3,927 research outputs found
The inverse cascade and nonlinear alpha-effect in simulations of isotropic helical hydromagnetic turbulence
A numerical model of isotropic homogeneous turbulence with helical forcing is
investigated. The resulting flow, which is essentially the prototype of the
alpha^2 dynamo of mean-field dynamo theory, produces strong dynamo action with
an additional large scale field on the scale of the box (at wavenumber k=1;
forcing is at k=5). This large scale field is nearly force-free and exceeds the
equipartition value. As the magnetic Reynolds number R_m increases, the
saturation field strength and the growth rate of the dynamo increase. However,
the time it takes to built up the large scale field from equipartition to its
final super-equipartition value increases with magnetic Reynolds number. The
large scale field generation can be identified as being due to nonlocal
interactions originating from the forcing scale, which is characteristic of the
alpha-effect. Both alpha and turbulent magnetic diffusivity eta_t are
determined simultaneously using numerical experiments where the mean-field is
modified artificially. Both quantities are quenched in a R_m-dependent fashion.
The evolution of the energy of the mean field matches that predicted by an
alpha^2 dynamo model with similar alpha and eta_t quenchings. For this model an
analytic solution is given which matches the results of the simulations. The
simulations are numerically robust in that the shape of the spectrum at large
scales is unchanged when changing the resolution from 30^3 to 120^3 meshpoints,
or when increasing the magnetic Prandtl number (viscosity/magnetic diffusivity)
from 1 to 100. Increasing the forcing wavenumber to 30 (i.e. increasing the
scale separation) makes the inverse cascade effect more pronounced, although it
remains otherwise qualitatively unchanged.Comment: 21 pages, 26 figures, ApJ (accepted
Cocirculation of Hajj and non-Hajj strains among serogroup W meningococci in Italy, 2000 to 2016
In Italy, B and C are the predominant serogroups among meningococci causing invasive diseases. Nevertheless, in the period from 2013 to 2016, an increase in serogroup W Neisseria meningitidis (MenW) was observed. This study intends to define the main characteristics of 63 MenW isolates responsible of invasive meningococcal disease (IMD) in Italy from 2000 to 2016. We performed whole genome sequencing on bacterial isolates or single gene sequencing on culturenegative samples to evaluate molecular heterogeneity. Our main finding was the cocirculation of the Hajj and the South American sublineages belonging to MenW/ clonal complex (cc)11, which gradually surpassed the MenW/cc22 in Italy. All MenW/cc11 isolates were fully susceptible to cefotaxime, ceftriaxone, ciprofloxacin, penicillin G and rifampicin. We identified the fulllength NadA protein variant 2/3, present in all the MenW/cc11. We also identified the fHbp variant 1, which we found exclusively in the MenW/cc11/Hajj sublineage. Concern about the epidemic potential of MenW/cc11 has increased worldwide since the year 2000. Continued surveillance, supported by genomic characterisation, allows high-resolution tracking of pathogen dissemination and the detection of epidemicassociated strains
Vorticity alignment results for the three-dimensional Euler and Navier-Stokes equations
We address the problem in Navier-Stokes isotropic turbulence of why the
vorticity accumulates on thin sets such as quasi-one-dimensional tubes and
quasi-two-dimensional sheets. Taking our motivation from the work of Ashurst,
Kerstein, Kerr and Gibson, who observed that the vorticity vector
{\boldmath\omega} aligns with the intermediate eigenvector of the strain
matrix , we study this problem in the context of both the three-dimensional
Euler and Navier-Stokes equations using the variables \alpha =
\hat{{\boldmath\xi}}\cdot S\hat{{\boldmath\xi}} and {\boldmath\chi} =
\hat{{\boldmath\xi}}\times S\hat{{\boldmath\xi}} where
\hat{{\boldmath\xi}} = {\boldmath\omega}/\omega. This introduces the
dynamic angle , which lies between
{\boldmath\omega} and S{\boldmath\omega}. For the Euler equations a
closed set of differential equations for and {\boldmath\chi} is
derived in terms of the Hessian matrix of the pressure . For
the Navier-Stokes equations, the Burgers vortex and shear layer solutions turn
out to be the Lagrangian fixed point solutions of the equivalent
(\alpha,{\boldmath\chi}) equations with a corresponding angle .
