Jueves 13 de abril 2012

10h:30-11h:30 “Tunneling Spectroscopy of Fluctuating and Localized Preformed Cooper Pairs in Highly Disordered Superconducting Films

Prof: Claude Chapelier  (SPSMS, UMR-E9001,CEA-INAC/UJF, Grenoble-France)


We have performed tunneling spectroscopy on superconducting titanium nitride and indium oxide films in the vicinity of the disorder-driven superconductor-insulator transition (SIT). Tunnelling spectroscopy highlights a rather unusual superconducting state with a pseudogap regime above the critical temperature Tc [1]. We demonstrated that this pseudogap is the signature of short lived Cooper pairs that are preformed above Tc. It evolves at low temperature into an inhomogeneous superconducting system due to spatial fluctuations of the disorder at the mesoscopic scale in both materials [2,3]. However, the SIT in TiN and InO films display different characters. Ultrathin TiN films remain bad metals with dominating two-dimensional thermodynamic fluctuations when disorder is increased. In this case Tc goes to zero at the critical disorder of the SIT, whereas in InO, Tc remains above 1 K on the superconducting side of the SIT. In this latter situation, localization takes over and the preformed Cooper pairs above Tc can locally remain localized at zero temperature.

1. B. Sacépé, C. Chapelier, T.I. Baturina, V.M. Vinokur, M.R. Baklanov, and M. Sanquer, Nature Commun. 1 :140 doi : 10.1038/ncommns1140 (2010).
2. B. . Sacépé, C. Chapelier T.I. Baturina, V.M. Vinokur, M.R. Baklanov, and M. Sanquer, Phys. Rev. Lett. 101, 157006 (2008).
3. B. Sacépé, Th. Dubouchet, C.Chapelier,  M. Sanquer, M. Ovadia, Dan Shahar, M. Feigel’man, and L.B. Ioffe, Nature Physics 7, 239 (2011).