Field-angle-dependent specific heat in the unconventional heavy-fermion superconductor [formula omitted

Abstract

We have measured the field-angle-dependent specific heat, C ( H , θ ) , in the unconventional heavy-fermion superconductor CeCoIn 5 down to ∼ 0.29 K . In a magnetic field rotating in the tetragonal c-plane, a clear fourfold angular oscillation is observed in C ( H , θ ) with the minima (maxima) occurring along the [1 0 0] ([1 1 0]) directions. Oscillation persists down to low fields H ⪡ H c 2 , thus directly proving the existence of gap nodes. Position of the minima in C ( H , θ ) indicates that the gap node is located along the [1 0 0] and [0 1 0] directions, implying that the superconducting gap symmetry of CeCoIn 5 is most probably of d xy type.