1,174 research outputs found
The process of irreversible nucleation in multilayer growth. II. Exact results in one and two dimensions
We study irreversible dimer nucleation on top of terraces during epitaxial
growth in one and two dimensions, for all values of the step-edge barrier. The
problem is solved exactly by transforming it into a first passage problem for a
random walker in a higher-dimensional space. The spatial distribution of
nucleation events is shown to differ markedly from the mean-field estimate
except in the limit of very weak step-edge barriers. The nucleation rate is
computed exactly, including numerical prefactors.Comment: 22 pages, 10 figures. To appear in Phys. Rev.
Energy diffusion in hard-point systems
We investigate the diffusive properties of energy fluctuations in a
one-dimensional diatomic chain of hard-point particles interacting through a
square--well potential. The evolution of initially localized infinitesimal and
finite perturbations is numerically investigated for different density values.
All cases belong to the same universality class which can be also interpreted
as a Levy walk of the energy with scaling exponent 3/5. The zero-pressure limit
is nevertheless exceptional in that normal diffusion is found in tangent space
and yet anomalous diffusion with a different rate for perturbations of finite
amplitude. The different behaviour of the two classes of perturbations is
traced back to the "stable chaos" type of dynamics exhibited by this model.
Finally, the effect of an additional internal degree of freedom is
investigated, finding that it does not modify the overall scenarioComment: 16 pages, 15 figure
A new approach to partial synchronization in globally coupled rotators
We develop a formalism to analyze the behaviour of pulse--coupled identical
phase oscillators with a specific attention devoted to the onset of partial
synchronization. The method, which allows describing the dynamics both at the
microscopic and macroscopic level, is introduced in a general context, but then
the application to the dynamics of leaky integrate-and-fire (LIF) neurons is
analysed. As a result, we derive a set of delayed equations describing exactly
the LIF behaviour in the thermodynamic limit. We also investigate the weak
coupling regime by means of a perturbative analysis, which reveals that the
evolution rule reduces to a set of ordinary differential equations. Robustness
and generality of the partial synchronization regime is finally tested both by
adding noise and considering different force fields.Comment: 5 pages, 3 eps figure
The process of irreversible nucleation in multilayer growth. I. Failure of the mean-field approach
The formation of stable dimers on top of terraces during epitaxial growth is
investigated in detail. In this paper we focus on mean-field theory, the
standard approach to study nucleation. Such theory is shown to be unsuitable
for the present problem, because it is equivalent to considering adatoms as
independent diffusing particles. This leads to an overestimate of the correct
nucleation rate by a factor N, which has a direct physical meaning: in average,
a visited lattice site is visited N times by a diffusing adatom. The dependence
of N on the size of the terrace and on the strength of step-edge barriers is
derived from well known results for random walks. The spatial distribution of
nucleation events is shown to be different from the mean-field prediction, for
the same physical reason. In the following paper we develop an exact treatment
of the problem.Comment: 19 pages, 3 figures. To appear in Phys. Rev.
Irreversible nucleation in molecular beam epitaxy: From theory to experiments
Recently, the nucleation rate on top of a terrace during the irreversible
growth of a crystal surface by MBE has been determined exactly. In this paper
we go beyond the standard model usually employed to study the nucleation
process, and we analyze the qualitative and quantitative consequences of two
important additional physical ingredients: the nonuniformity of the
Ehrlich-Schwoebel barrier at the step-edge, because of the existence of kinks,
and the steering effects, due to the interaction between the atoms of the flux
and the substrate. We apply our results to typical experiments of second layer
nucleation.Comment: 11 pages. Table I corrected and one appendix added. To be published
in Phys. Rev. B (scheduled issue: 15 February 2003
Island nucleation in the presence of step edge barriers: Theory and applications
We develop a theory of nucleation on top of two-dimensional islands bordered
by steps with an additional energy barrier for descending atoms.
The theory is based on the concept of the residence time of an adatom on the
island,and yields an expression for the nucleation rate which becomes exact in
the limit of strong step edge barriers. This expression differs qualitatively
and quantitatively from that obtained using the conventional rate equation
approach to nucleation [J. Tersoff et al., Phys. Rev. Lett.72, 266 (1994)]. We
argue that rate equation theory fails because nucleation is dominated by the
rare instances when two atoms are present on the island simultaneously. The
theory is applied to two distinct problems: The onset of second layer
nucleation in submonolayer growth, and the distribution of the sizes of top
terraces of multilayer mounds under conditions of strong step edge barriers.
Application to homoepitaxial growth on Pt(111) yields the estimate eV for the additional energy barrier at CO-decorated steps.Comment: 13 pages, 3 figure
ERROR PROPAGATION IN EXTENDED CHAOTIC SYSTEMS
A strong analogy is found between the evolution of localized disturbances in
extended chaotic systems and the propagation of fronts separating different
phases. A condition for the evolution to be controlled by nonlinear mechanisms
is derived on the basis of this relationship. An approximate expression for the
nonlinear velocity is also determined by extending the concept of Lyapunov
exponent to growth rate of finite perturbations.Comment: Tex file without figures- Figures and text in post-script available
via anonymous ftp at ftp://wpts0.physik.uni-wuppertal.de/pub/torcini/jpa_le
Absence of stable collinear configurations in Ni(001)ultrathin films: canted domain structure as ground state
Brillouin light scattering (BLS) measurements were performed for (17-120)
Angstrom thick Cu/Ni/Cu/Si(001) films. A monotonic dependence of the frequency
of the uniform mode on an in-plane magnetic field H was observed both on
increasing and on decreasing H in the range (2-14) kOe, suggesting the absence
of a metastable collinear perpendicular ground state. Further investigation by
magneto-optical vector magnetometry (MOKE-VM) in an unconventional canted-field
geometry provided evidence for a domain structure where the magnetization is
canted with respect to the perpendicular to the film. Spin wave calculations
confirm the absence of stable collinear configurations.Comment: 6 pages, 3 figures (text, appendix and 1 figure added
Quantum walks of correlated particles
Quantum walks of correlated particles offer the possibility to study
large-scale quantum interference, simulate biological, chemical and physical
systems, and a route to universal quantum computation. Here we demonstrate
quantum walks of two identical photons in an array of 21 continuously
evanescently-coupled waveguides in a SiOxNy chip. We observe quantum
correlations, violating a classical limit by 76 standard deviations, and find
that they depend critically on the input state of the quantum walk. These
results open the way to a powerful approach to quantum walks using correlated
particles to encode information in an exponentially larger state space
Coherent Time Evolution and Boundary Conditions of Two-Photon Quantum Walks
Multi-photon quantum walks in integrated optics are an attractive controlled
quantum system, that can mimic less readily accessible quantum systems and
exhibit behavior that cannot in general be accurately replicated by classical
light without an exponential overhead in resources. The ability to observe time
evolution of such systems is important for characterising multi-particle
quantum dynamics---notably this includes the effects of boundary conditions for
walks in spaces of finite size. Here we demonstrate the coherent evolution of
quantum walks of two indistinguishable photons using planar arrays of 21
evanescently coupled waveguides fabricated in silicon oxynitride technology. We
compare three time evolutions, that follow closely a model assuming unitary
evolution, corresponding to three different lengths of the array---in each case
we observe quantum interference features that violate classical predictions.
The longest array includes reflecting boundary conditions.Comment: 7 pages,7 figure
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