Evidence for nodeless superconducting gap in NaFe_(1-x)Co _xAs from low-temperature thermal conductivity measurements

摘要

The thermal conductivity of optimally doped NaFe_0972Co _0.028As (T_c ～ 20 K) and overdoped NaFe _0.925Co_0.075As (T_c ～ 11 K) single crystals were measured down to 50 mK. No residual linear term κ_0/T is found in zero magnetic field for both compounds. Applying a field H = 3 T (≈H_c2/12) does not noticeably increase κ_0/T in NaFe_(1.972)Co_0.028As. The κ_0/T of overdoped NaFe_(1.925)Co_0.075As shows a field dependence similar to the optimally doped BaFe1.85Co_0.15As2. All these results suggest nodeless superconducting gaps in NaFe_(1-x)Co_xAs from the optimal doping to the overdoped regime, with low anisotropy or ratio between the magnitudes of different gaps.