Superconducting fluctuations observed far above T$_\mathrm{c}$ in the isotropic superconductor K$_3$C$_{60}$

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 above Tc. We report on the observation of a sizeable magneto-thermoelectric (Nernst) effect in the normal state of K3C60, which displays the characteristics of superconducting fluctuations. The anomalous Nernst effect emerges from an ordinary quasiparticle background below a temperature of 80K, far above Tc = 20K. At the lowest fields and close to Tc, the scaling of the effect is captured by a model based on Gaussian fluctuations. The temperature up to which we observe fluctuations is exceptionally high for a three-dimensional isotropic system, where fluctuation effects are usually suppressed.