Fig. 4.

Valence fluctuations in PuCoGa₅. (A) The bulk modulus, (B) anomalous compressibility (inverse of the bulk modulus), (C) out-of-plane Poisson’s ratio (${\nu }_{xz}\equiv -S_{xz}/S_{xx}$, where $S_{ij}$ values are the elastic compliances from the inverted modulus tensor), and (D) in-plane Poisson’s ratio (${\nu }_{xy}\equiv -S_{xy}/S_{xx}$). The Poisson’s ratio ${\nu }_{xz}$ has a magnitude typical of most metals (29) and is only weakly temperature-dependent. The ratio ${\nu }_{xy}$, however, is strongly temperature-dependent and anomalously small, reminiscent of other mixed-valence systems (24). The anomalous contribution to the compressibility is shown in B along with a fit to $-a/\left(T_v-T\right)$, where $T_v=9\pm 1$ K. (E and F) Schematic Fermi surface of PuCoGa₅ (based on refs. 37 and 31) with hole pockets in red and electron pockets in blue. $A_{1g}$ symmetry Fermi surface distortions are shown in lighter shades, with fluctuations of the total carrier density in E and a distortion caused by hybridization fluctuations that preserves total carrier number in F.