Qunatum phase transition of 2D tantalum thin film tuned by b-field, disorder and effect of dc bias

Abstract

Superconductor-insulator transition has been investigated with tuning disorder (film thickness) or magnetic field. Recent electrical transport measurements of MoGe, Ta thin films revealed an unexpected intermediate metallic phase intervening between superconducting and insulating phases in a certain range of magnetic field. The non-zero resistance values with a positive slope in dR/dT are found above a critical value of magnetic field at low temperatures. In addition, Ta thin films show intriguing development in the IV characteristics as the film thickness or magnetic field is varied. Hysteretic IV characteristic curves in a superconducting phase change into non-linear response without hysteresis when thickness or magnetic field exceeds a critical value. The non-linear response disappears finally with the advent of the insulating phase. The origin of the interesting metallic phase was not understood clearly yet and the finite resistance at low temperatures is attributed to the existence of vortex motion or fermionic quasiparticles. In this dissertation, we report the scaling behavior during the superconducting transitions in Ta thin films. Critical exponents, $\nu$ and z, are obtained in the metallic and superconducting samples with wide ranges of disorder. The critical exponents in disorder-induced metallic phase are very different from those obtained in the superconducting samples. Dynamical exponent z of the superconducting samples is found to be approximately one which is good agreement with theoretical prediction. However, z in metallic samples is found to be about 0.7, inconsistent with theoretical prediction. The irrelevant critical exponents from the scaling analysis on the metallic samples indicate that the superconductor-insulator transition cannot explain the intermediate metallic phases. Further studies on the possible scaling behavior in the Bose metal insulator transitions are necessary to understand the intermediate phase. Negative Magneto-Resi...