Density functional tight binding approach utilized to study X-ray-induced transitions in solid materials

Vladimir Lipp,Victor Tkachenko,Beata Ziaja,Bálint Aradi,Michal Stransky,Thomas Frauenheim

doi:10.1038/s41598-022-04775-1

Abstract

Intense X-ray pulses from free-electron lasers can trigger ultrafast electronic, structural and magnetic transitions in solid materials, within a material volume which can be precisely shaped through adjustment of X-ray beam parameters. This opens unique prospects for material processing with X rays. However, any fundamental and applicational studies are in need of computational tools, able to predict material response to X-ray radiation. Here we present a dedicated computational approach developed to study X-ray induced transitions in a broad range of solid materials, including those of high chemical complexity. The latter becomes possible due to the implementation of the versatile density functional tight binding code DFTB+ to follow band structure evolution in irradiated materials. The outstanding performance of the implementation is demonstrated with a comparative study of XUV induced graphitization in diamond.

Highlights

Recent advances in the development of free electron lasers (FEL) enable unprecedentedly precise observation of electron and nuclear dynamics in various samples[1,2], giving rise to a number of important applications, including crystallographic structure determination[3,4,5,6], investigation of valence- and core-electron dynamics[7,8,9] and studies of ultrafast phase transitions[10]
The tight binding (TTB) parameterization in the band structure calculation module of the XTANT code can be replaced with any other model that can calculate electron levels, {ǫi} as a function of the nuclei positions
Essential that {ǫi} only depend on the nuclei position and not on other parameters such as, e.g., the electronic temperature. This assumption holds with a good accuracy for the case of the versatile Density Functional Tight Binding (DFTB)+ code[26]

Summary

Introduction

Recent advances in the development of free electron lasers (FEL) enable unprecedentedly precise observation of electron and nuclear dynamics in various samples[1,2], giving rise to a number of important applications, including crystallographic structure determination[3,4,5,6], investigation of valence- and core-electron dynamics[7,8,9] and studies of ultrafast phase transitions[10]. Such range of electronic energies is challenging to be treated with a fully ab-initio m ethod[27] With this in mind, our in-house simulation tool XTANT (X-ray-induced Thermal And Non-Thermal transitions)[28,29] has been developed. Our in-house simulation tool XTANT (X-ray-induced Thermal And Non-Thermal transitions)[28,29] has been developed It represents a computationally efficient hybrid approach which allows to study X-ray and XUV irradiated solids. The main principle behind the construction of XTANT is the separation of electrons into low and high energy fractions This is justified by the specific shape of the transient electron distribution, which is typically observed upon X-ray irradiation of solids. In Ref.[10], the comparison of the experimental and theoretical optical property, transient transmissivity, allowed to identify temporal timescales for various stages of X-ray induced ultrafast graphitization of diamond

Methods

Results

Conclusion