Equipe « Spectroscopie des nouveaux états quantiques »

Vendredi 24 octobre 2014 à 14 h - INSP - 4 place Jussieu - 75252 PARIS Cedex 05 - Barre 22-23, 3^e étage, salle 317

« Nano-scale electronic inhomogeneities in ultra-thin superconducting »

Abstract

In order to better understand the various processes taking place at the superconductor-insulator transition (SIT), we have probed simultaneously the global and local electronic properties of NbN ultrathin films, elaborated ex-situ on sapphire substrate. The transition was approached by reducing the films thickness, increasing the normal state square resistance to several kiloOhms. The corresponding Tc’s of the studied films ranged from about Tc≈15K, which is close to TC-bulk, to Tc≈3.8K (Tc/TC-bulk≈0.23). In the range 0.4TC-bulk Local scanning tunneling spectroscopy (STS) measurements show the emergence of granularity in the superconducting properties upon Tc reduction, as well as the progressive development of a strong Altshuler-Aronov background in the dI/dV(V) tunneling spectra, in agreement with Finkelstein scenario. For the most disordered film (Tc≈3.8K), simultaneous in-situ resistivity and STS measurements allowed us to follow at the same sample area the evolution of these emergent granular superconducting properties as a function of temperature and magnetic field, which demonstrate a percolative behavior of the transition to the bad-metal normal state. This local picture finds an interesting signature in the change of the dimensionality of the Aslamasov-Larkin fluctuations above Tc from 2D to 0D. Besides, such a study is particularly interesting to better understand the various phenomena occurring in and limiting the behavior of single-photon detectors patterned out of NbN films.

Composition du jury

 Dimitri Roditchev (directeur de thèse)
 Christophe Brun (co-directeur de thèse)
 Brigitte Leridon (membre invité)
 Konstantin Ilin (membre invité)
 Claude Chapelier (rapporteur)
 Claire Marrache-Kikuchi (rapportrice)
 Hélène Bouchiat (examinatrice)
 Abhay Shukla (examinateur)