7,199 research outputs found
On the force-velocity relationship of a bundle of rigid bio-filaments
In various cellular processes, bio-filaments like F-actin and F-tubulin are able to exploit chemical energy associated with polymerization to perform mechanical work against an obstacle loaded with an external force. The force-velocity relationship quantitatively summarizes the nature of this process. By a stochastic dynamical model, we give, together with the evolution of a staggered bundle of Nfrigid living filaments facing a loaded wall, the corresponding force-velocity relationship. We compute the evolution of the model in the infinite wall diffusion limit and in supercritical conditions (monomer density reduced by critical density ρ^1>1), and we show that this solution remains valid for moderate non-zero values of the ratio between the wall diffusion and the chemical time scales. We consider two classical protocols: the bundle is opposed either to a constant load or to an optical trap setup, characterized by a harmonic restoring force. The constant load case leads, for each F value, to a stationary velocity Vstat(F;Nf,ρ^1) after a relaxation with characteristic time τmicro(F). When the bundle (initially taken as an assembly of filament seeds) is subjected to a harmonic restoring force (optical trap load), the bundle elongates and the load increases up to stalling over a characteristic time τOT. Extracted from this single experiment, the force-velocity VOT(F;Nf,ρ^1) curve is found to coincide with Vstat(F;Nf,ρ^1), except at low loads. We show that this result follows from the adiabatic separation between τmicroand τOT, i.e., τmicro≈ τOT
Quantum relaxation and metastability of lattice bosons with cavity-induced long-range interactions
The coupling of cold atoms to the radiation field within a high-finesse
optical resonator, an optical cavity, induces long-range interactions which can
compete with an underlying optical lattice. The interplay between short- and
long-range interactions gives rise to new phases of matter including
supersolidity (SS) and density waves (DW), and interesting quantum dynamics.
Here it is shown that for hard-core bosons in one dimension the ground state
phase diagram and the quantum relaxation after sudden quenches can be
calculated exactly in the thermodynamic limit. Remanent DW order is observed
for quenches from a DW ground state into the superfluid (SF) phase below a
dynamical transition line. After sufficiently strong SF to DW quenches beyond a
static metastability line DW order emerges on top of remanent SF order, giving
rise to a dynamically generated supersolid state.Comment: 6 pages, 5 figure
AREPO-RT: Radiation hydrodynamics on a moving mesh
We introduce AREPO-RT, a novel radiation hydrodynamic (RHD) solver for the
unstructured moving-mesh code AREPO. Our method solves the moment-based
radiative transfer equations using the M1 closure relation. We achieve second
order convergence by using a slope limited linear spatial extrapolation and a
first order time prediction step to obtain the values of the primitive
variables on both sides of the cell interface. A Harten-Lax-Van Leer flux
function, suitably modified for moving meshes, is then used to solve the
Riemann problem at the interface. The implementation is fully conservative and
compatible with the individual timestepping scheme of AREPO. It incorporates
atomic Hydrogen (H) and Helium (He) thermochemistry, which is used to couple
the ultra-violet (UV) radiation field to the gas. Additionally, infrared
radiation is coupled to the gas under the assumption of local thermodynamic
equilibrium between the gas and the dust. We successfully apply our code to a
large number of test problems, including applications such as the expansion of
regions, radiation pressure driven outflows and the levitation
of optically thick layer of gas by trapped IR radiation. The new implementation
is suitable for studying various important astrophysical phenomena, such as the
effect of radiative feedback in driving galactic scale outflows, radiation
driven dusty winds in high redshift quasars, or simulating the reionisation
history of the Universe in a self consistent manner.Comment: v2, accepted for publication in MNRAS, changed to a Strang split
scheme to achieve second order convergenc
Is null-point reconnection important for solar flux emergence?
The role of null-point reconnection in a 3D numerical MHD model of solar
emerging flux is investigated. The model consists of a twisted magnetic flux
tube rising through a stratified convection zone and atmosphere to interact and
reconnect with a horizontal overlying magnetic field in the atmosphere. Null
points appear as the reconnection begins and persist throughout the rest of the
emergence, where they can be found mostly in the model photosphere and
transition region, forming two loose clusters on either side of the emerging
flux tube. Up to 26 nulls are present at any one time, and tracking in time
shows that there is a total of 305 overall, despite the initial simplicity of
the magnetic field configuration. We find evidence for the reality of the nulls
in terms of their methods of creation and destruction, their balance of signs,
their long lifetimes, and their geometrical stability. We then show that due to
the low parallel electric fields associated with the nulls, null-point
reconnection is not the main type of magnetic reconnection involved in the
interaction of the newly emerged flux with the overlying field. However, the
large number of nulls implies that the topological structure of the magnetic
field must be very complex and the importance of reconnection along separators
or separatrix surfaces for flux emergence cannot be ruled out.Comment: 26 pages, 12 figures. Added one referenc
Magnetic excitations in coupled Haldane spin chains near the quantum critical point
Two quasi-1-dimensional S=1 quantum antiferromagnetic materials, PbNi2V2O8
and SrNi2V2O8, are studied by inelastic neutron scattering on powder samples.
While magnetic interactions in the two systems are found to be very similar,
subtle differences in inter-chain interaction strengths and magnetic anisotropy
are detected. The latter are shown to be responsible for qualitatively
different ground state properties: magnetic long-range order in SrNi2V2O8 and
disordered ``spin liquid'' Haldane-gap state in PbNi2V2O8.Comment: 15 figures, Figs. 5,9, and 10 in color. Some figures in JPEG format.
Complete PostScript and PDF available from
http://papillon.phy.bnl.gov/publicat.ht
Fast, accurate solutions for curvilinear earthquake faults and anelastic strain
Imaging the anelastic deformation within the crust and lithosphere using
surface geophysical data remains a significant challenge in part due to the
wide range of physical processes operating at different depths and to various
levels of localization that they embody. Models of Earth's elastic properties
from seismological imaging combined with geodetic modeling may form the basis
of comprehensive rheological models of Earth's interior. However, representing
the structural complexity of faults and shear zones in numerical models of
deformation still constitutes a major difficulty. Here, we present numerical
techniques for high-precision models of deformation and stress around both
curvilinear faults and volumes undergoing anelastic (irreversible) strain in a
heterogenous elastic half-space. To that end, we enhance the software Gamra to
model triangular and rectangular fault patches and tetrahedral and cuboidal
strain volumes. This affords a means of rapid and accurate calculations of
elasto-static Green's functions for localized (e.g., faulting) and distributed
(e.g., viscoelastic) deformation in Earth's crust and lithosphere. We
demonstrate the correctness of the method with analytic tests, and we
illustrate its practical performance by solving for coseismic and postseismic
deformation following the 2015 Mw 7.8 Gorkha, Nepal earthquake to extremely
high precision
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