160,270 research outputs found
Towards an experimental von Karman dynamo: numerical studies for an optimized design
Numerical studies of a kinematic dynamo based on von Karman type flows
between two counterrotating disks in a finite cylinder are reported. The flow
has been optimized using a water model experiment, varying the driving
impellers configuration. A solution leading to dynamo action for the mean flow
has been found. This solution may be achieved in VKS2, the new sodium
experiment to be performed in Cadarache, France. The optimization process is
described and discussed, then the effects of adding a stationary conducting
layer around the flow on the threshold, on the shape of the neutral mode and on
the magnetic energy balance are studied. Finally, the possible processes
involved into kinematic dynamo action in a von Karman flow are reviewed and
discussed. Among the possible processes we highlight the joint effect of the
boundary-layer radial velocity shear and of the Ohmic dissipation localized at
the flow/outer-shell boundary
Scanning nano-spin ensemble microscope for nanoscale magnetic and thermal imaging
Quantum sensors based on solid-state spins provide tremendous opportunities
in a wide range of fields from basic physics and chemistry to biomedical
imaging. However, integrating them into a scanning probe microscope to enable
practical, nanoscale quantum imaging is a highly challenging task. Recently,
the use of single spins in diamond in conjunction with atomic force microscopy
techniques has allowed significant progress towards this goal, but
generalisation of this approach has so far been impeded by long acquisition
times or by the absence of simultaneous topographic information. Here we report
on a scanning quantum probe microscope which solves both issues, by employing a
nano-spin ensemble hosted in a nanodiamond. This approach provides up to an
order of magnitude gain in acquisition time, whilst preserving sub-100 nm
spatial resolution both for the quantum sensor and topographic images. We
demonstrate two applications of this microscope. We first image nanoscale
clusters of maghemite particles through both spin resonance spectroscopy and
spin relaxometry, under ambient conditions. Our images reveal fast magnetic
field fluctuations in addition to a static component, indicating the presence
of both superparamagnetic and ferromagnetic particles. We next demonstrate a
new imaging modality where the nano-spin ensemble is used as a thermometer. We
use this technique to map the photo-induced heating generated by laser
irradiation of a single gold nanoparticle in a fluid environment. This work
paves the way towards new applications of quantum probe microscopy such as
thermal/magnetic imaging of operating microelectronic devices and magnetic
detection of ion channels in cell membranes.Comment: 22 pages including Supporting Information. Changes to v1:
affiliations and funding information updated, plus minor revisions to the
main tex
Levy ratchets with dichotomic random flashing
Additive symmetric L\'evy noise can induce directed transport of overdamped
particles in a static asymmetric potential. We study, numerically and
analytically, the effect of an additional dichotomous random flashing in such
L\'evy ratchet system. For this purpose we analyze and solve the corresponding
fractional Fokker-Planck equations and we check the results with Langevin
simulations. We study the behavior of the current as function of the stability
index of the L\'evy noise, the noise intensity and the flashing parameters. We
find that flashing allows both to enhance and diminish in a broad range the
static L\'evy ratchet current, depending on the frequencies and asymmetry of
the multiplicative dichotomous noise, and on the additive L\'evy noise
parameters. Our results thus extend those for dichotomous flashing ratchets
with Gaussian noise to the case of broadly distributed noises.Comment: 15 pages, 6 figure
Violation of the fluctuation-dissipation theorem in glassy systems: basic notions and the numerical evidence
This review reports on the research done during the past years on violations
of the fluctuation-dissipation theorem (FDT) in glassy systems. It is focused
on the existence of a quasi-fluctuation-dissipation theorem (QFDT) in glassy
systems and the currently supporting knowledge gained from numerical simulation
studies. It covers a broad range of non-stationary aging and stationary driven
systems such as structural-glasses, spin-glasses, coarsening systems,
ferromagnetic models at criticality, trap models, models with entropy barriers,
kinetically constrained models, sheared systems and granular media. The review
is divided into four main parts: 1) An introductory section explaining basic
notions related to the existence of the FDT in equilibrium and its possible
extension to the glassy regime (QFDT), 2) A description of the basic analytical
tools and results derived in the framework of some exactly solvable models, 3)
A detailed report of the current evidence in favour of the QFDT and 4) A brief
digression on the experimental evidence in its favour. This review is intended
for inexpert readers who want to learn about the basic notions and concepts
related to the existence of the QFDT as well as for the more expert readers who
may be interested in more specific results.Comment: 120 pages, 37 figures. Topical review paper . Several typos and
misprints corrected, new references included and others updated. to be
published in J. Phys. A (Math. Gen.
Magnetic field structure of relativistic jets without current sheets
We present an analytical class of equilibrium solutions for the structure of
relativistic sheared and rotating magnetized jets that contain no boundary
current sheets. We demonstrate the overall dynamical stability of these
solutions and, most importantly, a better numerical resistive stability than
the commonly employed force-free structures which inevitably require the
presence of dissipative surface currents. The jet is volumetrically confined by
the external pressure, with no pressure gradient on the surface. We calculate
the expected observed properties of such jets. Given the simplicity of these
solution we suggest them as useful initial conditions for relativistic jet
simulations.Comment: 13 pages, 13 figures, Accepted by MNRA
Self-Consistent Cosmological Simulations of DGP Braneworld Gravity
We perform cosmological N-body simulations of the Dvali-Gabadadze-Porrati
braneworld model, by solving the full non-linear equations of motion for the
scalar degree of freedom in this model, the brane bending mode. While coupling
universally to matter, the brane-bending mode has self-interactions that become
important as soon as the density field becomes non-linear. These
self-interactions lead to a suppression of the field in high-density
environments, and restore gravity to General Relativity. The code uses a
multi-grid relaxation scheme to solve the non-linear field equation in the
quasi-static approximation. We perform simulations of a flat self-accelerating
DGP model without cosmological constant. The results of the DGP simulations are
compared with standard gravity simulations assuming the same expansion history,
and with DGP simulations using the linearized equation for the brane bending
mode. This allows us to isolate the effects of the non-linear self-couplings of
the field which are noticeable already on quasi-linear scales. We present
results on the matter power spectrum and the halo mass function, and discuss
the behavior of the brane bending mode within cosmological structure formation.
We find that, independently of CMB constraints, the self-accelerating DGP model
is strongly constrained by current weak lensing and cluster abundance
measurements.Comment: 21 pages; 10 figures. Revised version matching published versio
Scaling of the glassy dynamics of soft repulsive particles: a mode-coupling approach
We combine the hyper-netted chain approximation of liquid state theory with
the mode-coupling theory of the glass transition to analyze the structure and
dynamics of soft spheres interacting via harmonic repulsion. We determine the
locus of the fluid-glass dynamic transition in a temperature -- volume fraction
phase diagram. The zero-temperature (hard sphere) glass transition influences
the dynamics at finite temperatures in its vicinity. This directly implies a
form of dynamic scaling for both the average relaxation time and dynamic
susceptibilities quantifying dynamic heterogeneity. We discuss several
qualitative disagreements between theory and existing simulations at
equilibrium. Our theoretical results are, however, very similar to numerical
results for the driven athermal dynamics of repulsive spheres, suggesting that
`mean-field' mode-coupling approaches might be good starting points to describe
these nonequilibrium dynamics.Comment: 11 pages, 8 figure
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