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Fluctuating superconductivity in organic molecular metals close to the Mott transition


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 κ-(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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Figure 1: Structure and phase diagram of κ-(BEDT-TTF) 2X.
Figure 2: The Nernst effect in κ-NCS.
Figure 3: The Nernst effect in κ-Br.


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Work at Oxford is funded by the EPSRC. Work at Argonne National Laboratory is supported by the Office of Basic Energy Sciences, Division of Materials Science, US Department of Energy. A.A. is supported by the Royal Society. We thank J. M. Bhaseen, K. Burnett, P. M. Chaikin, J. T. Chalker, L. Forro, D. Jaksch, N. P. Ong and I. A. Walmsley for discussions.

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Correspondence to Arzhang Ardavan.

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Nam, MS., Ardavan, A., Blundell, S. et al. Fluctuating superconductivity in organic molecular metals close to the Mott transition. Nature 449, 584–587 (2007).

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