59Co NQR study in superconducting CeCo2

We have performed 59 Co NQR study in superconducting CeCo 2 with T c ∼ 1.4 K in order to investigate the superconducting property. Nuclear spin-lattice relaxation rate of Co, 1/ T 1 was measured down to 150 mK. 1/ T 1 is proportional to temperature ( T ) in the normal state, and decreases exponentially in the superconducting state with the Hebel-Slichter peak just below T c . T -dependence of 1/ T 1 in the superconducting state is a typical feature of s-wave superconductivity with 2 Δ / k B T c ∼ 2.26, which is remarkably smaller than 3.5 in the BCS superconductor.