3,636 research outputs found
Electronic and atomic kinetics in solids irradiated with free-electron lasers or swift-heavy ions
In this brief review we discuss the transient processes in solids under
irradiation with femtosecond X-ray free-electron-laser (FEL) pulses and
swift-heavy ions (SHI). Both kinds of irradiation produce highly excited
electrons in a target on extremely short timescales. Transfer of the excess
electronic energy into the lattice may lead to observable target modifications
such as phase transitions and damage formation. Transient kinetics of material
excitation and relaxation under FEL or SHI irradiation are comparatively
discussed. The same origin for the electronic and atomic relaxation in both
cases is demonstrated. Differences in these kinetics introduced by the
geometrical effects ({\mu}m-size of a laser spot vs nm-size of an ion track)
and initial irradiation (photoabsorption vs an ion impact) are analyzed. The
basic mechanisms of electron transport and electron-lattice coupling are
addressed. Appropriate models and their limitations are presented.
Possibilities of thermal and nonthermal melting of materials under FEL and SHI
irradiation are discussed
Weihrauch goes Brouwerian
We prove that the Weihrauch lattice can be transformed into a Brouwer algebra
by the consecutive application of two closure operators in the appropriate
order: first completion and then parallelization. The closure operator of
completion is a new closure operator that we introduce. It transforms any
problem into a total problem on the completion of the respective types, where
we allow any value outside of the original domain of the problem. This closure
operator is of interest by itself, as it generates a total version of Weihrauch
reducibility that is defined like the usual version of Weihrauch reducibility,
but in terms of total realizers. From a logical perspective completion can be
seen as a way to make problems independent of their premises. Alongside with
the completion operator and total Weihrauch reducibility we need to study
precomplete representations that are required to describe these concepts. In
order to show that the parallelized total Weihrauch lattice forms a Brouwer
algebra, we introduce a new multiplicative version of an implication. While the
parallelized total Weihrauch lattice forms a Brouwer algebra with this
implication, the total Weihrauch lattice fails to be a model of intuitionistic
linear logic in two different ways. In order to pinpoint the algebraic reasons
for this failure, we introduce the concept of a Weihrauch algebra that allows
us to formulate the failure in precise and neat terms. Finally, we show that
the Medvedev Brouwer algebra can be embedded into our Brouwer algebra, which
also implies that the theory of our Brouwer algebra is Jankov logic.Comment: 36 page
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
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
A model of linear chain submonolayer structures. Application to Li/W(112) and Li/Mo(112)
We propose a lattice gas model to account for linear chain structures
adsorbed on (112) faces of W and Mo. The model includes a dipole-dipole
interaction as well as a long-ranged indirect interaction. We have explicitly
demonstrated that the periodic ground states depend on a competition between
dipole-dipole and indirect interaction. The effect of temperature is studied
within the molecular-field approximation. The numerical results show that for
dipole-dipole interaction only, all long periodic linear chain phases are
suppressed to low temperatures. However, when the long-range indirect
interaction becomes important, the long-periodic linear chain phases start to
fill up the phase diagram and develop a high thermal stability. Model
parameters are chosen to reconstruct a sequence of long-periodic phases as
observed experimentally for Li/Mo(112) and Li/W(112).Comment: RevTeX 9 pages + 5 Postscript figures (included), uses newdoc.sty
(included), to be published in Surface Scienc
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