Unconventional superconductivity in CeCoIn₅ studied by the specific heat and magnetization measurements

We measured the low-temperature specific heat in magnetic fields up to 80 kOe, together with the magnetization for the heavy-fermion superconductor CeCoIn₅ with the transition temperature T_c = 2.25 K. CeCoIn₅ is a strong-coupling superconductor with a large jump in the specific heat ΔC/C_n(T_c) = 4.7. The heavy-fermion state is significantly formed below 1 K, reaching at least 1070 mJ/K²·mol in magnetic fields. This large electronic specific heat coefficient at 0.25 K is, however, reduced with increasing magnetic field, ranging from 1070 mJ/K²·mol at 50 kOe to 820 mJ/K²·mol at 80 kOe. The upper critical field H_c2 at O K is estimated to be 116 kOe for H ∥ [100] and 49.5 kOe for H ∥ [001]. The present anisotropy of H_c2 is well explained by the anisotropic effective mass, but the magnitude of H_c2 is reduced strongly by the paramagnetic effect for both field directions.