Unconventional Heavy-Fermion Superconductivity of a New Transuranium Compound NpPd₅Al₂

Abstract

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 ΔC⁄γT_c=2.33 [T_c=4.9 K and γ=200 mJ/(K²·mol)]. The upper critical field H_c2 at 0 K 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 T³-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.