Phase separation in the vicinity of the surface of $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br by fast cooling

TL;DR

This study reveals that fast cooling induces surface insulating regions in the organic superconductor $$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, leading to partial suppression of superconductivity and explaining the large in-plane penetration depth.

Contribution

It demonstrates the formation of macroscopic insulating regions near the surface due to fast cooling, providing insight into surface effects on superconductivity in this material.

Findings

Insulating regions are found near the surface after fast cooling.

Surface insulating regions have diameters of 50-100 μm and depths of a few μm.

The large in-plane penetration depth can be explained by these insulating regions.

Abstract

Partial suppression of superconductivity by fast cooling has been observed in the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br by two means: a marked sample size effect on the magnetic susceptibility and direct imaging of insulating regions by scanning microregion infrared reflectance spectroscopy. Macroscopic insulating regions are found in the vicinity of the crystalline surface after fast cooling, with diameters of 50--100 $\mu$m and depths of a few $\mu$m. The very large in-plane penetration depth reported to date ($\sim$ 24--100 $\mu$m) can be explained by the existence of the insulating regions.