94 research outputs found
Multistep transition of diamond to warm dense matter state revealed by femtosecond X-ray diffraction
Diamond bulk irradiated with a free-electron laser pulse of 6100 eV photon
energy, 5 fs duration, at the eV/atom absorbed doses, is studied
theoretically on its way to warm dense matter state. Simulations with our
hybrid code XTANT show disordering on sub-100 fs timescale, with the
diffraction peak (220) vanishing faster than the peak (111). The warm dense
matter formation proceeds as a nonthermal damage of diamond with the band gap
collapse triggering atomic disordering. Short-living graphite-like state is
identified during a few femtoseconds between the disappearance of (220) peak
and the disappearance of (111) peak. The results obtained are compared with the
data from the recent experiment at SACLA, showing qualitative agreement.
Challenges remaining for the accurate modeling of the transition of solids to
warm dense matter state and proposals for supplementary measurements are
discussed in detail.Comment: Preprint, submitte
XTANT-3: X-ray-induced Thermal And Nonthermal Transitions in matter: theory, numerical details, user manual
This is the user manual for the hybrid code XTANT-3, simulating intense
femtosecond X-ray irradiation of matter. The code combines a few models into
one with feedbacks: transport Monte Carlo simulation, Boltzmann collision
integrals, and tight binding molecular dynamics. Such a combination allows the
simulation of nonequilibrium, nonadiabatic, and nonthermal effects in
electronically excited matter, and the synergy and interplay of these effects.
This text contains a description of the theoretical basis of the model and the
practical user manual. The detailed description should allow new users,
students, and non-specialists to access the ideas behind the code and make the
learning curve less steep.Comment: XTANT-3 user manua
Electronic nonequilibrium effect in ultrafast-laser-irradiated solids
This paper describes the effects of electronic nonequilibrium in a simulation
of ultrafast laser irradiation of materials. The simulation scheme based on
tight-binding molecular dynamics, in which the electronic populations are
traced with a combined Monte Carlo and Boltzmann equation, enables the modeling
of nonequilibrium, nonthermal, and nonadiabatic (electron-phonon coupling)
effects simultaneously. The electron-electron thermalization is described
within the relaxation-time approximation, which automatically restores various
known limits such as instantaneous thermalization (the thermalization time
) and Born-Oppenheimer approximation (). The results of the simulation suggest that the
non-equilibrium state of the electronic system slows down electron-phonon
coupling with respect to the electronic equilibrium case in all studied
materials: metals, semiconductors, and insulators. In semiconductors and
insulators, it also alters the damage threshold of ultrafast nonthermal phase
transitions induced by modification of the interatomic potential due to
electronic excitation
Electron-phonon coupling in semiconductors at high electronic temperatures
A nonperturbative dynamical coupling approach based on tight-binding
molecular dynamics is used to evaluate the electron-ion (electron-phonon)
coupling parameter in irradiated semiconductors as a function of the electronic
temperature up to ~25,000 K. The method accounts for arbitrary electronic
distribution function via the Boltzmann equation, enabling a comparative
analysis of various models: fully equilibrium electronic distribution,
band-resolved local equilibria (distinct temperatures and chemical potential of
electrons in the valence and the conduction band), and a full nonequilibrium
distribution. It is demonstrated that the nonequilibrium produces the
electron-phonon coupling parameter different by at most ~35% from its
equilibrium counterpart for identical deposited energy density, allowing to use
the coupling parameter as a function of the single electronic equivalent (or
kinetic) temperature. The following 14 semiconductors are studied here - group
IV: Si, Ge, SiC; group III-V: AlAs, AlP, GaP, GaAs, GaSb; oxides: ZnO, TiO2,
Cu2O; layered PbI2; ZnS and B4C
Electron-ion coupling in semiconductors beyond Fermi's golden rule
In the present work, a theoretical study of electron-phonon (electron-ion)
coupling rates in semiconductors driven out of equilibrium is performed.
Transient change of optical coefficients reflects the band gap shrinkage in
covalently bonded materials, and thus, the heating of atomic lattice. Utilizing
this dependence, we test various models of electron-ion coupling. The
simulation technique is based on tight-binding molecular dynamics. Our
simulations with the dedicated hybrid approach (XTANT) indicate that the widely
used Fermi's golden rule can break down describing material excitation on
femtosecond time scales. In contrast, dynamical coupling proposed in this work
yields a reasonably good agreement of simulation results with available
experimental data
Contribution of inter- and intraband transitions into electron-phonon coupling in metals
We recently developed an approach for calculation of the electron-phonon
(electron-ion in a more general case) coupling in materials based on
tight-binding molecular dynamics simulations. In the present work we utilize
this approach to study partial contributions of inter- and intraband electron
scattering events into total electron-phonon coupling in Al, Au, Cu elemental
metals and in AlCu alloy. We demonstrate that the interband scattering plays an
important role in electron-ion energy exchange process in Al and AlCu, whereas
intraband transitions are dominant in Au and Cu. Moreover, inter- and
intraband transitions exhibit qualitatively different dependencies on the
electron temperature. Our findings should be taken into account for
interpretation of experimental results on electron-phonon coupling parameter.Comment: To be submitte
Metallic water: transient state under ultrafast electronic excitation
The modern means of controlled irradiation by femtosecond lasers or swift
heavy ion beams can transiently produce such energy densities in samples that
reach collective electronic excitation levels of the warm dense matter state
where the potential energy of interaction of the particles is comparable to
their kinetic energies (temperatures of a few eV). Such massive electronic
excitation severely alters the interatomic potentials, producing unusual
nonequilibrium states of matter and different chemistry. We employ density
functional theory and tight binding molecular dynamics formalisms to study the
response of bulk water to ultrafast excitation of its electrons. After a
certain threshold electronic temperature, the water becomes electronically
conducting via the collapse of its band gap. At high doses, it is accompanied
by nonthermal acceleration of ions to a temperature of a few thousand Kelvins
within sub-100 fs timescales. We identify the interplay of this nonthermal
mechanism with the electron-ion coupling, enhancing the electron-to-ions energy
transfer. Various chemically active fragments are formed from the
disintegrating water molecules, depending on the deposited dose.Comment: to be submitte
Damage threshold in pre-heated materials exposed to intense X-rays
Materials exposed to ultrashort intense x-ray irradiation may experience
various damaging conditions depending on the in-situ temperature. A pre-heated
target exposed to intense x-rays plays a crucial role in numerous systems of
physical-technical importance, ranging from the heavily-, and repeatedly
radiation-loaded optics at x-ray free-electron laser facilities, to the first
wall of prospective inertial fusion reactors. We study theoretically the damage
threshold dependence on the temperature in different classes of materials: an
insulator (diamond), a semiconductor (silicon), a metal (tungsten), and an
organic polymer (PMMA). The numerical techniques used here enable us to trace
the evolution of both, an electronic state and atomic dynamics of the
materials. It includes damage mechanisms such as thermal damage (induced by an
increase of the atomic temperature due to energy transfer from x-ray-excited
electrons) and nonthermal phase transitions (induced by changes in the
interatomic potential due to excitation of electrons). We demonstrate that in
the pre-heated materials, typically, the thermal damage threshold stays the
same or lowers with the increase of the in-situ temperature, whereas nonthermal
damage thresholds may be lowered or raised, depending on the particular
material and specifics of the damage kinetics
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