474 research outputs found
Stability of pulse-like earthquake ruptures
Pulse-like ruptures arise spontaneously in many elastodynamic rupture
simulations and seem to be the dominant rupture mode along crustal faults.
Pulse-like ruptures propagating under steady-state conditions can be
efficiently analysed theoretically, but it remains unclear how they can arise
and how they evolve if perturbed. Using thermal pressurisation as a
representative constitutive law, we conduct elastodynamic simulations of
pulse-like ruptures and determine the spatio-temporal evolution of slip, slip
rate and pulse width perturbations induced by infinitesimal perturbations in
background stress. These simulations indicate that steady-state pulses driven
by thermal pressurisation are unstable. If the initial stress perturbation is
negative, ruptures stop; conversely, if the perturbation is positive, ruptures
grow and transition to either self-similar pulses (at low background stress) or
expanding cracks (at elevated background stress). Based on a dynamic
dislocation model, we develop an elastodynamic equation of motion for slip
pulses, and demonstrate that steady-state slip pulses are unstable if their
accrued slip is a decreasing function of the uniform background stress
. This condition is satisfied by slip pulses driven by thermal
pressurisation. The equation of motion also predicts quantitatively the growth
rate of perturbations, and provides a generic tool to analyse the propagation
of slip pulses. The unstable character of steady-state slip pulses implies that
this rupture mode is a key one determining the minimum stress conditions for
sustainable ruptures along faults, i.e., their ``strength''. Furthermore, slip
pulse instabilities can produce a remarkable complexity of rupture dynamics,
even under uniform background stress conditions and material properties
Number of Common Sites Visited by N Random Walkers
We compute analytically the mean number of common sites, W_N(t), visited by N
independent random walkers each of length t and all starting at the origin at
t=0 in d dimensions. We show that in the (N-d) plane, there are three distinct
regimes for the asymptotic large t growth of W_N(t). These three regimes are
separated by two critical lines d=2 and d=d_c(N)=2N/(N-1) in the (N-d) plane.
For d<2, W_N(t)\sim t^{d/2} for large t (the N dependence is only in the
prefactor). For 2<d<d_c(N), W_N(t)\sim t^{\nu} where the exponent \nu=
N-d(N-1)/2 varies with N and d. For d>d_c(N), W_N(t) approaches a constant as
t\to \infty. Exactly at the critical dimensions there are logaritmic
corrections: for d=2, we get W_N(t)\sim t/[\ln t]^N, while for d=d_c(N),
W_N(t)\sim \ln t for large t. Our analytical predictions are verified in
numerical simulations.Comment: 5 pages, 3 .eps figures include
Structure of the silicon vacancy in 6H-SiC after annealing identified as the carbon vacancy–carbon antisite pair
We investigated radiation-induced defects in neutron-irradiated and subsequently annealed 6H-silicon carbide (SiC) with electron paramagnetic resonance (EPR), the magnetic circular dichroism of the absorption (MCDA), and MCDA-detected EPR (MCDA-EPR). In samples annealed beyond the annealing temperature of the isolated silicon vacancy we observed photoinduced EPR spectra of spin S=1 centers that occur in orientations expected for nearest neighbor pair defects. EPR spectra of the defect on the three inequivalent lattice sites were resolved and attributed to optical transitions between photon energies of 999 and 1075 meV by MCDA-EPR. The resolved hyperfine structure indicates the presence of one single carbon nucleus and several silicon ligand nuclei. These experimental findings are interpreted with help of total energy and spin density data obtained from the standard local-spin density approximation of the density-functional theory, using relaxed defect geometries obtained from the self-consistent charge density-functional theory based tight binding scheme. We have checked several defect models of which only the photoexcited spin triplet state of the carbon antisite–carbon vacancy pair (CSi-VC) in the doubly positive charge state can explain all experimental findings. We propose that the (CSi-VC) defect is formed from the isolated silicon vacancy as an annealing product by the movement of a carbon neighbor into the vacancy
Acoustic radiation controls friction: Evidence from a spring-block experiment
Brittle failures of materials and earthquakes generate acoustic/seismic waves
which lead to radiation damping feedbacks that should be introduced in the
dynamical equations of crack motion. We present direct experimental evidence of
the importance of this feedback on the acoustic noise spectrum of
well-controlled spring-block sliding experiments performed on a variety of
smooth surfaces. The full noise spectrum is quantitatively explained by a
simple noisy harmonic oscillator equation with a radiation damping force
proportional to the derivative of the acceleration, added to a standard viscous
term.Comment: 4 pages including 3 figures. Replaced with version accepted in PR
Time-Resolved Studies of Stick-Slip Friction in Sheared Granular Layers
Sensitive and fast force measurements are performed on sheared granular
layers undergoing stick-slip motion, along with simultaneous imaging. A full
study has been done for spherical particles with a +-20% size distribution.
