Specific heat and anisotropy of the nonconventional superconductorsPuCoGa5andPuRhGa5

Abstract

We report on specific-heat measurements under a magnetic field on oriented single crystals of the nonconventional superconductors ${\mathrm{PuCoGa}}_{5}$ and ${\mathrm{PuRhGa}}_{5}$ with superconducting transition temperature ${T}_{c}\ensuremath{\sim}18$ and $9\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively. In the superconducting state, the electronic specific heat follows a quadratic temperature dependence, which indicates an axial state with line nodes in the superconducting gap structure. The specific heat measured under a magnetic field applied along different crystallographic directions reveals anisotropic behavior. For both compounds, ${T}_{c}$ decreases faster for fields applied along the $c$ axis than for fields perpendicular to it. The value of the slope of the critical field at ${T}_{c}$ is higher in the case of ${\mathrm{PuCoGa}}_{5}$, while the anisotropy effect is stronger in ${\mathrm{PuRhGa}}_{5}$. Further results are discussed in the context of the isostructural Ce- and Np-based materials. The possibility of the appearance of a Fulde-Ferrell-Larkin-Ovchinnikov state and/or strong spin-orbit coupling, related to the observed low-temperature linear shape of the critical field deviating from the common $d$-wave superconductor behavior, is considered, especially in the ${\mathrm{PuRhGa}}_{5}$ case.