Gaps and Pseudogaps across the inhomogeneous superconductor to paired insulator transition

The mechanism for the disorder-tuned superconductor to insulator transition (SIT) in thin films and the nature of the resulting insulator are still debated, despite decades of research. We use quantum Monte Carlo simulations [1] that treat, on an equal footing, inhomogeneous amplitude variations and phase fluctuations, and go beyond our earlier Bogoliubov-deGennes analysis [2]. We gain new microscopic insights into the SIT, compare our theory with experiments [3] and make testable predictions for local spectroscopic probes. The energy gap in the single-particle density of states survives across the transition, but coherence peaks exist only in the superconducting state. A characteristic pseudogap persists above the critical disorder and critical temperature, in contrast to conventional theories. Surprisingly, the insulator has signatures of pairing with a two-particle gap scale that vanishes at the superconductor--insulator transition, despite a robust single-particle gap. The impact of rare regions on the gaps will also be discussed. In collaboration with K. Bouadim, Y.L.Loh and N. Trivedi. \\[4pt] [1] K. Bouadim, Y.L.Loh, M. Randeria and N. Trivedi, Nature Phys. 7, 884 (2011). \\[0pt] [2] A. Ghosal, M. Randeria, and N. Trivedi, Phys. Rev. B 65, 014501 (2001). \\[0pt] [3] B. Sacepe et al., Nature Comm. 1, 140 (2010); Nature Phys. 7, 239 (2011); M. Mondal et al., Phys. Rev. Lett. 106, 047001 (2011).