Stick-slip motion due to repetitive fluidization of the layer occurs for low
driving velocities. Between major slip events, slight creep occurs that is
variable from one event to the next. The effects of changing the stiffness k
and velocity V of the driving system are studied in detail. The stick-slip
motion is almost periodic for spherical particles over a wide range of
parameters, but becomes irregular when k is large and V is relatively small. At
larger V, the motion becomes smoother and is affected by the inertia of the
upper plate bounding the layer. Measurements of the period T and amplitude A of
the relative motion are presented as a function of V. At a critical value Vc, a
transition to continuous sliding motion occurs that is discontinuous for k not
too large. The time dependence of the instantaneous velocity of the upper plate
and the frictional force produced by the granular layer are determined within
individual slipping events. The force is a multi-valued function of the
instantaneous velocity, with pronounced hysteresis and a sudden drop prior to
resticking. Measurements of vertical displacement reveal a small dilation of
the material (about one tenth of the mean particle size in a layer 20 particles
deep) associated with each slip event. Finally, optical imaging reveals that
localized microscopic rearrangements precede (and follow) each slip event. The
behavior of smooth particles is contrasted with that of rough particles.Comment: 20, pages, 17 figures, to appear in Phys. Rev.
Quantum transport in ultracold atoms
Ultracold atoms confined by engineered magnetic or optical potentials are
ideal systems for studying phenomena otherwise difficult to realize or probe in
the solid state because their atomic interaction strength, number of species,
density, and geometry can be independently controlled. This review focuses on
quantum transport phenomena in atomic gases that mirror and oftentimes either
better elucidate or show fundamental differences with those observed in
mesoscopic and nanoscopic systems. We discuss significant progress in
performing transport experiments in atomic gases, contrast similarities and
differences between transport in cold atoms and in condensed matter systems,
and survey inspiring theoretical predictions that are difficult to verify in
conventional setups. These results further demonstrate the versatility offered
by atomic systems in the study of nonequilibrium phenomena and their promise
for novel applications.Comment: 24 pages, 7 figures. A revie
Channeling macrophage polarization by rocaglates increases macrophage resistance to Mycobacterium tuberculosis
Macrophages contribute to host immunity and tissue homeostasis via alternative activation programs. M1-like macrophages control intracellular bacterial pathogens and tumor progression. In contrast, M2-like macrophages shape reparative microenvironments that can be conducive for pathogen survival or tumor growth. An imbalance of these macrophages phenotypes may perpetuate sites of chronic unresolved inflammation, such as infectious granulomas and solid tumors. We have found that plant-derived and synthetic rocaglates sensitize macrophages to low concentrations of the M1-inducing cytokine IFN-gamma and inhibit their responsiveness to IL-4, a prototypical activator of the M2-like phenotype. Treatment of primary macrophages with rocaglates enhanced phagosome-lysosome fusion and control of intracellular mycobacteria. Thus, rocaglates represent a novel class of immunomodulators that can direct macrophage polarization toward the M1-like phenotype in complex microenvironments associated with hypofunction of type 1 and/or hyperactivation of type 2 immunity, e.g., chronic bacterial infections, allergies, and, possibly, certain tumors.R35 GM118173 - NIGMS NIH HHS; R01 HL126066 - NHLBI NIH HHS; R01 GM120272 - NIGMS NIH HHS; R01 CA218500 - NCI NIH HHS; R01 HL133190 - NHLBI NIH HHS; R33 AI105944 - NIAID NIH HHSPublished versio
Evolution of spiral and scroll waves of excitation in a mathematical model of ischaemic border zone
Abnormal electrical activity from the boundaries of ischemic cardiac tissue
is recognized as one of the major causes in generation of ischemia-reperfusion
arrhythmias. Here we present theoretical analysis of the waves of electrical
activity that can rise on the boundary of cardiac cell network upon its
recovery from ischaemia-like conditions. The main factors included in our
analysis are macroscopic gradients of the cell-to-cell coupling and cell
excitability and microscopic heterogeneity of individual cells. The interplay
