Large-scale atomistic simulations of laser-excited bismuth with density functional theory accuracy: unified description of nonthermal and thermal ultrafast relaxation effects

• Applicants: Dr. Bernd Bauerhenne; Professor Dr. Martin Ezequiel Garcia
• Subject Area: Theoretical Condensed Matter Physics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 518272843

When a solid is irradiated by an intense femtosecond laser pulse, the electrons are abruptly heated to an extremely high temperature. This leads to significant changes in the atomic bonds and to structural changes that have a non-thermal character. The transient state with hot electrons and cold ions decays on a picosecond time scale due to collisions between electrons and ions. These collisions cause the electrons to cool down and release energy to the ion lattice. As a result, a thermal state is formed, in which structural changes can occur (thermal structural changes). We plan, for the first time, to simulate thermal and non-thermal structural changes following a femtosecond laser pulse excitation of bismuth at the same level of accuracy (density functional theory accuracy) at length- and time scales of experimental relevance. Bismuth crystallizes under normal conditions in a Peierls-distorted A7 structure. Due to this particular crystal structure, many femtosecond laser-induced phenomena can be and have been experimentally observed: bond softening, a solid-to-solid transition, excitation of coherent phonons, ultrafast melting, and ablation. Moreover, bismuth exhibits a large scattering cross section, which makes it attractive for pump probe experiments, like time-resolved crystallography. In those experiments, thin bismuth films are excited with a femtosecond laser pulse and then probed, with a delay of a few hundred femtoseconds, with an ultrashort X-ray or electron pulse. From the time-resolved diffraction patterns, conclusions can be drawn about the atomic motions following a femtosecond laser excitation. However, the precise motions of individual atoms cannot be reconstructed. We plan to close this gap and provide a "molecular movie" of the structural dynamics in bismuth after femtosecond laser excitation: To do so, we will perform molecular dynamics simulations considering several millions of atoms to compare one-to-one with existing experiments and to make predictions regarding laser-induced material transformations. In bismuth, however, relativistic effects play a significant role and therefore pose a challenge for large scale atomistic simulations. In the present project we will simulate laser induced thermal and non-thermal structural changes in bismuth including spin-orbit coupling. In order to realize these simulations, we will construct the first electron temperature-dependent interatomic potential for bismuth based on extensive density functional theory data, which we will produce. This interatomic potential will then be implemented in large-scale two temperature model molecular dynamics (TTM-MD) simulations.

• DFG Programme: Research Grants