13,636 research outputs found
Divide-and-Conquer Method for Instanton Rate Theory
Ring-polymer instanton theory has been developed to simulate the quantum
dynamics of molecular systems at low temperatures. Chemical reaction rates can
be obtained by locating the dominant tunneling pathway and analyzing
fluctuations around it. In the standard method, calculating the fluctuation
terms involves the diagonalization of a large matrix, which can be unfeasible
for large systems with a high number of ring-polymer beads. Here we present a
method for computing the instanton fluctuations with a large reduction in
computational scaling. This method is applied to three reactions described by
fitted, analytic and on-the-fly ab initio potential-energy surfaces and is
shown to be numerically stable for the calculation of thermal reaction rates
even at very low temperature
Dewetting of solid films with substrate mediated evaporation
The dewetting dynamics of an ultrathin film is studied in the presence of
evaporation - or reaction - of adatoms on the substrate. KMC simulations are in
good agreement with an analytical model with diffusion, rim facetting, and
substrate sublimation. As sublimation is increased, we find a transition from
the usual dewetting regime where the front slows down with time, to a
sublimation-controlled regime where the front velocity is approximately
constant. The rim width exhibits an unexpected non-monotonous behavior, with a
maximum in time.Comment: 6 pages, 6 figure
Local electromigration model for crystal surfaces
We analyze the dynamics of crystal surfaces in the presence of
electromigration. From a phase field model with a migration force which depends
on the local geometry, we derive a step model with additional contributions in
the kinetic boundary conditions. These contributions trigger various surface
instabilities, such as step meandering, bunching and pairing on vicinal
surfaces. Experiments are discussed
Nonlinear evolution of step meander during growth of a vicinal surface with no desorption
Step meandering due to a deterministic morphological instability on vicinal
surfaces during growth is studied. We investigate nonlinear dynamics of a step
model with asymmetric step kinetics, terrace and line diffusion, by means of a
multiscale analysis. We give the detailed derivation of the highly nonlinear
evolution equation on which a brief account has been given [Pierre-Louis et.al.
PRL(98)]. Decomposing the model into driving and relaxational contributions, we
give a profound explanation to the origin of the unusual divergent scaling of
step meander ~ 1/F^{1/2} (where F is the incoming atom flux). A careful
numerical analysis indicates that a cellular structure arises where plateaus
form, as opposed to spike-like structures reported erroneously in Ref.
[Pierre-Louis et.al. PRL(98)]. As a robust feature, the amplitude of these
cells scales as t^{1/2}, regardless of the strength of the Ehrlich-Schwoebel
effect, or the presence of line diffusion. A simple ansatz allows to describe
analytically the asymptotic regime quantitatively. We show also how
sub-dominant terms from multiscale analysis account for the loss of up-down
symmetry of the cellular structure.Comment: 23 pages, 10 figures; (Submitted to EPJ B
Dewetting of a solid monolayer
We report on the dewetting of a monolayer on a solid substrate, where mass
transport occurs via surface diffusion. For a wide range of parameters, a
labyrinthine pattern of bilayer islands is formed. An irreversible regime and a
thermodynamic regime are identified. In both regimes, the velocity of a
dewetting front, the wavelength of the bilayer island pattern, and the rate of
nucleation of dewetted zones are obtained. We also point out the existence of a
scaling behavior, which is analyzed by means of a geometrical model.Comment: to be published in PhysRevLet
Energy relaxation due to magnetic impurities in mesoscopic wires: Logarithmic approach
The transport in mesoscopic wires with large applied bias voltage has
recently attracted great interest by measuring the energy distribution of the
electrons at a given point of the wire, in Saclay. In the diffusive limit with
negligible energy relaxation that shows two sharp steps at the Fermi energies
of the two contacts, which are broadened due to the energy relaxation. In some
of the experiments the broadening is reflecting an anomalous energy relaxation
rate proportional to instead of valid for Coulomb
electron-electron interaction, where is the energy transfer. Later it has
been suggested that such relaxation rate can be due to electron-electron
interaction mediated by Kondo impurities. In the present paper the latter is
systematically studied in the logarithmic approximation valid above the Kondo
temperature. In the case of large applied bias voltage Kondo resonances are
formed at the steps of the distribution function and they are narrowed by
increasing the bias. An additional Korringa energy broadening occurs for the
spins which smears the Kondo resonances, and the renormalized coupling can be
replaced by a smooth but essentially enhanced average coupling (factor of
8-10). Thus the experimental data can be described by formulas without
logarithmic Kondo corrections, but with enhanced coupling. In certain regions
of large bias, that averaged coupling depends weakly on the bias. In those
cases the distribution function depends only on the ratio of the electron
energy and the bias, showing scaling behavior. The impurity concentrations
estimated from those experiments and other dephasing experiments can be very
different, and a possible explanation considering the surface spin anisotropy
due to strong spin-orbit interaction is the subject of our earlier paper.Comment: 12 pages, RevTex
Double symmetry breaking and 2D quantum phase diagram in spin-boson systems
The quantum ground state properties of two independent chains of spins
(two-levels systems) interacting with the same bosonic field are theoretically
investigated. Each chain is coupled to a different quadrature of the field,
leading to two independent symmetry breakings for increasing values of the two
spin-boson interaction constants and . A phase diagram is
provided in the plane (,) with 4 different phases that can
be characterized by the complex bosonic coherence of the ground states and can
be manipulated via non-abelian Berry effects. In particular, when
and are both larger than two critical values, the fundamental
subspace has a four-fold degeneracy. Possible implementations in
superconducting or atomic systems are discussed
Scattering rates and lifetime of exact and boson excitons
Although excitons are not exact bosons, they are commonly treated as such
provided that their composite nature is included in effective scatterings
dressed by exchange. We here \emph{prove} that, \emph{whatever these
scatterings are}, they cannot give both the scattering rates and
the exciton lifetime , correctly: A striking factor 1/2 exists between
and the sum of 's, which originates from the
composite nature of excitons, irretrievably lost when they are bosonized. This
result, which appears as very disturbing at first, casts major doubts on
bosonization for problems dealing with \emph{interacting} excitons
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