6,551 research outputs found
Delayed feedback as a means of control of noise-induced motion
Time--delayed feedback is exploited for controlling noise--induced motion in
coherence resonance oscillators. Namely, under the proper choice of time delay,
one can either increase or decrease the regularity of motion. It is shown that
in an excitable system, delayed feedback can stabilize the frequency of
oscillations against variation of noise strength. Also, for fixed noise
intensity, the phenomenon of entrainment of the basic oscillation period by the
delayed feedback occurs. This allows one to steer the timescales of
noise-induced motion by changing the time delay.Comment: 4 pages, 4 figures. In the replacement file Fig. 2 and Fig. 4(b),(d)
were amended. The reason is numerical error found, that affected the
quantitative estimates of correlation time, but did not affect the main
messag
Many-body effects on the capacitance of multilayers made from strongly correlated materials
Recent work by Kopp and Mannhart on novel electronic systems formed at oxide
interfaces has shown interesting effects on the capacitances of these devices.
We employ inhomogeneous dynamical mean-field theory to calculate the
capacitance of multilayered nanostructures. These multilayered nanostructures
are composed of semi-infinite metallic leads coupled via a strongly correlated
dielectric barrier region. The barrier region can be adjusted from a metallic
regime to a Mott insulator through adjusting the interaction strength. We
examine the effects of varying the barrier width, temperature, potential
difference, screening length, and chemical potential. We find that the
interaction strength has a relatively strong effect on the capacitance, while
the potential and temperature show weaker dependence.Comment: 19 pages, 7 figures, REVTe
Age and Growth of the Sandbar Shark (Carcharhinus plumbeus) from the Northern Gulf of Mexico and the Western North Atlantic Ocean
Age and growth analysis of the sandbar shark, Carcharhinus plumbeus, from the northern Gulf of Mexico and western North Atlantic Ocean was completed with vertebral samples (n = 1,194). Three parameter von Bertalanffy growth curves were run for male and female sandbar sharks separately, and growth parameters were estimated: theoretical maximum length (Lâ) = 172.9 ± 1.3 cm straight-line fork length [FL], growth coefficient (k) = 0.15 ± 0.01 yr-1, x-intercept (t0) = 22.3 ± 0.2 SE (male); and Lâ = 181.2 ± 1.5 cm FL, k = 0.12 ± 0.01 yr-1, t0 = 23.1 ± 0.2 SE (female). The oldest sandbar shark was a 27-yr-old female, and the oldest male was 22 yr old. The age and growth parameters estimated during this study differed from those in previous studies. The differences in the age and growth parameters may indicate growth overfishing, or they may be due to the bias in sampling from a fishery that targets a limited size range of sharks
Magnetic Helicity Conservation and Astrophysical Dynamos
We construct a magnetic helicity conserving dynamo theory which incorporates
a calculated magnetic helicity current. In this model the fluid helicity plays
a small role in large scale magnetic field generation. Instead, the dynamo
process is dominated by a new quantity, derived from asymmetries in the second
derivative of the velocity correlation function, closely related to the `twist
and fold' dynamo model. The turbulent damping term is, as expected, almost
unchanged. Numerical simulations with a spatially constant fluid helicity and
vanishing resistivity are not expected to generate large scale fields in
equipartition with the turbulent energy density. The prospects for driving a
fast dynamo under these circumstances are uncertain, but if it is possible,
then the field must be largely force-free. On the other hand, there is an
efficient analog to the dynamo. Systems whose turbulence is
driven by some anisotropic local instability in a shearing flow, like real
stars and accretion disks, and some computer simulations, may successfully
drive the generation of strong large scale magnetic fields, provided that
. We show that this
criterion is usually satisfied. Such dynamos will include a persistent,
spatially coherent vertical magnetic helicity current with the same sign as
, that is, positive for an accretion disk and negative for
the Sun. We comment on the role of random magnetic helicity currents in storing
turbulent energy in a disordered magnetic field, which will generate an
equipartition, disordered field in a turbulent medium, and also a declining
long wavelength tail to the power spectrum. As a result, calculations of the
galactic `seed' field are largely irrelevant.Comment: 28 pages, accepted by The Astrophysical Journa
An Alternative Parameterization of R-matrix Theory
An alternative parameterization of R-matrix theory is presented which is
mathematically equivalent to the standard approach, but possesses features
which simplify the fitting of experimental data. In particular there are no
level shifts and no boundary-condition constants which allows the positions and
partial widths of an arbitrary number levels to be easily fixed in an analysis.
