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).

Abstract

On cooling through the transition temperature T_c 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 -(BEDT-TTF)₂X organic molecular metals^{3, 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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