Field-angle dependence of the zero-energy density of states in the unconventional heavy-fermion superconductor CeCoIn5

Field-angle-dependent specific heat measurements have been made on the heavy-fermion superconductor CeCoIn5 down to similar to0.29 K, in a magnetic field rotating in the tetragonal c-plane. A clear fourfold angular oscillation is observed in the specific heat with the minima (maxima) occurring along the [100] ([110]) directions. Oscillation persists down to low fields H much less than H-c2, thus directly proving the existence of gap nodes. The results indicate that the superconducting gap symmetry is most probably of d(xy) type.