Under certain assumptions for more general flows it is shown that there is an
attracting fixed point of the (\alpha,\bchi) equations which corresponds to
positive vortex stretching and for which the cosine of the corresponding angle
is close to unity. This indicates that near alignment is an attracting state of
the system and is consistent with the formation of Burgers-like structures.Comment: To appear in Nonlinearity Nov. 199
Flame front propagation IV: Random Noise and Pole-Dynamics in Unstable Front Propagation II
The current paper is a corrected version of our previous paper
arXiv:adap-org/9608001. Similarly to previous version we investigate the
problem of flame propagation. This problem is studied as an example of unstable
fronts that wrinkle on many scales. The analytic tool of pole expansion in the
complex plane is employed to address the interaction of the unstable growth
process with random initial conditions and perturbations. We argue that the
effect of random noise is immense and that it can never be neglected in
sufficiently large systems. We present simulations that lead to scaling laws
for the velocity and acceleration of the front as a function of the system size
and the level of noise, and analytic arguments that explain these results in
terms of the noisy pole dynamics.This version corrects some very critical
errors made in arXiv:adap-org/9608001 and makes more detailed description of
excess number of poles in system, number of poles that appear in the system in
unit of time, life time of pole. It allows us to understand more correctly
dependence of the system parameters on noise than in arXiv:adap-org/9608001Comment: 23 pages, 4 figures,revised, version accepted for publication in
journal "Combustion, Explosion and Shock Waves". arXiv admin note:
substantial text overlap with arXiv:nlin/0302021, arXiv:adap-org/9608001,
arXiv:nlin/030201
Flow and magnetic structures in a kinematic ABC-dynamo
Dynamo theory describes the magnetic field induced by the rotating, convecting and electrically conducting fluid in a celestial body. The classical ABC-flow model represents fast dynamo action, required to sustain such a magnetic field. In this letter, Lagrangian coherent structures (LCSs) in the ABC-flow are detected through Finite-time Lyapunov exponents (FTLE). The flow skeleton is identified by extracting intersections between repelling and attracting LCSs. For the case A = B = C = 1, the skeleton structures are made up from lines connecting two different types of stagnation points in the ABC-flow. The corresponding kinematic ABC-dynamo problem is solved using a spectral method, and the distribution of cigar-like magnetic structures visualized. Inherent links are found to exist between LCSs in the ABC-flow and induced magnetic structures, which provides insight into the mechanism behind the ABC-dynamo
Control of bulk superconductivity via surface-bound electric fields in ion-gated niobium nitride thin films
Ionic gating is a very popular tool to investigate and control the electric transport and electronic ground state in a wide variety of different materials. This is due to its capability to induce large modulations of the surface charge density by means of the electric-double-layer field-effect transistor (EDL-FET) architecture, often reaching values comparable to those occurring in metallic systems. Despite finding large success in tuning the phase diagram of low-carrier density systems, including cuprates and iron-based superconductors, its applicability to conventional metallic superconductors has received significantly less attention. In my talk, I will present the work which has been carried out in my research group over several years to investigate how ionic gating can tune the properties of metallic superconductor, using niobium nitride (NbN) as an emblematic case.
By fabricating EDL-FETs on NbN thin films with thickness ranging between 10 and 40 nm, we observed that small positive and negative shifts in the critical temperature Tc could be induced by changing the gate-voltage polarity, and that the magnitude of these shifts increased upon decreasing the film thickness. These findings indicated that, despite the gate-induced electric field being confined in a thin layer at the surface by electrostatic screening, the perturbation to the superconducting state extends in a region much larger than a single unit cell. Indeed, the dependence of Tc on the gate voltage and thickness could be reconciled with the Eliashberg theory of superconductivity only if this thin surface layer is coupled to the underlying, unperturbed bulk via proximity effect. We also determined that the thickness of this surface layer (i.e. the screening length of the electric field) strongly increases for large gate electric fields, reaching values of the order of 3 nm at the highest doping. Ab-initio DFT calculations reproduced these results and linked this anomalous increase of the screening length to a distortion of the pristine charge density in the material upon the application of sufficiently large electric fields.
This proximity-effect-induced transformation of the quasi-2D perturbation to the electron density into a 3D bulk modification of the superconducting properties seems to be a general behavior in gated superconductors that could hinder the possibility to obtain large Tc shifts in films thicker than the screening length. Consequently, we are currently focusing on exploring the tunability of ultrathin (< 5nm-thick) NbN films in order to maximize the gate-induced Tc shift, where we developed a novel technique of self-encapsulation in ultrathin niobium oxide to ensure the full reversibility of the gate modulation in these extremely sensitive devices
Measurement of the t t-bar production cross section in the dilepton channel in pp collisions at sqrt(s) = 7 TeV
The t t-bar production cross section (sigma[t t-bar]) is measured in
proton-proton collisions at sqrt(s) = 7 TeV in data collected by the CMS
experiment, corresponding to an integrated luminosity of 2.3 inverse
femtobarns. The measurement is performed in events with two leptons (electrons
or muons) in the final state, at least two jets identified as jets originating
from b quarks, and the presence of an imbalance in transverse momentum. The
measured value of sigma[t t-bar] for a top-quark mass of 172.5 GeV is 161.9 +/-
2.5 (stat.) +5.1/-5.0 (syst.) +/- 3.6(lumi.) pb, consistent with the prediction
of the standard model.Comment: Replaced with published version. Included journal reference and DO
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