Two-dimensional superconducting states near zero temperature quantum phase transitions.

摘要

The competition between localization and superconductivity in two dimensions has puzzled physicists for decades. In two dimensions, only two electronic phases are predicted to exist at zero temperature superconducting and insulating. Contrary to this theoretical expectation, previous transport measurements on 2-dimensional (2D) thin films have found evidence for metallic resistivity down to extremely low temperatures when using either disorder or magnetic field to tune between the superconducting and insulating phases. In this thesis we further investigate the mechanism for the superconductor to insulator transition at zero temperature. We study the destruction of superconductivity due to the application of sufficient magnetic field in 2D films of MoGe and InOx.;Unlike previous works, which concentrated on four point resistivity measurements, we focus primarily on measuring the AC conductivity of the 2D films, which probes the superfluid response. Using a contactless technique, we have been able to measure the superconducting transition as a function of temperature and demonstrate the existence of a Kosterlitz-Thouless (KT) transition in zero magnetic field. Applying a magnetic field to the sample, we have been able to observe the suppression of the superconducting response. Temperature sweeps in a magnetic field showed a similar discontinuity as observed in the zero field KT transition, suggesting a similar process for the destruction of superconductivity in both the zero and non-zero magnetic field cases. Analysis of the change in the critical temperature as a function of magnetic field suggests a quantum phase transition at zero temperature.