Unconventional heavy-fermion superconductivity of a new transuranium compound NpPd5Al2

We succeeded in growing a new transuranium compound NpPd5Al2 with the tetragonal crystal structure by the Pb-flux method, and measured the electrical resistivity, specific heat, magnetic susceptibility and magnetization. NpPd5Al2 is found to be a paramagnetic heavy-fermion superconductor with strong coupling Delta C/gamma T-c = 2.33 [T-c = 4.9 K and gamma = 200 mJ/(K(2 center dot)mol)]. The upper critical field H-c2 at 0K is large and highly anisotropic: H-c2(0) = 37 kOe for H \\ [100] and H-c2(0) = 143 kOe for H \\ [001]. H-c2 is found to be strongly suppressed by the magnetic field, and the magnetization curve indicates a step-like increase at H-c2, which are due to the paramagnetic effect. The low-temperature electronic specific heat approximately follows the T-3-dependence, indicating the existence of point nodes in the energy gap. The d-wave superconductivity with point nodes is most likely realized in this compound.