846 research outputs found
Phase transitions in the one-dimensional frustrated quantum XY model and Josephson-junction ladders
A one-dimensional quantum version of the frustrated XY (planar rotor) model
is considered which can be physically realized as a ladder of
Josephson-junctions at half a flux quantum per plaquette. This system undergoes
a superconductor to insulator transition at zero temperature as a function of
charging energy. The critical behavior is studied using a Monte Carlo transfer
matrix applied to the path-integral representation of the model and a
finite-size-scaling analysis of data on small system sizes. Depending on the
ratio between the interchain and intrachain couplings the system can have
single or double transitions which is consistent with the prediction that its
critical behavior should be described by the two-dimensional classical XY-Ising
model.Comment: 13 pages, Revtex, J. Appl. Phys. (to appear), Inpe-las-00
Equilibrium Shape and Size of Supported Heteroepitaxial Nanoislands
We study the equilibrium shape, shape transitions and optimal size of
strained heteroepitaxial nanoislands with a two-dimensional atomistic model
using simply adjustable interatomic pair potentials. We map out the global
phase diagram as a function of substrate-adsorbate misfit and interaction. This
phase diagram reveals all the phases corresponding to different well-known
growth modes. In particular, for large enough misfits and attractive substrate
there is a Stranski-Krastanow regime, where nano-sized islands grow on top of
wetting films. We analyze the various terms contributing to the total island
energy in detail, and show how the competition between them leads to the
optimal shape and size of the islands. Finally, we also develop an analytic
interpolation formula for the various contributions to the total energy of
strained nanoislands.Comment: 9 pages, 7 figure
Phase-coherence threshold and vortex-glass state in diluted Josephson-junction arrays in a magnetic field
We study numerically the interplay of phase coherence and vortex-glass state
in two-dimensional Josephson-junction arrays with average rational values of
flux quantum per plaquette and random dilution of junctions. For ,
we find evidence of a phase coherence threshold value , below the
percolation concentration of diluted junctions , where the superconducting
transition vanishes. For the array behaves as a
zero-temperature vortex glass with nonzero linear resistance at finite
temperatures. The zero-temperature critical currents are insensitive to
variations in in the vortex glass region while they are strongly
dependent in the phase coherent region.Comment: 6 pages, 4 figures, to appear in Phys. Rev.
Dynamical transitions and sliding friction of the phase-field-crystal model with pinning
We study the nonlinear driven response and sliding friction behavior of the
phase-field-crystal (PFC) model with pinning including both thermal
fluctuations and inertial effects. The model provides a continuous description
of adsorbed layers on a substrate under the action of an external driving force
at finite temperatures, allowing for both elastic and plastic deformations. We
derive general stochastic dynamical equations for the particle and momentum
densities including both thermal fluctuations and inertial effects. The
resulting coupled equations for the PFC model are studied numerically. At
sufficiently low temperatures we find that the velocity response of an
initially pinned commensurate layer shows hysteresis with dynamical melting and
freezing transitions for increasing and decreasing applied forces at different
critical values. The main features of the nonlinear response in the PFC model
are similar to the results obtained previously with molecular dynamics
simulations of particle models for adsorbed layers.Comment: 7 pages, 8 figures, to appear in Physcial Review
Equilibrium shape and dislocation nucleation in strained epitaxial nanoislands
We study numerically the equilibrium shapes, shape transitions and
dislocation nucleation of small strained epitaxial islands with a
two-dimensional atomistic model, using simple interatomic pair potentials. We
first map out the phase diagram for the equilibrium island shapes as a function
of island size (up to N = 105 atoms) and lattice misfit with the substrate and
show that nanoscopic islands have four generic equilibrium shapes, in contrast
with predictions from the continuum theory of elasticity. For increasing
substrate-adsorbate attraction, we find islands that form on top of a finite
wetting layer as observed in Stranski-Krastanow growth. We also investigate
energy barriers and transition paths for transitions between different shapes
of the islands and for dislocation nucleation in initially coherent islands. In
particular, we find that dislocations nucleate spontaneously at the edges of
the adsorbate-substrate interface above a critical size or lattice misfit.Comment: 4 pages, 3 figures, uses wrapfig.sty and epsfig.st
Nonlinear sliding friction of adsorbed overlayers on disordered substrates
We study the response of an adsorbed monolayer on a disordered substrate
under a driving force using Brownian molecular-dynamics simulation. We find
that the sharp longitudinal and transverse depinning transitions with
hysteresis still persist in the presence of weak disorder. However, the
transitions are smeared out in the strong disorder limit. The theoretical
results here provide a natural explanation for the recent data for the
depinning transition of Kr films on gold substrate.Comment: 8 pages, 8 figs, to appear in Phys. Rev.
Current-voltage scaling of a Josephson-junction array at irrational frustration
Numerical simulations of the current-voltage characteristics of an ordered
two-dimensional Josephson junction array at an irrational flux quantum per
plaquette are presented. The results are consistent with an scaling analysis
which assumes a zero temperature vortex glass transition. The thermal
correlation length exponent characterizing this transition is found to be
significantly different from the corresponding value for vortex-glass models in
disordered two-dimensional superconductors. This leads to a current scale where
nonlinearities appear in the current-voltage characteristics decreasing with
temperature roughly as in contrast with the behavior expected
for disordered models.Comment: RevTex 3.0, 12 pages with Latex figures, to appear in Phys. Rev. B
54, Rapid. Com
Diluted Josephson-junction arrays in a magnetic field: phase coherence and vortex glass thresholds
The effects of random dilution of junctions on a two-dimensional
Josephson-junction array in a magnetic field are considered. For rational
values of the average flux quantum per plaquette , the superconducting
transition temperature vanishes, for increasing dilution, at a critical value
, while the vortex ordering remains stable up to , much
below the value corresponding to the geometric percolation threshold. For
, the array behaves as a zero-temperature vortex-glass.
Numerical results for from defect energy calculations are presented
which are consistent with this scenario.Comment: 4 pages, 4 figures, to appear in Phys. Rev.
Stress release mechanisms for Cu on Pd(111) in the submonolayer and monolayer regimes
We study the strain relaxation mechanisms of Cu on Pd(111) up to the
monolayer regime using two different computational methodologies, basin-hopping
global optimization and energy minimization with a repulsive bias potential.
Our numerical results are consistent with experimentally observed
layer-by-layer growth mode. However, we find that the structure of the Cu layer
is not fully pseudomorphic even at low coverages. Instead, the Cu adsorbates
forms fcc and hcp stacking domains, separated by partial misfit dislocations.
We also estimate the minimum energy path and energy barriers for transitions
from the ideal epitaxial state to the fcc-hcp domain pattern.Comment: 4 pages, 4 figure
Phase Diagram and Commensurate-Incommensurate Transitions in the Phase Field Crystal Model with an External Pinning Potential
We study the phase diagram and the commensurate-incommensurate transitions in
a phase field model of a two-dimensional crystal lattice in the presence of an
external pinning potential. The model allows for both elastic and plastic
deformations and provides a continuum description of lattice systems, such as
for adsorbed atomic layers or two-dimensional vortex lattices. Analytically, a
mode expansion analysis is used to determine the ground states and the
commensurate-incommensurate transitions in the model as a function of the
strength of the pinning potential and the lattice mismatch parameter. Numerical
minimization of the corresponding free energy shows good agreement with the
analytical predictions and provides details on the topological defects in the
transition region. We find that for small mismatch the transition is of
first-order, and it remains so for the largest values of mismatch studied here.
Our results are consistent with results of simulations for atomistic models of
adsorbed overlayers
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