Unconventional superconductivity of NpPd$_5$Al$_2$

The 5f electrons in actinide compounds has an intermediate character between the 4f-localized state and the 3d-itinerant state. This leads to a variety of exotic phenomena, such as non-Fermi liquid behavior, multipole ordering, hidden order state and unconventional superconductivity. The discovery of superconductivity in PuCoGa$_5$ and PuRhGa$_5$ with high critical temperatures provides a new perspective on the physics of actinide compounds. It is generally believed that Np compounds have more 5f-itinerant characteristic features like d-electron metals compared with U compounds. In fact, the results of dHvA experiments in Np-115 compounds are in good agreement with the 5f-itinerant band model. However, the complicated magnetic properties in Np-115 are well explained by the mean field theory including the orbital ordering based on the 5f-localized model. This indicates the dual nature of 5f electrons. NpPd$_5$Al$_2$ is the first Np-based heavy fermion superconductor with the ZrNi$_2$Al$_5$-type tetragonal structure. The superconductivity was found below $T_{\rm sc}=5\,{\rm K}$. The non-Fermi liquid behavior and the large specific heat coefficient ($\gamma=200\,{\rm mJ/K^2 mol}$) were detected. The upper critical field $H_{\rm c2}$ at $0\,{\rm K}$ is large and highly anisotropic: $37\,{\rm kOe}$ for $H \parallel [100]$ and $143\,{\rm kOe}$ for $H \parallel [001]$. $H_{\rm c2}$ is strongly suppressed by the magnetic field in the $H_{\rm c2}$--$T$ phase diagram for both field direction, indicating the strong Pauli paramagnetic effect. The d-wave spin-singlet superconductivity is most likely realized. The large specific heat jump $\Delta C/\gamma T_{\rm sc} = 2.33$ suggests the superconductivity with strong coupling. The results are compared with the well known heavy fermion superconductor CeCoIn$_5$.