Scanning Tunneling Spectroscopy of thin films close to the SI Transition

by Prof. Benjamin Sacepe (DPMC, University of Geneva, Switzerland)

Wednesday 3 Feb 2010, 14:00 → 16:00 Europe/Rome

Aula Careri (Dipartimento di Fisica-Ed. G.Marconi)

Homogeneously disordered superconducting TiN thin films close to the superconductor-insulator transition (SIT) have been investigated by scanning tunneling spectroscopy. At low temperature, we observed spatial fluctuations of the superconducting gap Δ in agreement with theoretical predictions. When the sample gets closer to the critical disorder, its superconducting critical temperature Tc is significantly depressed towards zero while these inhomogeneities are reinforced with a local strong Δ/Tc ratio. This non vanishing spectral gap demonstrates the persistence of local superconducting pairing across the disorder-tuned SIT. At higher temperature, a pseudogap state has been revealed above Tc. The thermal evolution of the density of states at the Fermi level in this pseudogap regime, analyzed in the framework of disorder-enhanced two-dimensional superconducting fluctuations, gives a clear signature of short-lived Cooper pairs well above Tc. The interplay of these superconducting fluctuations with the Coulomb interaction will be discussed