Unconventional superconductivity of NpPd5Al2

The high quality single crystals of NpPd 5 Al 2 with the body-centered tetragonal structure were grown by the Pb flux method. NpPd 5 Al 2 was found to be the first Np-based heavy fermion superconductor with the relatively high critical temperature T sc = 4.9 K. The upper critical field H c2 is large and highly anisotropic. Corresponding to the heavy electronic state, the initial slope of H c2 is large, but H c2 at low temperatures is suppressed by the magnetic field, indicating a strong Pauli paramagnetic effect and the first-order transition at H c2 . These results imply that NpPd 5 Al 2 is located at the proximity of the antiferromagnetic ordering, which might be hidden by the superconductivity. The d-wave superconductivity with a spin singlet state is most likely realized in NpPd 5 Al 2 -.