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

TL;DR

This study reports the detection of superconducting fluctuations far above the critical temperature in the isotropic superconductor K$_3$C$_{60}$, revealing unusual fluctuation effects in a three-dimensional system.

Contribution

It provides the first observation of high-temperature superconducting fluctuations in an isotropic 3D superconductor, expanding understanding of fluctuation phenomena.

Findings

Superconducting fluctuations observed up to 80K, well above T$_c$=20K.

Magneto-thermoelectric (Nernst) effect characteristic of fluctuations detected.

Gaussian fluctuation model explains behavior near T$_c$.

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 T$_\text{c}$. We report on the observation of a sizeable magneto-thermoelectric (Nernst) effect in the normal state of K$_3$C$_{60}$, which displays the characteristics of superconducting fluctuations. The anomalous Nernst effect emerges from an ordinary quasiparticle background below a temperature of 80K, far above T$_\text{c}$ = 20K. At the lowest fields and close to T$_\text{c}$, 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.