Superconductivity in Cerium Containing High-Entropy Alloys

High-entropy alloys are a class of materials that are defined as containing five or more constituent elements, each with atomic percentages of 5-35%. They are of interest not only because of their novel structural/chemical composition, but also because they exhibit extraordinary properties, including high stress and strain strength, high corrosive strength, and the retention of these characteristics across broad temperature and pressure ranges. Superconductivity has also been observed in some high-entropy alloys, with d – electron systems being extensively researched. In contrast, f – electron systems have remained largely unstudied. Following earlier work by W. Lucas Nelson that addressed [TaNb]1-x(UHfTi)x [13], the substitution series [TaNb]1-x(CeHfTi)x was synthesized for concentrations x = 0.2 – 0.4. X-ray diffraction measurements show the cerium HEA forms a body centered cubic phase with minimal impurities. Differences were discovered in the XRD of cold worked v. as cast samples, and the inducement of impurities through cold working was documented. In order to investigate the electronic and magnetic properties of these materials, magnetic susceptibility χ (T), magnetization M (H), electrical resistivity ρ (T), and heat capacity C (T) measurements were performed for the as-cast specimens. Resistivity measurements showed disordered metallic behavior, with the occurrence of superconductivity at a critical temperature Tc ≈ 7 K for all concentrations, while the upper critical field Hc2 increases with increasing x. Magnetic susceptibility χ (T) measurements confirm the existence of a superconducting state, and magnetization as a function of field, M (H), suggest type II superconductivity. An additional magnetically hysteretic state spanning 10 – 225 K is also seen in χ (T) for some concentrations. This aspect of the behavior is not fully explored in this work, and will require further investigation. The superconductivity is shown to occur in the bulk for x = 0.3 through heat capacity C (T) measurements, with a bulk-like jump being observed at Tc. Taken together, these data show the cerium HEA to be a bulk type II superconductor for x = 0.2 and 0.3 while larger concentrations show weakened superconductivity with evidence for cerium magnetism. This opens a path towards investigating f – electron superconductivity in these and other high entropy alloys, where it remains to be seen: (i) whether higher transition temperatures can be accessed and (ii) whether strong electronic correlations that are related to the f – electron state relevant to the superconductivity and other physical properties.