To identify the superconducting gap symmetry in ${\mathrm{CeCoIn}}_5$ ($T_c=2.3\mathrm{K}$), we measured the angle-resolved specific heat ($C_{\varphi }$) in a field rotated around the $c$ axis down to a very low temperature, $0.05T_c$, and made detailed theoretical calculations. In a field of 1 T, a sign reversal of the fourfold angular oscillation in $C_{\varphi }$ was observed at $T\simeq 0.1T_c$ upon entering a quasiclassical regime where the maximum of $C_{\varphi }$ corresponds to the antinodal direction, coinciding with the angle-resolved density of states (ADOS) calculation. The $C_{\varphi }$ behavior, which exhibits minima along the [110] directions, unambiguously allows us to conclude $d_{x^2-y^2}$ symmetry of this system. The ADOS-quasiclassical region is confined to a narrow $T$ and $H$ domain within $T/T_c\sim 0.1$ and 1.5 T ($0.13H_{c2}$).

• Received 10 November 2009

DOI:https://doi.org/10.1103/PhysRevLett.104.037002