Nature 449, 584-587 (4 October 2007) | doi:10.1038/nature06182; Received 4 June 2007; Accepted 15 August 2007

Fluctuating superconductivity in organic molecular metals close to the Mott transition

Moon-Sun Nam1, Arzhang Ardavan1, Stephen J. Blundell1 & John A. Schlueter2

  1. Clarendon Laboratory, Department of Physics, University of Oxford, OX1 3PU, UK
  2. Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

Correspondence to: Arzhang Ardavan1 Correspondence and requests for materials should be addressed to A.A. (Email: arzhang.ardavan@physics.ox.ac.uk).

On cooling through the transition temperature Tc of a conventional superconductor, an energy gap develops as the normal-state charge carriers form Cooper pairs; these pairs form a phase-coherent condensate that exhibits the well-known signatures of superconductivity: zero resistivity and the expulsion of magnetic flux (the Meissner effect1). However, in many unconventional superconductors, the formation of the energy gap is not coincident with the formation of the phase-coherent superfluid. Instead, at temperatures above the critical temperature a range of unusual properties, collectively known as 'pseudogap phenomena', are observed2. Here we argue that a key pseudogap phenomenon—fluctuating superconductivity occurring substantially above the transition temperature—could be induced by the proximity of a Mott-insulating state. The Mott-insulating state in the kappa-(BEDT-TTF)2X organic molecular metals3, 4, 5 can be tuned, without doping, through superconductivity into a normal metallic state as a function of the parameter t/U, where t is the tight-binding transfer integral characterizing the metallic bandwidth and U is the on-site Coulomb repulsion. By exploiting a particularly sensitive probe of superconducting fluctuations, the vortex-Nernst effect, we find that a fluctuating regime develops as t/U decreases and the role of Coulomb correlations increases.


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