Angular Position of Nodes in the Superconducting Gap of Quasi-2D Heavy-Fermion Superconductor CeCoIn5

The thermal conductivity of the heavy-fermion superconductor CeCoIn₅ has been studied in a magnetic field rotating within the 2D planes. A clear fourfold symmetry of the thermal conductivity which is characteristic of a superconducting gap with nodes along the ( +/-π,+/-π) directions is resolved. The thermal conductivity measurement also reveals a first-order transition at H_c2, indicating a Pauli limited superconducting state. These results indicate that the symmetry most likely belongs to d_x²-y², implying that the anisotropic antiferromagnetic fluctuation is relevant to the superconductivity.