Heavy Fermion Superconductivity with the Strong Pauli Paramagnetic Effect on NpPd5Al2

Abstract

We measured the electrical resistivity, specific heat, magnetic susceptibility and magnetization in the first Np-based superconductor NpPd 5 Al 2 with the tetragonal crystal structure. NpPd 5 Al 2 is a paramagnet and becomes the superconductivity at T c =4.9 K. The large electronic specific heat coefficient of γ= 200 mJ/K 2 mol at T c indicates that NpPd 5 Al 2 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 NpPd 5 Al 2 .