5,211 research outputs found
Phase field theory of interfaces and crystal nucleation in a eutectic system of fcc structure: II. Nucleation in the metastable liquid immiscibility region
The official version of this Article can be accessed from the link below - Copyright @ 2007 American Institute of PhysicsIn the second part of our paper, we address crystal nucleation in the metastable liquid miscibility region of eutectic systems that is always present, though experimentally often inaccessible. While this situation resembles the one seen in single component crystal nucleation in the presence of a metastable vapor-liquid critical point addressed in previous works, it is more complex because of the fact that here two crystal phases of significantly different compositions may nucleate. Accordingly, at a fixed temperature below the critical point, six different types of nuclei may form: two liquid-liquid nuclei: two solid-liquid nuclei; and two types of composite nuclei, in which the crystalline core has a liquid "skirt," whose composition falls in between the compositions of the solid and the initial liquid phases, in addition to nuclei with concentric alternating composition shells of prohibitively high free energy. We discuss crystalline phase selection via exploring/identifying the possible pathways for crystal nucleation.This work has been supported by the Hungarian Academy of Sciences under contract No. OTKA-K-62588 and by the ESA PECS Nos. 98021 and 98043
Vortex Fractionalization in a Josephson Ladder
We show numerically that, in a Josephson ladder with periodic boundary
conditions and subject to a suitable transverse magnetic field, a vortex
excitation can spontaneously break up into two or more fractional excitations.
If the ladder has N plaquettes, and N is divisible by an integer q, then in an
applied transverse field of 1/q flux quanta per plaquette the ground state is a
regular pattern of one fluxon every q plaquettes. When one additional fluxon is
added to the ladder, it breaks up into q fractional fluxons, each carrying 1/q
units of vorticity. The fractional fluxons are basically walls between
different domains of the ground state of the underlying 1/q lattice. The
fractional fluxons are all depinned at the same applied current and move as a
unit. For certain applied fields and ladder lengths, we show that there are
isolated fractional fluxons. It is shown that the fractional fluxons would
produce a time-averaged voltage related in a characteristic way to the ac
voltage frequency.Comment: 13 Figures. 10 page
Linear scaling electronic structure calculations and accurate sampling with noisy forces
Numerical simulations based on electronic structure calculations are finding
ever growing applications in many areas of physics. A major limiting factor is
however the cubic scaling of the algorithms used. Building on previous work [F.
R. Krajewski and M. Parrinello, Phys.Rev. B71, 233105 (2005)] we introduce a
novel statistical method for evaluating the inter-atomic forces which scales
linearly with system size and is applicable also to metals. The method is based
on exact decomposition of the fermionic determinant and on a mapping onto a
field theoretical expression. We solve exactly the problem of sampling the
Boltzmann distribution with noisy forces. This novel approach can be used in
such diverse fields as quantum chromodynamics, quantum Monte Carlo or colloidal
physics.Comment: 5 pages, 2 figure
Stochastic Mean-Field Theory: Method and Application to the Disordered Bose-Hubbard Model at Finite Temperature and Speckle Disorder
We discuss the stochastic mean-field theory (SMFT) method which is a new
approach for describing disordered Bose systems in the thermodynamic limit
including localization and dimensional effects. We explicate the method in
detail and apply it to the disordered Bose-Hubbard model at finite temperature,
with on-site box disorder, as well as experimentally relevant unbounded speckle
disorder. We find that disorder-induced condensation and reentrant behavior at
constant filling are only possible at low temperatures, beyond the reach of
current experiments [Pasienski et al., arXiv:0908.1182]. Including off-diagonal
hopping disorder as well, we investigate its effect on the phase diagram in
addition to pure on-site disorder. To make contact to present experiments on a
quantitative level, we also combine SMFT with an LDA approach and obtain the
condensate fraction in the presence of an external trapping potential.Comment: 19 pages, 15 figures. Extended definition of Bose glass phase, taking
collective excitations into account. 1 figure added, extended and updated
reference
Nonuniqueness in spin-density-functional theory on lattices
In electronic many-particle systems, the mapping between densities and spin
magnetizations, {n(r), m(r)}, and potentials and magnetic fields, {v(r), B(r)},
is known to be nonunique, which has fundamental and practical implications for
spin-density-functional theory (SDFT). This paper studies the nonuniqueness
(NU) in SDFT on arbitrary lattices. Two new, non-trivial cases are discovered,
here called local saturation and global noncollinear NU, and their properties
are discussed and illustrated. In the continuum limit, only some well-known
special cases of NU survive.Comment: 4 pages, 1 figur
Wavelet transforms in a critical interface model for Barkhausen noise
We discuss the application of wavelet transforms to a critical interface
model, which is known to provide a good description of Barkhausen noise in soft
ferromagnets. The two-dimensional version of the model (one-dimensional
interface) is considered, mainly in the adiabatic limit of very slow driving.
