11,280 research outputs found
Concentration profiles for fine and coarse sediments suspended by waves over ripples: An analytical study with the 1-DV gradient diffusion model
Field and laboratory measurements of suspended sediments over wave ripples
show, for time-averaged concentration profiles in semi-log plots, a contrast
between upward convex profiles for fine sand and upward concave profiles for
coarse sand. Careful examination of experimental data for coarse sand shows a
near-bed upward convex profile beneath the main upward concave profile.
Available models fail to predict these two profiles for coarse sediments. The
1-DV gradient diffusion model predicts the main upward concave profile for
coarse sediments thanks to a suitable (y)-function (where is the
inverse of the turbulent Schmidt number and y is the distance from the bed). In
order to predict the near-bed upward convex profile, an additional parameter
{\alpha} is needed. This parameter could be related to settling velocity
( equal to inverse of dimensionless settling velocity) or to convective
sediment entrainment process. The profiles are interpreted by a relation
between second derivative of the logarithm of concentration and derivative of
the product between sediment diffusivity and
A review of Monte Carlo simulations of polymers with PERM
In this review, we describe applications of the pruned-enriched Rosenbluth
method (PERM), a sequential Monte Carlo algorithm with resampling, to various
problems in polymer physics. PERM produces samples according to any given
prescribed weight distribution, by growing configurations step by step with
controlled bias, and correcting "bad" configurations by "population control".
The latter is implemented, in contrast to other population based algorithms
like e.g. genetic algorithms, by depth-first recursion which avoids storing all
members of the population at the same time in computer memory. The problems we
discuss all concern single polymers (with one exception), but under various
conditions: Homopolymers in good solvents and at the point, semi-stiff
polymers, polymers in confining geometries, stretched polymers undergoing a
forced globule-linear transition, star polymers, bottle brushes, lattice
animals as a model for randomly branched polymers, DNA melting, and finally --
as the only system at low temperatures, lattice heteropolymers as simple models
for protein folding. PERM is for some of these problems the method of choice,
but it can also fail. We discuss how to recognize when a result is reliable,
and we discuss also some types of bias that can be crucial in guiding the
growth into the right directions.Comment: 29 pages, 26 figures, to be published in J. Stat. Phys. (2011
Shape-dependent universality in percolation
The shape-dependent universality of the excess percolation cluster number and
cross-configuration probability on a torus is discussed. Besides the aspect
ratio of the torus, the universality class depends upon the twist in the
periodic boundary conditions, which for example are generally introduced when
triangular lattices are used in simulations.Comment: 11 pages, 3 figures, to be published in Physica
Atomic-scale control of magnetic anisotropy via novel spin-orbit coupling effect in La2/3Sr1/3MnO3/SrIrO3 superlattices
Magnetic anisotropy (MA) is one of the most important material properties for
modern spintronic devices. Conventional manipulation of the intrinsic MA, i.e.
magnetocrystalline anisotropy (MCA), typically depends upon crystal symmetry.
Extrinsic control over the MA is usually achieved by introducing shape
anisotropy or exchange bias from another magnetically ordered material. Here we
demonstrate a pathway to manipulate MA of 3d transition metal oxides (TMOs) by
digitally inserting non-magnetic 5d TMOs with pronounced spin-orbit coupling
(SOC). High quality superlattices comprised of ferromagnetic La2/3Sr1/3MnO3
(LSMO) and paramagnetic SrIrO3 (SIO) are synthesized with the precise control
of thickness at atomic scale. Magnetic easy axis reorientation is observed by
controlling the dimensionality of SIO, mediated through the emergence of a
novel spin-orbit state within the nominally paramagnetic SIO.Comment: Proceedings of the National Academy of Sciences, May 201
Chiral Perturbation Theory, Large-N_c and the eta' Mass
Using chiral perturbation theory and the large- expansion, we obtain
expressions for the mass and mixing in terms of
low-energy chiral Lagrangian parameters. This is accomplished through an
intermediate step of `matching' the topological susceptibility in the
large- and chiral Lagrangian descriptions. By inserting the values of
well-measured parameters we obtain predictions involving the the second order
parameters and . The prediction for is quite restrictive
even after allowing for corrections.Comment: 11 pages, Latex, 3 figures using eps
Motion of Contact Line of a Crystal Over the Edge of Solid Mask in Epitaxial Lateral Overgrowth
Mathematical model that allows for direct tracking of the homoepitaxial
crystal growth out of the window etched in the solid, pre-deposited layer on
the substrate is described. The growth is governed by the normal (to the
crystal-vapor interface) flux from the vapor phase and by the interface
diffusion. The model accounts for possibly inhomogeneous energy of the mask
surface and for strong anisotropies of crystal-vapor interfacial energy and
kinetic mobility. Results demonstrate that the motion of the crystal-mask
contact line slows down abruptly as radius of curvature of the mask edge
approaches zero. Numerical procedure is suggested to overcome difficulties
associated with ill-posedness of the evolution problem for the interface with
strong energy anisotropy.
Keywords: Thin films, epitaxy, MOCVD, surface diffusion, interface dynamics,
contact lines, rough surfaces, wetting, regularization of ill-posed evolution
problems.Comment: 21 pages, 11 figures; to appear in Computational Materials Scienc
Connection between low energy effective Hamiltonians and energy level statistics
We study the level statistics of a non-integrable one dimensional interacting
fermionic system characterized by the GOE distribution. We calculate
numerically on a finite size system the level spacing distribution and
the Dyson-Mehta correlation. We observe that its low energy spectrum
follows rather the Poisson distribution, characteristic of an integrable
system, consistent with the fact that the low energy excitations of this system
are described by the Luttinger model. We propose this Random Matrix Theory
analysis as a probe for the existence and integrability of low energy effective
Hamiltonians for strongly correlated systems.Comment: REVTEX, 5 postscript figures at the end of the fil
Superconductivity and single crystal growth of Ni0:05TaS2
Superconductivity was discovered in a Ni0:05TaS2 single crystal. A Ni0:05TaS2
single crystal was successfully grown via the NaCl/KCl flux method. The
obtained lattice constant c of Ni0:05TaS2 is 1.1999 nm, which is significantly
smaller than that of 2H-TaS2 (1.208 nm). Electrical resistivity and
magnetization measurements reveal that the superconductivity transition
temperature of Ni0:05TaS2 is enhanced from 0.8 K (2H-TaS2) to 3.9 K. The
charge-density-wave transition of the matrix compound 2H-TaS2 is suppressed in
Ni0:05TaS2. The success of Ni0:05TaS2 single crystal growth via a NaCl/KCl flux
demonstrates that NaCl/KCl flux method will be a feasible method for single
crystal growth of the layered transition metal dichalcogenides.Comment: 13pages, 6 figures, Published in SS
Scaling of Star Polymers with one to 80 Arms
We present large statistics simulations of 3-dimensional star polymers with
up to arms, and with up to 4000 monomers per arm for small values of
. They were done for the Domb-Joyce model on the simple cubic lattice. This
is a model with soft core exclusion which allows multiple occupancy of sites
but punishes each same-site pair of monomers with a Boltzmann factor . We
use this to allow all arms to be attached at the central site, and we use the
`magic' value to minimize corrections to scaling. The simulations are
made with a very efficient chain growth algorithm with resampling, PERM,
modified to allow simultaneous growth of all arms. This allows us to measure
not only the swelling (as observed from the center-to-end distances), but also
the partition sum. The latter gives very precise estimates of the critical
exponents . For completeness we made also extensive simulations of
linear (unbranched) polymers which give the best estimates for the exponent
.Comment: 7 pages, 7 figure
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