| This thesis examines the transport properties of two-dimensional (2D) and quasi-2D superconductors. In particular, the effects of perpendicular magnetic fields on the superconducting transitions have been investigated for both low temperature (Nb) and high temperature ({dollar}rm Bisb2Srsb2CaCusb2Osb{lcub}8+delta{rcub}{dollar}) superconducting systems. Studies have also been done in order to better understand the influence of overlayed materials on the 2D superconductors.; Chap. 2 of this thesis gives a summary of the sample preparation for the ultrathin Nb films (d = 20 to 30 A) and Nb/NbSi bilayer films. All thin film samples were made by electron beam evaporation with the Nb being single crystals and the NbSi being amorphous.; Chap. 3 studies various transport properties of an ultrathin Nb film (d {dollar}approx{dollar} 20 A), whose thickness is less than all other relevant length scales except the lattice constant. We have observed effects such as vortex motion in perpendicular magnetic fields, 2D fluctuation conductivity above {dollar}Tsb c{dollar}, the Kosterlitz-Thouless transition in zero field, the signature of weak localization when external fields are larger than the upper critical field, and the distinction between parallel and perpendicular fields. Results indicate the sample is 2D in both superconducting and normal states.; In Chap. 4, we investigate the effect of amorphous NbSi near the metal-insulator transition on the resistive transitions of 2D superconducting Nb films. An enhancement of {dollar}Tsb c{dollar} was observed for insulating NbSi. The magnitude of the enhancement has been explained through a simple extension of the classical proximity effect theory.; The last chapter of the thesis concentrates on the effect of perpendicular magnetic fields on the resistive transitions of {dollar}rm Bisb2Srsb2CaCusb2Osb{lcub}8+delta{rcub}{dollar} single crystals with different oxygen doping. Unusual features, which correlate with the oxygen doping, have been observed in the resistance vs. temperature curves. |
