7,969 research outputs found
Long-wavelength density fluctuations as nucleation precursors
Recent theories of nucleation that go beyond Classical Nucleation Theory
predict that diffusion-limited nucleation of both liquid droplets and of
crystals from a low-density vapor (or weak solution) begins with
long-wavelength density fluctuations. This means that in the early stages of
nucleation, 'clusters' can have low density but large spatial extent, which is
at odds with the classical picture of arbitrarily small clusters of the
condensed phase. We present the results of kinetic Monte Carlo simulations
using Forward Flux Sampling to show that these predictions are confirmed:
namely that on average, nucleation begins in the presence of low-amplitude, but
spatially extended density fluctuations thus confirming a significant
prediction of the non-classical theory
Solvent-mediated interactions between nanostructures: from water to Lennard-Jones liquid
Solvent-mediated interactions emerge from complex mechanisms that depend on
the solute structure, its wetting properties and the nature of the liquid.
While numerous studies have focused on the two first influences, here, we
compare results from water and Lennard-Jones liquid in order to reveal to what
extent solvent-mediated interactions are universal with respect to the nature
of the liquid. Besides the influence of the liquid, results were obtained with
classical density functional theory and brute-force molecular dynamics
simulations which allows us to contrast these two numerical techniques
Lattice induced crystallization of nanodroplets: the role of finite-size effects and substrate properties in controlling polymorphism
Targeting specific technological applications requires the control of
nanoparticle properties, especially the crystalline polymorph. Freezing a
nanodroplet deposited on a solid substrate leads to the formation of
crystalline structures. We study the inherent mechanisms underlying this
general phenomenon by means of molecular dynamics simulations. Our work shows
that different crystal structures can be selected by finely tuning the solid
substrate lattice parameter. Indeed, while for our system, face-centered cubic
is usually the most preponderant structure, the growth of two distinct
polymorphs, hexagonal centered packing and body-centered cubic, was also
observed even when the solid substrate was face-centered cubic. Finally, we
also demonstrated that the growth of hexagonal centered packing is conditioned
by the appearance of large enough body-centered cubic clusters thus suggesting
the presence of a cross-nucleation pathway. Our results provide insights into
the impact of nanoscale effects and solid substrate properties towards the
growth of polymorphic nanomaterials
Finite-Size Scaling Exponents of the Lipkin-Meshkov-Glick Model
We study the ground state properties of the critical Lipkin-Meshkov-Glick
model. Using the Holstein-Primakoff boson representation, and the continuous
unitary transformation technique, we compute explicitly the finite-size scaling
exponents for the energy gap, the ground state energy, the magnetization, and
the spin-spin correlation functions. Finally, we discuss the behavior of the
two-spin entanglement in the vicinity of the phase transition.Comment: 4 pages, published versio
Microscopic prediction of skyrmion lattice state in clean interface superconductors
When an in-plane field is applied to a clean interface superconductor, a
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like phase is stabilized. This phase
has a symmetry and, in principle, this
symmetry allows for flux carrying topological excitations different from
Abrikosov vortices (which are the simplest defects associated with maps). However, in practice, largely due to electromagnetic and other
intercomponent interactions, such topological excitations are very rare in
superconducting systems. Here we demonstrate that a realistic microscopic
theory for interface superconductors, such as SrTiO/LaAlO, predicts an
unconventional magnetic response where the flux-carrying objects are skyrmions,
characterized by homotopy invariants of maps. Additionally, we
show that this microscopic theory predicts that stable fractional vortices form
near the boundary of these superconductors. It also predicts the appearance of
type-1.5 superconductivity for some range of parameters. Central to these
results is the assumption that the Rashba spin orbit coupling is much larger
than the superconducting gap.Comment: Replaced with a version in print in Phys. Rev. B; Improved and
extended as compared to the first version; 10 pages, 6 figure
Compact and Flexible Basis Functions for Quantum Monte Carlo Calculations
Molecular calculations in quantum Monte Carlo frequently employ a mixed basis
consisting of contracted and primitive Gaussian functions. While standard basis
sets of varying size and accuracy are available in the literature, we
demonstrate that reoptimizing the primitive function exponents within quantum
Monte Carlo yields more compact basis sets for a given accuracy. Particularly
large gains are achieved for highly excited states. For calculations requiring
non-diverging pseudopotentials, we introduce Gauss-Slater basis functions that
behave as Gaussians at short distances and Slaters at long distances. These
basis functions further improve the energy and fluctuations of the local energy
for a given basis size. Gains achieved by exponent optimization and
Gauss-Slater basis use are exemplified by calculations for the ground state of
carbon, the lowest lying excited states of carbon with , ,
, symmetries, carbon dimer, and naphthalene. Basis size
reduction enables quantum Monte Carlo treatment of larger molecules at high
accuracy.Comment: 8 Pages, 2 Figures, 9 Table
Intermediate Mass Black Holes and Nearby Dark Matter Point Sources: A Critical Reassessment
The proposal of a galactic population of intermediate mass black holes
(IMBHs), forming dark matter (DM) ``mini-spikes'' around them, has received
considerable attention in recent years. In fact, leading in some scenarios to
large annihilation fluxes in gamma rays, neutrinos and charged cosmic rays,
these objects are sometimes quoted as one of the most promising targets for
indirect DM searches. In this letter, we apply a detailed statistical analysis
to point out that the existing EGRET data already place very stringent limits
on those scenarios, making it rather unlikely that any of these objects will be
observed with, e.g., the Fermi/GLAST satellite or upcoming Air Cherenkov
telescopes. We also demonstrate that prospects for observing signals in
neutrinos or charged cosmic rays seem even worse. Finally, we address the
question of whether the excess in the cosmic ray positron/electron flux
recently reported by PAMELA/ATIC could be due to a nearby DM point source like
a DM clump or mini-spike; gamma-ray bounds, as well as the recently released
Fermi cosmic ray electron and positron data, again exclude such a possibility
for conventional DM candidates, and strongly constrain it for DM purely
annihilating into light leptons.Comment: 4 pages revtex4, 4 figures. Improved analysis and discussion, added
constraints from Fermi data, corrected figures and updated reference
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