Evidence for nodeless superconducting gap in NaFe$_{1-x}$Co$_x$As from   low-temperature thermal conductivity measurements
  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. 
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, and S. Y. Li
cond-mat.supr-con cond-mat.str-el