Superconductivity in spinel oxide LiTi2O4 epitaxial thin films

LiTi2O4 is a unique material in that it is the only known oxide spinel superconductor. Although bulk studies have demonstrated that superconductivity can be generally described by the Bardeen-Cooper-Schreiffer theory, the microscopic mechanisms of superconductivity are not yet resolved fully. The sensitivity of the superconducting properties to various defects of the spinel crystal structure provides insight into such mechanisms. Epitaxial films of LiTi2O4 on single crystalline substrates of MgAl2O4, MgO, and SrTiO3 provide model systems to systematically explore the effects of lattice strain and microstructural disorder. Lattice strain that affects bandwidth gives rise to limited variations in the superconducting and normal state properties. Microstructural disorder such as antiphase boundaries that give rise to Ti network disorder can reduce the critical temperature, but Ti network disorder combined with Mg interdiffusion can affect the superconducting state much more dramatically. Thickness dependent transport studies indicate a superconductor-insulator transition as a function of film thickness regardless of lattice strain and microstructure. In addition, surface sensitive X-ray absorption spectroscopy has identified Ti to retain site symmetry and average valence of the bulk material regardless of film thickness.