22,913 research outputs found
Spatially hybrid computations for streamer discharges: II. Fully 3D simulations
We recently have presented first physical predictions of a spatially hybrid
model that follows the evolution of a negative streamer discharge in full three
spatial dimensions; our spatially hybrid model couples a particle model in the
high field region ahead of the streamer with a fluid model in the streamer
interior where electron densities are high and fields are low. Therefore the
model is computationally efficient, while it also follows the dynamics of
single electrons including their possible run-away. Here we describe the
technical details of our computations, and present the next step in a
systematic development of the simulation code. First, new sets of transport
coefficients and reaction rates are obtained from particle swarm simulations in
air, nitrogen, oxygen and argon. These coefficients are implemented in an
extended fluid model to make the fluid approximation as consistent as possible
with the particle model, and to avoid discontinuities at the interface between
fluid and particle regions. Then two splitting methods are introduced and
compared for the location and motion of the fluid-particle-interface in three
spatial dimensions. Finally, we present first results of the 3D spatially
hybrid model for a negative streamer in air
Localization and Coherence in Nonintegrable Systems
We study the irreversible dynamics of nonlinear, nonintegrable Hamiltonian
oscillator chains approaching their statistical asympotic states. In systems
constrained by more than one conserved quantity, the partitioning of the
conserved quantities leads naturally to localized and coherent structures. If
the phase space is compact, the final equilibrium state is governed by entropy
maximization and the final coherent structures are stable lumps. In systems
where the phase space is not compact, the coherent structures can be collapses
represented in phase space by a heteroclinic connection to infinity.Comment: 41 pages, 15 figure
Laws of crack motion and phase-field models of fracture
Recently proposed phase-field models offer self-consistent descriptions of
brittle fracture. Here, we analyze these theories in the quasistatic regime of
crack propagation. We show how to derive the laws of crack motion either by
using solvability conditions in a perturbative treatment for slight departure
from the Griffith threshold, or by generalizing the Eshelby tensor to
phase-field models. The analysis provides a simple physical interpretation of
the second component of the classic Eshelby integral in the limit of vanishing
crack propagation velocity: it gives the elastic torque on the crack tip that
is needed to balance the Herring torque arising from the anisotropic interface
energy. This force balance condition reduces in this limit to the principle of
local symmetry in isotropic media and to the principle of maximum energy
release rate for smooth curvilinear cracks in anisotropic media. It can also be
interpreted physically in this limit based on energetic considerations in the
traditional framework of continuum fracture mechanics, in support of its
general validity for real systems beyond the scope of phase-field models.
Analytical predictions of crack paths in anisotropic media are validated by
numerical simulations. Simulations also show that these predictions hold even
if the phase-field dynamics is modified to make the failure process
irreversible. In addition, the role of dissipative forces on the process zone
scale as well as the extension of the results to motion of planar cracks under
pure antiplane shear are discussed
Transport regimes of cold gases in a two-dimensional anisotropic disorder
We numerically study the dynamics of cold atoms in a two-dimensional
disordered potential. We consider an anisotropic speckle potential and focus on
the classical regime, which is relevant to some recent experiments. First, we
study the behavior of particles with a fixed energy and identify different
transport regimes. For low energy, the particles are classically localized due
to the absence of a percolating cluster. For high energy, the particles undergo
normal diffusion and we show that the diffusion constants scale algebraically
with the particle energy, with an anisotropy factor which significantly differs
from that of the disordered potential. For intermediate energy, we find a
transient sub-diffusive regime, which is relevant to the time scale of typical
experiments. Second, we study the behavior of a cold-atomic gas with an
arbitrary energy distribution, using the above results as a groundwork. We show
that the density profile of the atomic cloud in the diffusion regime is
strongly peaked and, in particular, that it is not Gaussian. Its behavior at
large distances allows us to extract the energy-dependent diffusion constants
from experimental density distributions. For a thermal cloud released into the
disordered potential, we show that our numerical predictions are in agreement
with experimental findings. Not only does this work give insights to recent
experimental results, but it may also serve interpretation of future
experiments searching for deviation from classical diffusion and traces of
Anderson localization.Comment: 19 pages, 16 figure
An orbitally derived single-atom magnetic memory
A single magnetic atom on a surface epitomizes the scaling limit for magnetic
information storage. Indeed, recent work has shown that individual atomic spins
can exhibit magnetic remanence and be read out with spin-based methods,
demonstrating the fundamental requirements for magnetic memory. However, atomic
spin memory has been only realized on thin insulating surfaces to date,
removing potential tunability via electronic gating or distance-dependent
exchange-driven magnetic coupling. Here, we show a novel mechanism for
single-atom magnetic information storage based on bistability in the orbital
population, or so-called valency, of an individual Co atom on semiconducting
black phosphorus (BP). Distance-dependent screening from the BP surface
stabilizes the two distinct valencies and enables us to electronically
manipulate the relative orbital population, total magnetic moment and spatial
charge density of an individual magnetic atom without a spin-dependent readout
mechanism. Furthermore, we show that the strongly anisotropic wavefunction can
be used to locally tailor the switching dynamics between the two valencies.
This orbital memory derives stability from the energetic barrier to atomic
relaxation and demonstrates the potential for high-temperature single-atom
information storage
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