Probing the superconducting condensate on a nanometer scale

Institut des Nano-Sciences de Paris, INSP, Universités Paris 6 et Paris 7, et CNRS (UMR 75 88) - 140 rue de Lourmel, Campus Boucicaut, 75015 Paris, France

Received: 12 October 2005
Accepted: 31 January 2006

Abstract

Superconductivity is a rare example of a quantum system in which the wave function has a macroscopic quantum effect, due to the unique condensate of electron pairs. The amplitude of the wave function is directly related to the pair density, but both amplitude and phase enter the Josephson current: the coherent tunneling of pairs between superconductors. Very sensitive devices exploit the superconducting state, however properties of the condensate on the local scale are largely unknown, for instance, in unconventional high-T_c cuprate, multiple gap, and gapless superconductors. The technique of choice would be Josephson STS, based on Scanning Tunneling Spectroscopy (STS), where the condensate is directly probed by measuring the local Josephson current (JC) between a superconducting tip and sample. However, Josephson STS is an experimental challenge since it requires stable superconducting tips, and tunneling conditions close to atomic contact. We demonstrate how these difficulties can be overcome and present the first spatial mapping of the JC on the nanometer scale. The case of an film, subject to a normal magnetic field, is considered.