Fig. 1.

Vibrational spectrum of PuCoGa₅ at room temperature. (A) Transmitted ultrasonic signal as a function of frequency (real and imaginary components in blue and black, respectively, on the right vertical axis) showing the first 65 resonances at room temperature. The position of each resonance is indicated by a red dot (left vertical axis): the elastic moduli are calculated precisely by fitting these resonance positions—a highly overdetermined problem with six moduli and 65 resonances. The calculated resonance positions (black crosses) not only have a small residual error but also, reproduce the correct structure of the data. (B) The PuCoGa₅ unit cell and the five irreducible representations of strain allowed by tetragonal symmetry and their associated elastic moduli. The sixth modulus ($c_{13}$) is the coupling coefficient between the two $A_{1g}$ strains. These six moduli at $T=295\hspace{0.17em}\text{K}$ are measured to be $\left(c_{11}+c_{12}\right)/2=119.8$, $c_{13}=64.5$, $c_{33}=176.1$, $\left(c_{11}-c_{12}\right)/2=73.1$, $c_{44}=60.8$, and $c_{66}=54.2$ (all values in gigapascals).