Avoided valence transition in a plutonium superconductor

Significance One way to search for new superconductors is to find a magnetic metal and then suppress the magnetism using chemical doping or pressure. Heavy-fermion superconductors are the archetypal family of magnetic superconductors, but PuCoGa5—the heavy fermion with the highest Tc (18.5 K)—has no static magnetism. What other mechanism, then, is driving superconductivity in PuCoGa5? We measured the elastic constants of PuCoGa5 and found that the bulk modulus softens dramatically before Tc—evidence for fluctuations of the plutonium valence as opposed to magnetic fluctuations associated with the suppression of magnetic order. Valence fluctuations resolve the missing magnetism conundrum in PuCoGa5 by providing an alternative mechanism for the high-temperature superconductivity. The d and f electrons in correlated metals are often neither fully localized around their host nuclei nor fully itinerant. This localized/itinerant duality underlies the correlated electronic states of the high-Tc cuprate superconductors and the heavy-fermion intermetallics and is nowhere more apparent than in the 5f valence electrons of plutonium. Here, we report the full set of symmetry-resolved elastic moduli of PuCoGa5—the highest Tc superconductor of the heavy fermions (Tc = 18.5 K)—and find that the bulk modulus softens anomalously over a wide range in temperature above Tc. The elastic symmetry channel in which this softening occurs is characteristic of a valence instability—therefore, we identify the elastic softening with fluctuations of the plutonium 5f mixed-valence state. These valence fluctuations disappear when the superconducting gap opens at Tc, suggesting that electrons near the Fermi surface play an essential role in the mixed-valence physics of this system and that PuCoGa5 avoids a valence transition by entering the superconducting state. The lack of magnetism in PuCoGa5 has made it difficult to reconcile with most other heavy-fermion superconductors, where superconductivity is generally believed to be mediated by magnetic fluctuations. Our observations suggest that valence fluctuations play a critical role in the unusually high Tc of PuCoGa5.