Magnetic Impurity Effects on Cooper Pair Localization

The Cooper pair insulator(CPI) state has been observed and probed by various experiments in the past decade. Nevertheless, the microscopic mechanisms driving Cooper pair localization and giving rise to its signature activated transport and giant magnetoresistance peak remain unelucidated. In this talk, I will show our recent studies using magnetic impurity doping, which weakens Cooper pairing, to probe the microscopic properties of quasi-two dimensional nano-patterned films to gain insight into the microscopic properties of the CPI state. The films are patterned with a hole array that allows the application of magnetic frustration to the CPI state. With increasing doping concentration in situ, the low temperature resistance increases and peaks much like the giant magneto-resistance(MR) peak. In addition, the evolution of the transport activation energy with doping shows an unexpected magnetic frustration dependence. I will describe how a theoretical model based on a competition between a localizing charging energy renormalized by quasiparticle tunneling and a delocalizing Josephson energy can account for these results.