Evidence for nodeless superconducting gap in NaFe$_{1-x}$Co$_x$As from low-temperature thermal conductivity measurements

TL;DR

This study uses low-temperature thermal conductivity measurements to demonstrate that NaFe$_{1-x}$Co$_x$As superconductors have a nodeless gap structure, with doping affecting gap anisotropy and magnitude ratios.

Contribution

It provides direct experimental evidence for nodeless superconducting gaps in NaFe$_{1-x}$Co$_x$As and explores how doping influences gap anisotropy and magnitude ratios.

Findings

No residual linear term in thermal conductivity indicates nodeless gaps.

Field dependence of thermal conductivity suggests multiple isotropic gaps.

Doping increases gap anisotropy or the ratio of gap magnitudes.

Abstract

The thermal conductivity of optimally doped NaFe$_{0.972}$Co$_{0.028}$As ($T_c \sim$ 20 K) and overdoped NaFe$_{0.925}$Co$_{0.075}$As ($T_c \sim$ 11 K) single crystals were measured down to 50 mK. No residual linear term $\kappa_0/T$ is found in zero magnetic field for both compounds, which is an evidence for nodeless superconducting gap. Applying field up to $H$ = 9 T ($\approx H_{c2}/4$) does not noticeably increase $\kappa_0/T$ in NaFe$_{1.972}$Co$_{0.028}$As, which is consistent with multiple isotropic gaps with similar magnitudes. The $\kappa_0/T$ of overdoped NaFe$_{1.925}$Co$_{0.075}$As shows a relatively faster field dependence, indicating the increase of the ratio between the magnitudes of different gaps, or the enhancement of gap anisotropy upon increasing doping.