Heavy fermion superconductivity with the strong pauli paramagnetic effect on NpPd₅Al₂

We measured the electrical resistivity, specific heat, magnetic susceptibility and magnetization in the first Np-based superconductor NpPd ₅Al₂ with the tetragonal crystal structure. NpPd ₅Al₂ is a paramagnet and becomes the superconductivity at T_c = 4.9 K. The large electronic specific heat coefficient of γ = 200 mJ/K²mol at T_c indicates that NpPd ₅Al₂ is a heavy fermion superconductor. The upper critical field H_c2 is large and highly anisotropic: H_c2(0) = 37 kOe for H ∥ [100] and H_c2(0) = 143 kOe for H ∥ [001]. Although the initial slope of H_c2 at T_c is very large, H_c2 is strongly suppressed against the magnetic field for both field direction, indicating the existence of strong Pauli paramagnetic effect. Furthermore, the magnetization curve shows the step-like behaviors at H _c2, corresponding to the first order transition. The d-wave superconductivity with the spin-singlet state is most likely realized in NpPd₅Al₂.