期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：11
页码：3285-3289
DOI：10.1073/pnas.1421174112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceOne 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 [IMG]f1.gif" ALT="Formula" BORDER="0"> (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 [IMG]f1.gif" ALT="Formula" BORDER="0">--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-[IMG]f1.gif" ALT="Formula" BORDER="0"> cuprate superconductors and the heavy-fermion intermetallics and is nowhere more apparent than in the [IMG]f2.gif" ALT="Formula" BORDER="0"> valence electrons of plutonium. Here, we report the full set of symmetry-resolved elastic moduli of PuCoGa5--the highest [IMG]f1.gif" ALT="Formula" BORDER="0"> superconductor of the heavy fermions ([IMG]f1.gif" ALT="Formula" BORDER="0"> = 18.5 K)--and find that the bulk modulus softens anomalously over a wide range in temperature above [IMG]f1.gif" ALT="Formula" BORDER="0">. 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 [IMG]f1.gif" ALT="Formula" BORDER="0">, 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 [IMG]f1.gif" ALT="Formula" BORDER="0"> of PuCoGa5.
