In-plane torque measurements were performed on heavy fermion ${\text{CeCoIn}}_5$ single crystals in the temperature $T$ range $1.8\text{K}\le T\le 10\text{K}$ and applied magnetic field $H$ up to 14 T. The normal-state torque is given by ${\tau }_n\propto H^4{\left(1+T/T_K\right)}^{-1}\text{sin}4\phi$. The reversible part of the mixed-state torque, obtained after subtracting the corresponding normal-state torque, shows also a fourfold symmetry. In addition, sharp peaks are present in the irreversible torque at angles of $\pi /4$, $3\pi /4$, $5\pi /4$, $7\pi /4$, etc. Both the fourfold symmetry in the reversible torque and the sharp peaks in the irreversible torque of the mixed state imply $d_{xy}$ symmetry of the superconducting order parameter. The field and temperature dependences of the reversible mixed-state torque provide further evidence for $d_{xy}$ wave symmetry. The fourfold symmetry in the normal state has a different origin since it has different field and temperature dependences than the one in the mixed state. The possible reasons of the normal-state fourfold symmetry are discussed.

• Received 15 April 2007

DOI:https://doi.org/10.1103/PhysRevB.78.014510