Anomalous sound speed in warm dense matter

Recent experimental measurements reported anomalously low sound speed in liquid deuterium shocked to 200 GPa [D. Fratanduono et al, Phys. Plasmas 26, 012710 (2019)] compared to all theoretical models. This experimental sound speed is shown to coincide with the propagation speed of electrostatic perturbations corresponding to the ion-acoustic plasma mode. The thermodynamic conditions where such anomalous sound speed is expected will be described for hydrogen, beryllium, and carbon.