Conventional s-Wave Superconductivity in BiS2-Based NdO0.71F0.29BiS2 Revealed by Thermal Transport Measurements

To study the superconducting gap structure of BiS2-based layered compound NdO0.71F0.29BiS2 (Tc = 5 K), we measured the thermal conductivity κ, which is a sensitive probe of low-energy quasiparticle spectrum. In the absence of a magnetic field, residual linear term in the thermal conductivity κ0/T at T → 0 is vanishingly small, indicating that the residual normal fluid, which is expected for nodal superconductors, is absent. Moreover, the applied magnetic field hardly affects thermal conductivity in wide range of the vortex state, indicating the absence of Doppler shifted quasiparticles. These results provide evidence that NdO0.71F0.29BiS2 is a fully gapped superconductor. The obtained gap structure, along with the robustness of the superconductivity against the impurity, suggest a conventional s-wave superconducting state in NdO0.71F0.29BiS2.