These alternative parameters can be converted to standard R-matrix parameters
by a straightforward matrix diagonalization procedure. In addition it is
possible to express the collision matrix directly in terms of the alternative
parameters.Comment: 8 pages; accepted for publication in Phys. Rev. C; expanded Sec. IV,
added Sec. VI, added Appendix, corrected typo
Evaluation of attenuated tumor antigens and the implications for peptide-based cancer vaccine development
Scattering and Iron Fluorescence Revealed During Absorption Dips in Circinus X-1
We show that dramatic spectral evolution associated with dips occurring near
phase zero in RXTE observations of Cir X-1 is well-fit by variable and at times
heavy absorption (N_H > 10^24 cm^-2) of a bright component, plus an underlying
faint component which is not attenuated by the variable column and whose flux
is ~10% of that of the unabsorbed bright component. A prominent Fe emission
line at ~6.5 keV is evident during the dips. The absolute line flux outside the
dips is similar to that during the dips, indicating that the line is associated
with the faint component. These results are consistent with a model in which
the bright component is radiation received directly from a compact source while
the faint component may be attributed to scattered radiation. Our results are
also generally consistent with those of Brandt et al., who found that a
partial- covering model could explain ASCA spectra of a low-to-high transition
in Cir X-1. The relative brightness of the two components in our model requires
a column density of ~2*10^23 cm^-2 if the faint component is due to Thomson
scattering in material that mostly surrounds the source. We find that
illumination of such a scattering cloud by the observed direct component would
produce an Fe K-alpha fluorescence flux that is in rough agreement with the
flux of the observed emission line. We also conclude that if the scattering
medium is not highly ionized, our line of sight to the compact source does not
pass through it. Finally, we discuss simple pictures of the absorbers
responsible for the dips themselves.Comment: Accepted for publication in The Astrophysical Journal (23 pages,
including 11 figures
Evolution of helicity in NOAA 10923 over three consecutive solar rotations
We have studied the evolution of magnetic helicity and chirality in an active
region over three consecutive solar rotations. The region when it first
appeared was named NOAA10923 and in subsequent rotations it was numbered NOAA
10930, 10935 and 10941. We compare the chirality of these regions at
photospheric, chromospheric and coronal heights. The observations used for
photospheric and chromospheric heights are taken from Solar Vector Magnetograph
(SVM) and H_alpha imaging telescope of Udaipur Solar Observatory (USO),
respectively. We discuss the chirality of the sunspots and associated H_alpha
filaments in these regions. We find that the twistedness of superpenumbral
filaments is maintained in the photospheric transverse field vectors also. We
also compare the chirality at photospheric and chromospheric heights with the
chirality of the associated coronal loops, as observed from the HINODE X-Ray
Telescope.Comment: 8 pages, 4 figure
Exponential decay for the damped wave equation in unbounded domains
We study the decay of the semigroup generated by the damped wave equation in
an unbounded domain. We first prove under the natural geometric control
condition the exponential decay of the semigroup. Then we prove under a weaker
condition the logarithmic decay of the solutions (assuming that the initial
data are smoother). As corollaries, we obtain several extensions of previous
results of stabilisation and control
Systematic Inclusion of High-Order Multi-Spin Correlations for the Spin- Models
We apply the microscopic coupled-cluster method (CCM) to the spin-
models on both the one-dimensional chain and the two-dimensional square
lattice. Based on a systematic approximation scheme of the CCM developed by us
previously, we carry out high-order {\it ab initio} calculations using
computer-algebraic techniques. The ground-state properties of the models are
obtained with high accuracy as functions of the anisotropy parameter.
Furthermore, our CCM analysis enables us to study their quantum critical
behavior in a systematic and unbiased manner.Comment: (to appear in PRL). 4 pages, ReVTeX, two figures available upon
request. UMIST Preprint MA-000-000
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