Superconductivity in disordered lanthanum-sulfide films

This thesis examines the interplay between superconductivity and geometrical and intrinsic disorder in thin films of lanthanum sulfide. La{dollar}sb3{dollar}S{dollar}sb4{dollar} is a simple superconducting material in which the carrier density and superconducting transition temperature can be varied by small changes in La concentration without changing the crystal structure or lattice constant. This makes it an ideal material system in which to study disorder effects on superconductivity.; Thin films of La-S were prepared by a novel adaptation of reactive magnetron sputtering necessary for highly reactive species. X-ray diffraction, SEM, and TEM studies show that single phase polycrystalline films have been produced with narrow columnar grains. Thermodynamic measurements indicate good compositional uniformity over large areas.; In contrast to bulk specimens of this material the metal-insulator transition occurs in a range of concentrations in which we still observe sharp resistive superconducting transitions. The metal-insulator transition and high resistivity of thin films is shown to be due to additional disorder at grain boundaries of the films. The interplay between superconductivity and localization is analyzed in terms of current theories of the metal-insulator transition.; The microstructure of the films plays an important role in many of the superconducting properties. In particular, we see evidence for geometrical effects on both the upper-critical field and the fluctuation conductivity. The perpendicular upper-critical field shows a large enhancement above the standard dirty-limit theory and the fluctuation conductivity is reduced in magnitude from the simple Alamasov-Larkin result. These results can be understood within a percolation model.; A technique for making planar tunnel junctions on these films has also been developed. This provides the first direct measurement of the superconducting gap and the quasi-particle density of states in this material as a function of carrier concentration.