Avoided Valence Transition in a Plutonium Superconductor

TL;DR

This study reveals that plutonium valence fluctuations influence the high-temperature superconductivity in PuCoGa_5, with elastic softening linked to valence instability, which is suppressed upon entering the superconducting state.

Contribution

It provides direct experimental evidence of valence fluctuations in a plutonium superconductor and links these fluctuations to its high critical temperature.

Findings

Anomalous softening of the bulk modulus above T_c

Disappearance of elastic softening at T_c

Valence fluctuations likely drive high T_c in PuCoGa_5

Abstract

Some of the most remarkable phenomena---and greatest theoretical challenges---in condensed matter physics arise when $d$ or $f$ electrons are 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 $PuCoGa_5$---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 $T_c$. Because the bulk modulus is known to couple strongly to the valence state, we propose that plutonium valence fluctuations drive this elastic softening. This elastic softening is observed to disappear when the superconducting gap opens at $T_c$, suggesting that plutonium valence fluctuations have a strong footprint on the Fermi surface, and that $PuCoGa_5$ avoids a valence-transition by entering the superconducting state. These measurements provide direct evidence of a valence instability in a plutonium compound, and suggest that the unusually high-$T_c$ in this system is driven by valence fluctuations.