Sound speed and Grüneisen parameter up to three terapascal in shock-compressed iron

The warm dense matter regime is generally found in materials compressed to several times their densities on the Earth's surface at temperatures hot enough to produce a conducting fluid, but not so hot that the cohesive binding is overwhelmed by the temperature. This region of phase space is notoriously difficult to constrain with theoretical models. Iron is the most stable element, and is relevant to astrophysics, planetary physics, and industry. The paper here describes experiments using giant kilojoule lasers to shock compress iron to warm dense matter conditions and then send acoustic waves through this compressed material. This method yields the first experimental measurements of sound speed and Gr\"uneisen parameter in shocked iron in the terapascal range, and reveals an interesting dependence on density and melting for these physically important derivatives of the equation of state.