2,149 research outputs found
Breakdown of metastable step-flow growth on vicinal surfaces induced by nucleation
We consider the growth of a vicinal crystal surface in the presence of a
step-edge barrier. For any value of the barrier strength, measured by the
length l_es, nucleation of islands on terraces is always able to destroy
asymptotically step-flow growth. The breakdown of the metastable step-flow
occurs through the formation of a mound of critical width proportional to
L_c=1/sqrt(l_es), the length associated to the linear instability of a
high-symmetry surface. The time required for the destabilization grows
exponentially with L_c. Thermal detachment from steps or islands, or a steeper
slope increase the instability time but do not modify the above picture, nor
change L_c significantly. Standard continuum theories cannot be used to
evaluate the activation energy of the critical mound and the instability time.
The dynamics of a mound can be described as a one dimensional random walk for
its height k: attaining the critical height (i.e. the critical size) means that
the probability to grow (k->k+1) becomes larger than the probability for the
mound to shrink (k->k-1). Thermal detachment induces correlations in the random
walk, otherwise absent.Comment: 10 pages. Minor changes. Accepted for publication in Phys. Rev.
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.
Lonely adatoms in space
There is a close relation between the problems of second layer nucleation in
epitaxial crystal growth and chemical surface reactions, such as hydrogen
recombination, on interstellar dust grains. In both cases standard rate
equation analysis has been found to fail because the process takes place in a
confined geometry. Using scaling arguments developed in the context of second
layer nucleation, I present a simple derivation of the hydrogen recombination
rate for small and large grains. I clarify the reasons for the failure of rate
equations for small grains, and point out a logarithmic correction to the
reaction rate when the reaction is limited by the desorption of hydrogen atoms
(the second order reaction regime)
Collective Atomic Recoil Laser as a synchronization transition
We consider here a model previously introduced to describe the collective
behavior of an ensemble of cold atoms interacting with a coherent
electromagnetic field. The atomic motion along the self-generated
spatially-periodic force field can be interpreted as the rotation of a phase
oscillator. This suggests a relationship with synchronization transitions
occurring in globally coupled rotators. In fact, we show that whenever the
field dynamics can be adiabatically eliminated, the model reduces to a
self-consistent equation for the probability distribution of the atomic
"phases". In this limit, there exists a formal equivalence with the Kuramoto
model, though with important differences in the self-consistency conditions.
Depending on the field-cavity detuning, we show that the onset of synchronized
behavior may occur through either a first- or second-order phase transition.
Furthermore, we find a secondary threshold, above which a periodic self-pulsing
regime sets in, that is immediately followed by the unlocking of the
forward-field frequency. At yet higher, but still experimentally meaningful,
input intensities, irregular, chaotic oscillations may eventually appear.
Finally, we derive a simpler model, involving only five scalar variables, which
is able to reproduce the entire phenomenology exhibited by the original model
Heralding two- and four-photon path entanglement on chip
Generating quantum entanglement is not only an important scientific endeavor,
but will be essential to realizing quantum-enhanced technologies, in
particular, quantum-enhanced measurements with precision beyond classical
limits. We investigate the heralded generation of multiphoton entanglement for
quantum metrology using a reconfigurable integrated waveguide device in which
projective measurement of auxiliary photons heralds the generation of
path-entangled states. We use four and six-photon inputs, to analyze the
heralding process of two- and four-photon NOON states-a superposition of N
photons in two paths, capable of enabling phase supersensitive measurements at
the Heisenberg limit. Realistic devices will include imperfections; as part of
the heralded state preparation, we demonstrate phase superresolution within our
chip with a state that is more robust to photon loss
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.
Fracture precursors in disordered systems
A two-dimensional lattice model with bond disorder is used to investigate the
fracture behaviour under stress-controlled conditions. Although the cumulative
energy of precursors does not diverge at the critical point, its derivative
with respect to the control parameter (reduced stress) exhibits a singular
behaviour. Our results are nevertheless compatible with previous experimental
findings, if one restricts the comparison to the (limited) range accessible in
the experiment. A power-law avalanche distribution is also found with an
exponent close to the experimental values.Comment: 4 pages, 5 figures. Submitted to Europhysics Letter
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