between these factors allows one to explain how spirals form, drift together
with the moving boundary, get transiently pinned to local inhomogeneities, and
finally penetrate into the bulk of the well-coupled tissue where they reach
macroscopic scale. The asymptotic theory of the drift of spiral and scroll
waves based on response functions provides explanation of the drifts involved
in this mechanism, with the exception of effects due to the discreteness of
cardiac tissue. In particular, this asymptotic theory allows an extrapolation
of 2D events into 3D, which has shown that cells within the border zone can
give rise to 3D analogues of spirals, the scroll waves. When and if such scroll
waves escape into a better coupled tissue, they are likely to collapse due to
the positive filament tension. However, our simulations have shown that such
collapse of newly generated scrolls is not inevitable and that under certain
conditions filament tension becomes negative, leading to scroll filaments to
expand and multiply leading to a fibrillation-like state within small areas of
cardiac tissue.Comment: 26 pages, 13 figures, appendix and 2 movies, as accepted to PLoS ONE
2011/08/0
Acute febrile illness is associated with Rickettsia spp infection in dogs
BACKGROUND: Rickettsia conorii is transmitted by Rhipicephalus sanguineus ticks and causes Mediterranean Spotted Fever (MSF) in humans. Although dogs are considered the natural host of the vector, the clinical and epidemiological significance of R. conorii infection in dogs remains unclear. The aim of this prospective study was to investigate whether Rickettsia infection causes febrile illness in dogs living in areas endemic for human MSF. METHODS: Dogs from southern Italy with acute fever (n = 99) were compared with case–control dogs with normal body temperatures (n = 72). Serology and real-time PCR were performed for Rickettsia spp., Ehrlichia canis, Anaplasma phagocytophilum/A. platys and Leishmania infantum. Conventional PCR was performed for Babesia spp. and Hepatozoon spp. Acute and convalescent antibodies to R. conorii, E. canis and A. phagocytophilum were determined. RESULTS: The seroprevalence rates at first visit for R. conorii, E. canis, A. phagocytophilum and L. infantum were 44.8%, 48.5%, 37.8% and 17.6%, respectively. The seroconversion rates for R. conorii, E. canis and A. phagocytophilum were 20.7%, 14.3% and 8.8%, respectively. The molecular positive rates at first visit for Rickettsia spp., E. canis, A. phagocytophilum, A. platys, L. infantum, Babesia spp. and Hepatozoon spp. were 1.8%, 4.1%, 0%, 2.3%, 11.1%, 2.3% and 0.6%, respectively. Positive PCR for E. canis (7%), Rickettsia spp. (3%), Babesia spp. (4.0%) and Hepatozoon spp. (1.0%) were found only in febrile dogs. The DNA sequences obtained from Rickettsia and Babesia PCRs positive samples were 100% identical to the R. conorii and Babesia vogeli sequences in GenBank®, respectively. Febrile illness was statistically associated with acute and convalescent positive R. conorii antibodies, seroconversion to R. conorii, E. canis positive PCR, and positivity to any tick pathogen PCRs. Fourteen febrile dogs (31.8%) were diagnosed with Rickettsia spp. infection based on seroconversion and/or PCR while only six afebrile dogs (12.5%) seroconverted (P = 0.0248). The most common clinical findings of dogs with Rickettsia infection diagnosed by seroconversion and/or PCR were fever, myalgia, lameness, elevation of C-reactive protein, thrombocytopenia and hypoalbuminemia. CONCLUSIONS: This study demonstrates acute febrile illness associated with Rickettsia infection in dogs living in endemic areas of human MSF based on seroconversion alone or in combination with PCR
Circumstellar Material in Type Ia Supernovae via Sodium Absorption Features
Type Ia supernovae are key tools for measuring distances on a cosmic scale.
They are generally thought to be the thermonuclear explosion of an accreting
white dwarf in a close binary system. The nature of the mass donor is still
uncertain. In the single-degenerate model it is a main-sequence star or an
evolved star, whereas in the double-degenerate model it is another white dwarf.
We show that the velocity structure of absorbing material along the line of
sight to 35 type Ia supernovae tends to be blueshifted. These structures are
likely signatures of gas outflows from the supernova progenitor systems. Thus
many type Ia supernovae in nearby spiral galaxies may originate in
single-degenerate systems.Comment: Accepted for publication in Science 5 July 201
- …