Superconducting Fluctuations Observed Far above Tc in the Isotropic Superconductor K3C60

Abstract

Alkali-doped fullerides are strongly correlated organic superconductors that exhibit high transition temperatures, exceptionally large critical magnetic fields, and a number of other unusual properties. The proximity to a Mott insulating phase is thought to be a crucial ingredient of the underlying physics and may also affect precursors of superconductivity in the normal state aboveTc. We report on the observation of a sizable magneto-thermoelectric (Nernst) effect in the normal state ofK3C60, which displays the characteristics of superconducting fluctuations. This nonquasiparticle Nernst effect emerges from an ordinary quasiparticle background below a temperature of 80 K, far aboveTc=20K. At the lowest fields and close toTc, the scaling of the effect is captured by a model based on Gaussian fluctuations. The behavior at higher magnetic fields displays a symmetry between the magnetic length and the correlation length of the system. The temperature up to which we observe fluctuations is exceptionally high for a three-dimensional isotropic system, where fluctuation effects are expected to be suppressed.

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Plain Language Summary

In superconductors, electrons partner up to form Cooper pairs, whose bond is so strong that they move through a material impervious to all obstacles. Below some critical temperature, many such pairs move in lockstep, allowing current to flow with no resistance. But when the material heats up, and superconductivity ceases, it is not clear what becomes of all those Cooper pairs. Here, we report on precursors of superconductivity at temperatures up to 4 times the critical temperature in the organic superconductorK3C60, evidence that some Cooper pairs form or survive well outside the superconducting regime.

In standard measurements of a material’s superconductivity, the effects of any lingering Cooper pairs are overshadowed by the conductivity of ordinary electrons. In our work, we turn to a very sensitive probe for Cooper pairs: When combining a strong magnetic field with a small temperature gradient across a material, a minuscule transverse voltage builds up. This effect is almost indifferent to the presence of ordinary electrons but is highly sensitive to Cooper pairs. While previous studies have looked for this effect in effectively 2D situations, our work is the first to do so in 3D.

Theoretical predictions have suggested that, above the critical temperature, Cooper pairs survive either as ephemeral bubbles that appear and disappear or in separated puddles that fail to synchronize. Using our results, researchers can now compare theoretical scenarios to understand what remains of superconductivity at high temperatures.

This work not only contributes to the fundamental understanding of superconductors but also may help researchers understand a recently discovered phenomenon: Interaction with light seems to massively increase the critical temperature for superconductivity inK3C60. The presence of Cooper pairs at high temperatures might be the key for comprehending this effect.