Angular Position of Nodes in the Superconducting Gap of Quasi-2D Heavy-Fermion Superconductor ${\mathrm{CeCoIn}}_5$

The thermal conductivity of the heavy-fermion superconductor ${\mathrm{CeCoIn}}_5$ 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 ( $\pm \pi ,\pm \pi$) directions is resolved. The thermal conductivity measurement also reveals a first-order transition at $H_{c2\mathrm{}}$, indicating a Pauli limited superconducting state. These results indicate that the symmetry most likely belongs to $d_{x^2{-y}^2}$, implying that the anisotropic antiferromagnetic fluctuation is relevant to the superconductivity.