Probing the superconducting condensate on a nanometer scale

Abstract

Superconductivity is a rare example of a quantum system in which the\nwavefunction has a macroscopic quantum effect, due to the unique condensate of\nelectron pairs. The amplitude of the wavefunction is directly related to the\npair density, but both amplitude and phase enter the Josephson current : the\ncoherent tunneling of pairs between superconductors. Very sensitive devices\nexploit the superconducting state, however properties of the {\\it condensate}\non the {\\it local scale} are largely unknown, for instance, in unconventional\nhigh-T$_c$ cuprate, multiple gap, and gapless superconductors.\n The technique of choice would be Josephson STS, based on Scanning Tunneling\nSpectroscopy (STS), where the condensate is {\\it directly} probed by measuring\nthe local Josephson current (JC) between a superconducting tip and sample.\nHowever, Josephson STS is an experimental challenge since it requires stable\nsuperconducting tips, and tunneling conditions close to atomic contact. We\ndemonstrate how these difficulties can be overcome and present the first\nspatial mapping of the JC on the nanometer scale. The case of an MgB$_2$ film,\nsubject to a normal magnetic field, is considered.\n