Enhanced Surface Superconductivity in Ba(Fe$_{0.95}$Co$_{0.05}$)$_2$As$_2$

TL;DR

This study provides direct evidence of enhanced superconductivity on the surface of Ba(Fe$_{0.95}$Co$_{0.05}$)$_2$As$_2$ crystals, showing a higher surface $T_c$ than the bulk, controllable by surface doping and sensitive to surface conditions.

Contribution

It demonstrates surface-enhanced superconductivity in an iron-based superconductor and shows how surface doping and residual gases influence this phenomenon.

Findings

Surface $T_c$ exceeds bulk $T_c$ by about 11K.

Potassium surface dosing suppresses the enhanced superconductivity.

Residual gases on the surface affect charge transfer and superconductivity.

Abstract

We present direct evidence for an enhanced superconducting $T_c$ on the surface of cleaved single crystals of Ba(Fe$_{0.95}$Co$_{0.05}$)$_2$As$_2$. Transport measurements performed on samples cleaved in ultra high vacuum (UHV) show a significantly enhanced superconducting transition when compared to equivalent measurements performed in air. Deviations from the bulk resistivity appear at 21K, well above the 10K bulk $T_c$ of the underdoped compound. We demonstrate that the excess conductivity above the bulk $T_c$ can be controllably suppressed by application of potassium ions on the cleaved surface, indicating that the enhanced superconductivity is strongly localized to the sample surface. Additionally, we find that the effects of the potassium surface dosing are strongly influenced by the presence of residual gas absorbates on the sample surface, which may prevent effective charge transfer from the potassium atoms to the FeAs plane. This is further support for the conclusion that the effects of the dosing (and enhanced superconductivity) are localized within a few layers of the surface.