Cooper Pair Insulator State Near Superconductor-Insulator Transition Probed by Magnetic ImpurityZhang, XuecreatorValles, JamesAdvisorMarston, BradReaderStein, DerekReaderBrown University. Department of Physicssponsor2019xviii, 231 p.born digitalThesis (Ph. D.)--Brown University, 2019thesesStudies of the superconductor-insulator transition (SIT) arose from the investigations of metal-insulator transitions (MIT) in the two dimensional limit. The SIT is a typical example of a quantum phase transition (QFT) which involves the change of a quantum ground state caused by tuning the parameters in a Hamiltonian. This transition is realized in several different systems due to different physical effects including especially localization effects induced by disorder or Coulomb interactions. The former creates an insulator due to Anderson localization and the latter creates a Mott-like insulator. One of the central questions in the SIT field is whether superconductivity is completely destroyed on the insulating side of the transition. In our lab, thin films are prepared on nano-porous anodic aluminum oxide (AAO) substrates. Little-Parks oscillations are detected in their insulating phase providing direct evidence for Cooper pairs persisting across the SIT. The corresponding insulating state near the critical point is called a Cooper Pair Insulator (CPI). Another salient feature of the SIT investigated in our lab, as well as with many other experimental platforms, is the simply activated low temperature transport of the Cooper pair insulator. The activation energy characterizes the barriers that localize the Cooper pairs. It vanishes at the SIT. In my presentation, I will talk about a unique method to investigate the microscopic origins of the activation energy. We doped the CPI with magnetic impurities, which strongly affects the superconducting gap. The behavior of the activation energy shows that extra dissipative processes accompany Cooper pair transport. These processes, which involve virtual quasiparticle tunneling, screen the localizing Coulomb interactions to reduce the energy barrier. This model also explains the transport behavior in magnetic field where a giant magnetoresistance peak is observed.Low temperaturesSuperconductivityquantum phase transitionsuperconductor insulator transitionEnglishRPB2019060310.26300/0551-b127In CopyrightCollection is open for research.dissertations