40,103 research outputs found
A model of driven and decaying magnetic turbulence in a cylinder
Using mean-field theory, we compute the evolution of the magnetic field in a
cylinder with outer perfectly conducting boundaries, an imposed axial magnetic
and electric field. The thus injected magnetic helicity in the system can be
redistributed by magnetic helicity fluxes down the gradient of the local
current helicity of the small-scale magnetic field. A weak reversal of the
axial magnetic field is found to be a consequence of the magnetic helicity flux
in the system. Such fluxes are known to alleviate so-called catastrophic
quenching of the {\alpha}-effect in astrophysical applications. Application to
the reversed field pinch in plasma confinement devices is discussed.Comment: 7 pages, 4 figures, submitted to Phys. Rev.
Leading Chiral Contributions to the Spin Structure of the Proton
The leading chiral contributions to the quark and gluon components of the
proton spin are calculated using heavy-baryon chiral perturbation theory.
Similar calculations are done for the moments of the generalized parton
distributions relevant to the quark and gluon angular momentum densities. These
results provide useful insight about the role of pions in the spin structure of
the nucleon, and can serve as a guidance for extrapolating lattice QCD
calculations at large quark masses to the chiral limit.Comment: 8 pages, 2 figures; a typo in Ref. 7 correcte
Large anomalous Hall effect in ferromagnetic insulator-topological insulator heterostructures
We demonstrate the van der Waals epitaxy of the topological insulator
compound Bi2Te3 on the ferromagnetic insulator Cr2Ge2Te6. The layers are
oriented with (001) of Bi2Te3 parallel to (001) of Cr2Ge2Te6 and (110) of
Bi2Te3 parallel to (100) of Cr2Ge2Te6. Cross-sectional transmission electron
microscopy indicates the formation of a sharp interface. At low temperatures,
bilayers consisting of Bi2Te3 on Cr2Ge2Te6 exhibit a large anomalous Hall
effect (AHE). Tilted field studies of the AHE indicate that the easy axis lies
along the c-axis of the heterostructure, consistent with magnetization
measurements in bulk Cr2Ge2Te6. The 61 K Curie temperature of Cr2Ge2Te6 and the
use of near-stoichiometric materials may lead to the development of spintronic
devices based on the AHE.Comment: Related papers at http://pettagroup.princeton.ed
A Scattered Light Echo around SN 1993J in M81
A light echo around SN 1993J was observed 8.2 years after explosion by a HST
WFPC2 observation, adding to the small family of supernovae with light echoes.
The light echo was formed by supernova light scattered from a dust sheet, which
lies 220 parsecs away from the supernova, 50 parsecs thick along the line of
sight, as inferred from radius and width of the light echo. The dust inferred
from the light echo surface brightness is 1000 times denser than the intercloud
dust. The graphite to silicate fraction can not be determined by our BVI
photometric measurements, however, a pure graphite model can be excluded based
on comparison with the data. With future observations, it will be possible to
measure the expansion rate of the light echo, from which an independent
distance to M81 can be obtained.Comment: 10 pages, 6 figures, in AASTeX format, submitted to ApJ Part
The Rotation Average in Lightcone Time-Ordered Perturbation Theory
We present a rotation average of the two-body scattering amplitude in the
lightcone time()-ordered perturbation theory. Using a rotation average
procedure, we show that the contribution of individual time-ordered diagram can
be quantified in a Lorentz invariant way. The number of time-ordered diagrams
can also be reduced by half if the masses of two bodies are same. In the
numerical example of theory, we find that the higher Fock-state
contribution is quite small in the lightcone quantization.Comment: 25 pages, REVTeX, epsf.sty, 69 eps file
Post-Oligarchic Evolution of Protoplanetary Embryos and the Stability of Planetary Systems
We investigate the orbit-crossing time (T_c) of protoplanet systems both with
and without a gas-disk background. The protoplanets are initially with equal
masses and separation (EMS systems) scaled by their mutual Hill's radii. In a
gas-free environment, we find log (T_c/yr) = A+B \log (k_0/2.3). Through a
simple analytical approach, we demonstrate that the evolution of the velocity
dispersion in an EMS system follows a random walk. The stochastic nature of
random-walk diffusion leads to (i) an increasing average eccentricity ~
t^1/2, where t is the time; (ii) Rayleigh-distributed eccentricities
(P(e,t)=e/\sigma^2 \exp(-e^2/(2\sigma^2)) of the protoplanets; (iii) a
power-law dependence of T_c on planetary separation. As evidence for the
chaotic diffusion, the observed eccentricities of known extra solar planets can
be approximated by a Rayleigh distribution. We evaluate the isolation masses of
the embryos, which determine the probability of gas giant formation, as a
function of the dust and gas surface densities.Comment: 15 pages, 13 figures (2 color ones), accepted for publication in Ap
Steady-state response of fluid-structure interactions in hydraulic piping system of passive interconnected suspensions
Copyright © 2016 Inderscience Enterprises Ltd. Pressure changes in the liquid-filled fluid circuit of a hydraulically interconnected suspension (HIS) system can induce vibrations of the whole pipeline and the associated structure, and hence become a source of structural noise which degrades ride comfort. This paper presents a numerical and experimental investigation into the vibration of the hydraulic piping system of a passive interconnected suspension. The transfer matrix method (TMM) is used to develop a mathematical model, which consists of various pipe sections, hose sections, concentrated masses, spring supports, elbows, damper valves, and accumulators. Laboratory experiments are performed on two liquid-filled piping systems. The measured steady-state responses of the hydraulic circuits are compared with those obtained from numerical simulations of the developed model. It is found that the developed model of the hydraulic system has a reasonable accuracy in the frequency range of interest, and thus can be employed to optimise the design of the hydraulic system
An optical fibre dynamic instrumented palpation sensor for the characterisation of biological tissue
AbstractThe diagnosis of prostate cancer using invasive techniques (such as biopsy and blood tests for prostate-specific antigen) and non-invasive techniques (such as digital rectal examination and trans-rectal ultrasonography) may be enhanced by using an additional dynamic instrumented palpation approach to prostate tissue classification. A dynamically actuated membrane sensor/actuator has been developed that incorporates an optical fibre Fabry–Pérot interferometer to record the displacement of the membrane when it is pressed on to different tissue samples. The membrane sensor was tested on a silicon elastomer prostate model with enlarged and stiffer material on one side to simulate early stage prostate cancer. The interferometer measurement was found to have high dynamic range and accuracy, with a minimum displacement resolution of ±0.4μm over a 721μm measurement range. The dynamic response of the membrane sensor when applied to different tissue types changed depending on the stiffness of the tissue being measured. This demonstrates the feasibility of an optically tracked dynamic palpation technique for classifying tissue type based on the dynamic response of the sensor/actuator
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