Evidence for nodeless superconducting gap in NaFe_1−xCo_xAs from low-temperature thermal conductivity measurements

¹ State Key Laboratory of Surface Physics, Department of Physics, and Laboratory of Advanced Materials, Fudan University - Shanghai 200433, PRC
² Hefei National Laboratory for Physical Science at Microscale and Department of Physics, University of Science and Technology of China - Hefei, Anhui 230026, PRC

^(a) shiyan_li@fudan.edu.cn

Received: 2 December 2012
Accepted: 20 December 2012

Abstract

The thermal conductivity of optimally doped NaFe_0.972Co_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 κ₀/T is found in zero magnetic field for both compounds. Applying a field H = 3 T (≈H_c2/12) does not noticeably increase κ₀/T in NaFe_1.972Co_0.028As. The κ₀/T of overdoped NaFe_1.925Co_0.075As shows a field dependence similar to the optimally doped BaFe_1.85Co_0.15As₂. All these results suggest nodeless superconducting gaps in NaFe_1−xCo_xAs from the optimal doping to the overdoped regime, with low anisotropy or ratio between the magnitudes of different gaps.