Effect of Nonequilibrium Transient Electronic Structures on Lattice Stability in Metals: Density Functional Theory Calculations

Qi Zhang,Xiaoxiang Yu,Hengyu Zhang,Cheng Gao,Dongdong Kang,Qiyu Zeng,Shen Zhang,Jiayu Dai,Jianhua Wu

doi:10.3389/fphy.2022.838568

Qi Zhang, Xiaoxiang Yu + Show 7 more

Open Access

https://doi.org/10.3389/fphy.2022.838568

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Abstract

The electronic structures of metals undergo transient nonequilibrium states during the photoexcitation process caused by isochoric heating of X-ray free-electron laser, and their lattice stability is, thus, significantly affected. By going beyond frozen core approximation, we manually introduced nonequilibrium electron distribution function in finite-temperature density functional theory with the framework of Kohn–Sham–Mermin to investigate such transient states, and their effect on lattice stability in metals is demonstrated by phonon dispersion calculated using the finite displacement method. We found that the perfect lattice of a metal collapses due to the exotic electronic structure of nonequilibrium transient state created by isochoric heating of X-ray free-electron laser. Further increase of the number of holes created in the sample (i.e., an increase of laser fluence) still results in lattice instability for aluminum, while for copper, it results in phonon hardening. The potential energy surface is calculated for the extreme case of both Al and Cu with exactly one hole created in its inner shell for each one of the atoms. A double-well structure is clearly observed for Al, while the potential energy surface becomes steeper for Cu.

Highlights

Lattice dynamics are of fundamental importance in a variety of fields, including phase transition [1], superconductivity [2], and thermal conductivity [3] It is reported that under intense laser radiation, the melting of the semiconductor will be athermal due to softening of interatomic bonds, which takes place long before the conventional thermal melting due to the process of energy transfer from electrons to ionic lattice [4]
We found that the perfect lattice of a metal collapses due to the exotic electronic structure of nonequilibrium transient state created by isochoric heating of X-ray free-electron laser
The specific holes can be created by predominant photoionization of 2s electrons of Al or 3s electrons of Cu to their Fermi surface, with other transitions being of minor importance

Summary

Introduction

Lattice dynamics are of fundamental importance in a variety of fields, including phase transition [1], superconductivity [2], and thermal conductivity [3] It is reported that under intense laser radiation, the melting of the semiconductor will be athermal due to softening of interatomic bonds, which takes place long before the conventional thermal melting due to the process of energy transfer from electrons to ionic lattice [4]. The chemical bond hardening in warm dense gold [13] is observed in experiment, which provides helpful information for understanding the generation process of warm dense matter [14, 15]. The equation of states, transport properties such as electrical and thermal conductivity, etc., as well as the lattice dynamics are fundamental properties of WDM, which are helpful in understanding the process of inertial confinement fusion [16,17,18] and the formation of celestial

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