On length scales shorter than a crossover length (which grows with the strength
of surface tension), the effective interface roughness exponent is
, close to the expected value for the universality class of the
quenched Edwards-Wilkinson model. We find that the waiting times between
avalanches are fully uncorrelated, as the wavelet transform of their
autocorrelations scales as white noise. Similarly, detrended size-size
correlations give a white-noise wavelet transform. Consideration of finite
driving rates, still deep within the intermittent regime, shows the wavelet
transform of correlations scaling as for intermediate frequencies.
This behavior is ascribed to intra-avalanche correlations.Comment: RevTeX, 10 pages, 9 .eps figures; Physical Review E, to be publishe
The effect of symmetry breaking on the dynamics near a structurally stable heteroclinic cycle between equilibria and a periodic orbit
The effect of small forced symmetry breaking on the dynamics near a structurally stable heteroclinic
cycle connecting two equilibria and a periodic orbit is investigated. This type of system is known
to exhibit complicated, possibly chaotic dynamics including irregular switching of sign of various
phase space variables, but details of the mechanisms underlying the complicated dynamics have
not previously been investigated. We identify global bifurcations that induce the onset of chaotic
dynamics and switching near a heteroclinic cycle of this type, and by construction and analysis
of approximate return maps, locate the global bifurcations in parameter space. We find there is a
threshold in the size of certain symmetry-breaking terms below which there can be no persistent
switching. Our results are illustrated by a numerical example
Quantitative modeling of \textit{in situ} x-ray reflectivity during organic molecule thin film growth
Synchrotron-based x-ray reflectivity is increasingly employed as an
\textit{in situ} probe of surface morphology during thin film growth, but
complete interpretation of the results requires modeling the growth process.
Many models have been developed and employed for this purpose, yet no detailed,
comparative studies of their scope and accuracy exists in the literature. Using
experimental data obtained from hyperthermal deposition of pentane and
diindenoperylene (DIP) on SiO, we compare and contrast three such models,
both with each other and with detailed characterization of the surface
morphology using ex-situ atomic force microscopy (AFM). These two systems each
exhibit particular phenomena of broader interest: pentacene/SiO exhibits a
rapid transition from rough to smooth growth. DIP/SiO, under the conditions
employed here, exhibits growth rate acceleration due to a different sticking
probability between the substrate and film. In general, \textit{independent of
which model is used}, we find good agreement between the surface morphology
obtained from fits to the \insitu x-ray data with the actual morphology at
early times. This agreement deteriorates at later time, once the root-mean
squared (rms) film roughness exceeds about 1 ML. A second observation is that,
because layer coverages are under-determined by the evolution of a single point
on the reflectivity curve, we find that the best fits to reflectivity data ---
corresponding to the lowest values of --- do not necessarily yield
the best agreement between simulated and measured surface morphologies.
Instead, it appears critical that the model reproduce all local extrema in the
data. In addition to showing that layer morphologies can be extracted from a
minimal set of data, the methodology established here provides a basis for
improving models of multilayer growth by comparison to real systems.Comment: 34 pages (double-spaced, including figures and references), 10
figures, 3 appendice
Emission spectrum of quasi-resonant laterally coupled quantum dots
We calculate the emission spectrum of neutral and charged excitons in a pair
of laterally coupled InGaAs quantum dots with nearly degenerate energy levels.
As the interdot distance decreases, a number of changes take place in the
emission spectrum which can be used as indications of molecular coupling. These
signatures ensue from the stronger tunnel-coupling of trions as compared to
that of neutral excitons.Comment: 7 pages, 7 figure
Groundstate and Collective Modes of a Spin-Polarized Dipolar Bose-Einstein Condensate in a Harmonic Trap
We report new results for the Thomas-Fermi groundstate and the quadrupolar
modes of density oscillations of a spin- polarized dipolar interacting
Bose-Einstein condensate for the case when the external magnetic field is not
orientated parallel to a principal axis of a harmonic anisotropic trap.Comment: Final version, published in Physical Review
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