SUPERCONDUCTING PROPERTIES OF ULTRATHIN NIOBIUM AND VANADIUM FILMS

Ultrathin films of Nb and V films have been prepared in order to study the nature of superconductivity in thin transition metal films. In both elements, superconductivity is suppressed with decreasing thickness. The objective of this thesis work has been to understand the origin of effects that oppose the usual electron-phonon mediated superconductivity. Superconducting tunneling junctions have been successfully fabricated on films as thin as 10(ANGSTROM) in order to achieve the objective.; The films are e-beam evaporated onto sapphire substrates that have undergone low energy Ar+ ion beam cleaning just prior to the deposition. The low-energy ion-beam cleaning is an essential step in obtaining homogeneous and uniform films of excellent superconducting property. From the in-situ XPS study, the ion beam cleaning is seen to remove the carbon atoms from the surface of the substrate. TEM study reveals a perfect lattice matching between the Nb film (of orientation (100)) and the sapphire substrate (of orientation (1102)) despite the large lattice mismatch.; The tunneling measurements show an increased low-energy electron-phonon coupling in both elements suggesting more favorable conditions for superconductivity. At the same time, however, an increase in electron-electron scattering is observed through the magnetoresistance measurement. The increase in the electron-electron scattering is attributed to the decrease in screening due to disorder and reduced dimensionality. The enhancement in the Coulomb interaction seems to overwhelm the increased low energy electron-phonon coupling to suppress superconducting properties.