Photoinduced Phase Transition of Diamond: A Nonadiabatic Quantum Molecular Dynamics Study

Photoinduced phase transition holds the key to realizing novel states of matter and transition pathways that do not exist otherwise. An example is ultrafast graphitization of diamond using femtosecond soft X-ray laser pulses, for which the structural transformation pathways have not been fully explored. Using first-principles nonadiabatic quantum molecular dynamics simulations, we found a progression from order-to-order (diamond-to-graphite) to order-to-disorder (diamond-to-amorphous) phase transitions at elevated laser intensities. We further found a novel graphitization pathway on the crystallographic (11̅2) plane at high intensities below the order-to-disorder intensity threshold, which is distinct from the common (111)-graphitization mechanism in thermal and low-intensity photopathways. This study reveals rich coexistence of phase-transition pathways under intense irradiation, thus expanding the notion of ultrafast laser control of material phases.