Superconductivity in actinides and related materials

In his plenary talk on actinide superconductors, Los Alamos’ Joe Thompson used the exciting discovery of PuCoGa₅ as a jumping-off point for a more general review of superconductivity mechanisms and potential insights into the mechanism of so-called unconventional superconductivity.

The basis for the presentation was immediately apparent in Thompson’s opening slide, which presented the now well-known nearly order-of-magnitude difference in the critical temperature (T_c) for superconductivity between the plutonium superconductor, PuCoGa₅, and 12 uranium-based superconductors, all of which show T_cs of less than 2.5 kelvin (K)—while the two known plutonium superconductors show T_cs of 18.7 K (PuCoGa₅) and 8.5 K (PuRhGa₅). As Thompson framed it, these discoveries motivate us to better understand the complexity of plutonium’s solid-state physics, particularly the ongoing mysteries of the behavior of its arguably both itinerant and localized 5f electrons.

A fundamental question posed in this talk was that of the mechanism of superconductivity for these plutonium compounds, i.e., are they conventional or unconventional superconductors? In the current understanding of so-called conventional superconductivity, a temporary lattice distortion provides an attractive interaction between conduction electrons of opposite spin and momentum, forming a so-called “Cooper pair,” which then behaves as a unit at temperatures below the Tc, condensing into a macroscopic quantum state that is energetically separated from that of all unpaired electrons in the solid by an energy gap that is finite over the entire Fermi surface. It requires only a small number of magnetic moments in a conventional superconductor to destroy superconductivity (essentially driving the Tc to 0 K).

But superconductivity in PuCoGa₅ has proven to be robust against the application of a magnetic field, hence arguing against conventional superconductivity. By contrast, electron pairing in unconventional superconductors comprises an attractive interaction between itinerant electrons mediated by an exchange of spin fluctuations. As a consequence, the superconducting energy gap between the Cooper pairs and the other electrons in the solid goes to zero at gap nodes on the Fermi surface.

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