Dynamical conductivity and spectral functions across the disorder-tuned superconductor-insulator transition

Nandini Trivedi

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. 

[1] A. Ghosal, M. Randeria and N. Trivedi, Phys. Rev. Lett. 81, 3940 (1998); Phys. Rev. B 65, 014501 (2001).

[2] K. Bouadim, Y.L. Loh, M. Randeria, and N. Trivedi, Nat. Phys. 7 884 (2011) 

[3] M. Swanson, Y.L. Loh, M. Randeria, and N. Trivedi, Phys. Rev. X 4, 021007 (2014)