The tetragonal ${\mathrm{CeCoIn}}_5$ is an unconventional $d$-wave heavy-fermion superconductor with $T_c=2.3\mathrm{K}$. Strong Pauli limiting causes a first-order superconducting transition for fields close to $H_{c2}\left(0\right)$ and evidence for a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state has been found at low temperatures. Here, we present a study of the in-plane angular dependence of the upper critical field. At $T=0.1\mathrm{K}$ a fourfold oscillation of 1.2% amplitude with maxima along the [100] direction is observed. This points toward a $d_{x^2-y^2}$ order-parameter symmetry. The data are compared with theoretical calculations including the FFLO state and a tiny anisotropy of the $g$ factor.

• Received 9 January 2006

DOI:https://doi.org/10.1103/PhysRevB.74.134511