Quantum phase transition of 2D tantalum thin film with periodic hole array

Abstract

The ground states of 2D electron system were generally considered to exist only in insulation state and superconducting state. Therefore, the quantum phase transition in the superconducting thin film is expected to transition directly from the superconducting state to the insulating state depending on the physical parameters such as the magnetic field and the disorder etc. But, after a report of an intervening metallic phase in amorphous MoGe thin film, an anomalous metallic state between superconducting and insulating states has been reported in studies of quantum phase transition in various superconducting thin films. Various characteristics of quantum phase transitions in superconducting thin films, such as the presence of intervening metallic phase, were considered to depend on the nature of the disorder of the superconducting thin film. But this seems not to simply depend on sheet resistance, a general measure of disorder, but to on the intrinsic disorder characteristics of superconducting film according to material or fabrication method. Thus, for systematic study to understand anomalous metallic phase, the other physical parameter is need, not disorder. In this dissertation, to investigate the influence of the vortex pinning effect, we have introduced the periodic hole array as strong pinning centers into tantalum thin film which is one of the representative superconducting thin films where anomalous metallic phase is observed. The low temperature transport characteristics of tantalum thin films with the same thickness but different areal number hole density were investigated. As density of holes increases, the density of the strongly pinned vortices increases because not only the density of the hole itself increases, but also the number of trapped vortices per hole increases due to the increase in the vortex-vortex repulsive force. High-density strong pinning centers affect significantly quantum vortex dynamics, so that the superconducting state expands and anomalous metallic phase is suppressed. We find that the flux distance between interstitial vortex and its adjacent flux at the respective superconducting and metallic phase boundary in perforated samples is constant regardless of areal number hole density. The result suggests that the dissipation of anomalous metallic phase is related to motion of weakly pinned interstitial vortices between holes, and that there is the critical flux distance of anomalous metal.