Coupling Shock Compression and Ultrafast X-ray Heating to Observe New Phase Transitions

Ultrafast heating by intense X-ray pulses is capable of driving rapid, nonthermal phase changes in materials. This is most obvious in covalently-bonded materials, where the excitation of large numbers of electrons out of the valence band can induce Coulomb forces between the ions, and drive rearrangement into a closer-packed structure.

Previous work at the SACLA X-ray Free Electron Laser demonstrated that its 2-color mode is able to both drive and observe such disordering on timescales of up to 300 fs. In this experiment, we aimed to observe the effect of atomic structure on this disordering, by combining the X-ray pump-probe with shock compression of silicon and germanium samples. At around 10 GPa, these undergo a phase transition from a diamond cubic structure to a closer-packed β-tin structure, which simulations suggest should disorder more slowly. This presentation will present the results of the experiment, and our preliminary analysis of the data collected.