Dynamical conductivity and spectral functions across the disorder-tuned superconductor-insulator transition
I will discuss the evolution of the spectral and dynamical response functions across the superconductor-insulator quantum phase transition (SIT) in a Josephson junction array driven by increasing charging effects or disorder. Specifically, I will identify characteristic energy scales in the superconducting and insulating phases that vanish at the transition due to enhanced quantum fluctuations, despite the persistence of a robust fermionic gap across the SIT[1,2]. I will discuss the behavior of the Goldstone collective modes and the Higgs modes as the SIT is approached from the superconducting side. At the transition I will determine the universal conductivity, compare the value with different methods, and discuss its significance.
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