Kondo behavior in superconducting NpPd5Al2

The unconventional superconductor ${\text{NpPd}}_{5}{\text{Al}}_{2}$, crystallizing in the tetragonal ${\text{ZrNi}}_{2}{\text{Al}}_{5}$-type structure, was studied by means of $^{237}\text{N}\text{p}$ M\"ossbauer spectroscopy, heat capacity, electrical resistivity, magnetoresistivity, and thermoelectric power measurements. The superconductivity is clearly observed below ${T}_{c}=5\text{ }\text{K}$. The M\"ossbauer studies confirm the absence of ordered magnetic Np moments in both the superconducting and normal phases. The value of the isomer shift suggests that the $5f$-electron count is close to ${n}_{5f}=4$, in agreement with electron band structure calculations. Above ${T}_{c}$ the electronic specific heat, the magnetic contribution to the electrical resistivity, the magnetoresistivity, and the Seebeck coefficient are governed by the interplay of Kondo and crystal-field effects. The Seebeck coefficient, negative in the whole temperature range, indicates electronlike conductivity. This perfectly agrees with the Hall effect data as well as with theoretical predictions.