The Effect Of Non-equilibrium States On Lattice Stability Of Warm Dense Matter

In recent years,with the development of X-ray free electron lasers,the technology of isochoric heating by free electron laser to produce warm dense matter has been widely used,making warm dense matter an important research field of atomic and molecular physics under extreme conditions.However,for complex systems,there are many important problems remaining to be solved for the non-equilibrium processes,such as electron excitation and lattice dynamics generated by ultrafast lasers.In this paper,based on first-principles method,the electronic structure of Na,Al,Cu,and Au corresponding to a nonequilibrium transient state is investigated using finite-temperature density-functional theory with the introduction of nonequilibrium distribution of electrons.The small displacement method is used to calculate the phonon dispersions of metal,which represents the effect of nonequilibrium transient electronic structures on lattice stability in metals.We find that,with the exciting of electrons in the inner shell orbitals of metal,the reduced screening results in the lowering of energy of the orbitals and the electrons are closer to the nucleus.However,the case of electronic localization in the valence band of aluminum and copper is very different.We attribute this difference to the existence of semi-local d band of Cu.The reduced screening makes the electrons of valence band become more localized for Cu.It is negligible for Al,however.In addition,we also investigate the lattice stability with different number of excited electrons.With the increased number of excited electrons in the inner shell,the lattice stability of Na,Al,Cu and Au will change from stable to unstable,the lattice stability of Na and Al remain unstable when symmetry is satisfied again.For Cu and Au,however,their stability is recovered,accompanied by the phenomenon of phonon hardening.In order to study the effect of anharmonicity on lattice stability,we also systematically calculate and analyze the potential energy surfaces of Al and Cu in the ground and excited states.Finally,the effect of electron excitation in the valence band and electron temperature on lattice stability are investigated.This work is helpful for further studies of the electronic structure and lattice dynamics of warm dense matter produced by isochoric heating of ultrafast laser.