Sarrao and co-workers found that the transition temperature (Tc) in the plutonium compound - the temperature at which the electrical resistance of a superconducting material drops to zero - is an order of magnitude higher than the highest seen in the heavy fermion systems (compounds based on uranium and cerium). The material also has a large critical current, which would be of technological importance if it were not for the hazardous radioactive properties of plutonium. This critical current comes from pinning centres due to defects in the material, created by radiation induced “self-damage”.
The team observed the superconductivity in measurements of magnetic susceptibility and specific heat. Further measurements on temperature-dependent magnetic susceptibility and electrical resistivity over a wide range of temperatures suggest that the degree of localization of the 5f electrons lies between that of compounds based on cerium and those based on uranium.
Plutonium is an actinide element located at the transition where the 5f electrons go from being delocalized to localized, which makes it one of the most complex materials known. The researchers believe that the superconductivity in plutonium comes directly from its anomalous electronic properties and that it is an intermediate addition, in terms of Tc, to the two other new classes of “magnetically mediated” superconductors - the heavy- fermion materials, which have Tcs of about 1 K and the copper oxides, which have Tcs of about 100 K.
The team hopes that future research will unearth superconductivity in other transuranic compounds with lower toxicity. “Experience tells us that where one superconductor is found, others are usually nearby, so there are many other related compounds to explore,” Sarrao told PhysicsWeb.