1204.3440 (S. Y. Zhou et al.)

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

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.

View original: http://arxiv.org/abs/1204.3440