Superconducting Fluctuations Observed Far above Tc in the Isotropic Superconductor K3C60

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 above Tc. We report on the observation of a sizable magneto-thermoelectric (Nernst) effect in the normal state of K3C60, which displays the characteristics of superconducting fluctuations. This nonquasiparticle Nernst effect emerges from an ordinary quasiparticle background below a temperature of 80 K, far above Tc 1/4 20 K. At the lowest fields and close to Tc, 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.

Automated summary

Alkali-doped fullerides are highly correlated organic superconductors with unique properties. The proximity to a Mott insulating phase is crucial for the physics underlying superconductivity. A magneto-thermoelectric effect has been observed in the normal state of K3C60, displaying characteristics of superconducting fluctuations. This effect emerges from a quasiparticle background and exhibits different behaviors at different temperatures and magnetic fields.