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

arxiv-cs:

Authors: S. Y. Zhou, X. C. Hong, X. Qiu, B. Y. Pan, Z. Zhang, X. L. Li, W. N. Dong, A. F. Wang, X. G. Luo, X. H. Chen, S. Y. Li

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.

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