XTANT-3: X-ray-induced Thermal And Nonthermal Transitions

A hybrid code for modeling femtosecond laser effects in matter. The code is capable of modeling the entire process of phase transition or damage formation in various materials irradiated with the ultrafast (femto- to pico-second) pulse. The following processes are modelled simultaneously and interconnectedly: * photon absorption * excitation of electrons * electron kinetics including impact ionization and elastic scattering * Auger-cascades of core holes * equilibration of the electronic ensemble (establishing Fermi-Dirac distribution) * coupling of electrons to the atomic system (electron-ion or electron-phonon coupling) * evolution of the electronic structure (band structure) * changes in the interatomic potential or potential-energy surface due to electronic excitation * atomic responce to the changes in the interatomic potential (including nonthermal melting) * atomic responce to the heating via electron-ion coupling (including thermal melting) * atomic dynamics including possible phase transition and/or ablation * possible cooling (via thermostats) relaxing and forming the final observable material state

Note: Although the code is primarily aimed at modelling X-ray-induced effects, it is also possible to model visible or NIR-laser irradiation, as long as the photon energy is larger than the bandgap of the modelled material, since only linear photon absorption is currently included (no multiphoton absorption); for metals, arbitrary non-relativistic photon energy is allowed

The code combines the following methods into one model with feed-backs:

a) Monte Carlo (MC) method for modeling photon-induced electron kinetics

b) Boltzmann equation for low-energy electrons: relaxation-time approximation (RTA) for electron-electron scattering; Boltzmann collision integrals (BCI) for nonadiabatic electron-ion (electron-phonon) coupling

c) Transferable tight binding (TB) for tracing electronic structure and interatomic forces

d) Molecular dynamics (MD) for tracing atomic response to modification of the interatomic potential due to electronic excitation

e) Kubo-Greenwood (or Random phase approximation, RPA) for calculation of the optical properties and electronic heat conductivity of the material

Disclaimer

This code is work in progress, anything might change without a notice, bugfixes and patches are expected!

Although we endeavour to ensure that the code XTANT and results delivered are correct, no warranty is given as to its accuracy (for details, see GPL-3.0 license). This code was developed for non-commercial peaceful purposes only, such as research and education.

Tables with calculated electron-phonon coupling

Two-tempreature model parameters: the tables with the calculated electron-ion (electron-phonon) coupling, electronic heat capacity, and electronic heat conductivity, at high electronic temepratures can be found here: https://github.com/N-Medvedev/XTANT-3couplingdata

How to cite

The use of the code is at your own risk. Should you choose to use it, please cite the code and/or the manual: * N. Medvedev (2023). XTANT-3 [Computer software]. https://doi.org/10.5281/zenodo.8392569

• N. Medvedev XTANT-3: X-ray-induced Thermal And Nonthermal Transitions in Matter: theory, numerical details, user manual (2023) https://doi.org/10.48550/arXiv.2307.03953

Journal citations may be used as follows:

• N. Medvedev, V. Tkachenko, V. Lipp, Z. Li, B. Ziaja, "Various damage mechanisms in carbon and silicon materials under femtosecond x-ray irradiation", 4open. 1 (2018) 3. https://doi.org/10.1051/fopen/2018003

Should you use electron-phonon coupling in the calculations, the following citation should be included in addition to the above-mentioned one:

• N. Medvedev, I. Milov, "Electron-phonon coupling in metals at high electronic temperatures", Phys. Rev. B. 102 (2020) 064302. https://doi.org/10.1103/PhysRevB.102.064302

In a publication, at least the following parameters should be mentioned for reproducibility of the results: Material, its initial structure, the number of atoms in the supercell, the initial conditions (atomic and electronic temperatures), an ensemble used, a type of boundary conditions, a scheme of low-energy electron thermalization (and value of thermalization type is nonequilibrium scheme is used), a type of cross sections in Monte Carlo simulation, a type of tight binding parametrization, whether the electron emission was included or not (if yes, whether Coulomb potential for atoms was accounted for and what model for electron emission was used), whether an additional short-range repulsive potential was used (and what type), time step of MD simulation, parameters of the incoming laser pulse (its photon energy, deposited dose, duration).