Thermal conductivity of overdoped BaFe$_{1.73}$Co$_{0.27}$As$_2$ single crystal: Evidence for nodeless multiple superconducting gaps and interband interactions

TL;DR

This study measures the thermal conductivity of overdoped BaFe$_{1.73}$Co$_{0.27}$As$_2$ single crystals, revealing nodeless gaps and evidence for interband interactions that influence the superconducting gap structure.

Contribution

It provides experimental evidence for an exotic, multiband superconducting gap structure driven by interband interactions in overdoped BaFe$_{1.73}$Co$_{0.27}$As$_2$.

Findings

Residual linear term $ /T$ is negligible at zero field.

Rapid increase of $ /T$ in magnetic field suggests unconventional gap structure.

Evidence for larger gaps on bands with smaller density of states.

Abstract

The in-plane thermal conductivity $\kappa$ of overdoped FeAs-based superconductor BaFe$_{1.73}$Co$_{0.27}$As$_2$ ($T_c$ = 8.1 K) single crystal was measured down to 80 mK. In zero field, the residual linear term $\kappa_0/T$ is negligible, suggesting a nodeless superconducting gap in the $ab$-plane. In magnetic field, $\kappa_0/T$ increases rapidly, very different from that of conventional s-wave superconductors. This anomalous $\kappa_0/T(H)$ may reveal an exotic superconducting gap structure in overdoped BaFe$_{1.73}$Co$_{0.27}$As$_2$: the vanishing hole ($\beta$) pocket has a much larger gap than the electron ($\gamma$ and $\delta$) pockets which contain most of the carriers. Such an exotic gap structure is an evidence for superconducting state induced by interband interactions, in which the band with the {\it smaller} density of states has a {\it larger} gap.