Systematics in superconductive compounds

In about twenty minutes, Franck Wastin of the Joint Research Center of the European Commission at the Institute for Transuranium Elements at Karlsruhe, Germany, reviewed and discussed a range of compounds of the AnTGa₅ and CeTIn₅ groups and the factors postulated to “tune” the critical temperature (T_c) for superconductivity in heavy-fermion superconductors. The best known of these is Los Alamos’ plutonium-cobalt-gallium superconductor, PuCoGa₅, with a T_c of 18.5 kelvin (K).

This research was prompted by the finding that PuRhGa₅ is also a superconductor with a T_c of approximately 9 K, but that the compound CeCoIn₅, essentially isostructural with PuCoGa₅, shows superconductivity only below 2 K. Hence, logic suggests that there is perhaps something about the electronic structure of the actinide-based superconductors that mediates a higher T_c.

Wastin reviewed a number of hypotheses to explain superconductivity and Tc in these compounds and succeeded in demonstrating flaws in each of them. In doing so, he considered the topics of lattice parameters, the effect of doping superconductive compounds with nonisoelectronic metals, and aging effects.

Wastin’s main finding was a correlation between superconductivity and the total count of outer-shell electrons: specifically that, with few exceptions, all AnMGa₅ superconducting compounds fall within a narrow band at 32 ± 0.2 total electrons, with compounds showing higher or lower totals failing to exhibit superconductivity (PuCoGa₅ having exactly 32). The talk, of course, raised as many questions as it addressed since the significance of the study’s finding remains to be investigated.

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