c due to disorder, we can infer that the film with the highest observed T-c has a relatively large disorder dependent electron-phonon interaction parameter similar to 1.2. Understanding microscopically why certain films display higher Tc will aid in the search for the trace high-T-c superconducting anomalies observed in lightly surface doped bronzes.