Avoided valence transition in a plutonium superconductor

作者：Ramshaw B J^*; Shekhter Arkady; McDonald Ross D; Betts Jon B; Mitchell J N; Tobash P H; Mielke C H; Bauer E D; Migliori Albert

来源：Proceedings of the National Academy of Sciences of the United States of America, 2015, 112(11): 3285-3289.

DOI：10.1073/pnas.1421174112

• unconventional superconductivity
• heavy fermions
• quantum criticality
• valence fluctuations
• resonant ultrasound spectroscopy

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-T-c 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 T-c superconductor of the heavy fermions (T-c = 18.5 K)-and find that the bulk modulus softens anomalously over a wide range in temperature above T-c. 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 T-c of PuCoGa5.

• 出版日期2015-3-17
• 单位Los Alamos