124 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
Thermal and nonthermal melting of silicon under femtosecond x-ray irradiation
As it is known from visible light experiments, silicon under femtosecond
pulse irradiation can undergo the so-called 'nonthermal melting' if the density
of electrons excited from the valence to the conduction band overcomes a
certain critical value. Such ultrafast transition is induced by strong changes
in the atomic potential energy surface, which trigger atomic relocation.
However, heating of a material due to the electron-phonon coupling can also
lead to a phase transition, called 'thermal melting'. This thermal melting can
occur even if the excited-electron density is much too low to induce
non-thermal effects. To study phase transitions, and in particular, the
interplay of the thermal and nonthermal effects in silicon under a femtosecond
x-ray irradiation, we propose their unified treatment by going beyond the
Born-Oppenheimer approximation within our hybrid model based on tight binding
molecular dynamics. With our extended model we identify damage thresholds for
various phase transitions in irradiated silicon. We show that electron-phonon
coupling triggers the phase transition of solid silicon into a low-density
liquid phase if the energy deposited into the sample is above eV per
atom. For the deposited doses of over eV per atom, solid silicon
undergoes a phase transition into high-density liquid phase triggered by an
interplay between electron-phonon heating and nonthermal effects. These
thresholds are much lower than those predicted with the Born-Oppenheimer
approximation ( eV/atom), and indicate a significant contribution of
electron-phonon coupling to the relaxation of the laser-excited silicon. We
expect that these results will stimulate dedicated experimental studies,
unveiling in detail various paths of structural relaxation within
laser-irradiated silicon
Spin structure function and the DHGHY integral at low : predictions from the GVMD model
Theoretical predictions for polarized nucleon structure function
at low are obtained in the framework of the Generalized Vector Meson
Dominance model. Contributions from both light and heavy vector mesons are
evaluated. In the photoproduction limit the first moment of is related to
the static properties of nucleon via the Drell-Hearn-Gerasimov-Hosoda-Yamamoto
sum rule. This property is employed to fix the magnitude of the light vector
meson contribution to , using the recent measurements in the region of
baryonic resonances. Results are compared to the data on . Finally,
the DHGHY moment function is calculated, and our theoretical
predictions are confronted with the recent preliminary data obtained at the
Jefferson Laboratory.Comment: 12 pages including 6 postscript figures, corrected ref.22, caption to
fig.4 and a few typo
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
Hydrodynamic model for picosecond propagation of laser-created nanoplasmas
The interaction of a free-electron-laser pulse with a moderate or large size
cluster is known to create a quasi-neutral nanoplasma, which then expands on
hydrodynamic timescale, i.e., ps. To have a better understanding of ion
and electron data from experiments derived from laser-irradiated clusters, one
needs to simulate cluster dynamics on such long timescales for which the
molecular dynamics approach becomes inefficient. We therefore propose a
two-step Molecular Dynamics-Hydrodynamic scheme. In the first step we use
molecular dynamics code to follow the dynamics of an irradiated cluster until
all the photo-excitation and corresponding relaxation processes are finished
and a nanoplasma, consisting of ground-state ions and thermalized electrons, is
formed. In the second step we perform long-timescale propagation of this
nanoplasma with a computationally efficient hydrodynamic approach.
In the present paper we examine the feasibility of a hydrodynamic two-fluid
approach to follow the expansion of spherically symmetric nanoplasma, without
accounting for the impact ionization and three-body recombination processes at
this stage. We compare our results with the corresponding molecular dynamics
simulations. We show that all relevant information about the nanoplasma
propagation can be extracted from hydrodynamic simulations at a significantly
lower computational cost when compared to a molecular dynamics approach.
Finally, we comment on the accuracy and limitations of our present model and
discuss possible future developments of the two-step strategy.Comment: 14 pages, 6 figure
Quantum-mechanical calculation of ionization potential lowering in dense plasmas
The charged environment within a dense plasma leads to the phenomenon of
ionization potential depression (IPD) for ions embedded in the plasma. Accurate
predictions of the IPD effect are of crucial importance for modeling atomic
processes occurring within dense plasmas. Several theoretical models have been
developed to describe the IPD effect, with frequently discrepant predictions.
Only recently, first experiments on IPD in Al plasma have been performed with
an x-ray free-electron laser (XFEL), where their results were found to be in
disagreement with the widely-used IPD model by Stewart and Pyatt. Another
experiment on Al, at the Orion laser, showed disagreement with the model by
Ecker and Kr\"oll. This controversy shows a strong need for a rigorous and
consistent theoretical approach to calculate the IPD effect. Here we propose
such an approach: a two-step Hartree-Fock-Slater model. With this
parameter-free model we can accurately and efficiently describe the
experimental Al data and validate the accuracy of standard IPD models. Our
model can be a useful tool for calculating atomic properties within dense
plasmas with wide-ranging applications to studies on warm dense matter, shock
experiments, planetary science, inertial confinement fusion and studies of
non-equilibrium plasmas created with XFELs.Comment: 13 pages, 9 figures, to be published in Phys. Rev. X; added
references [46,47
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