9,643 research outputs found
Unstable coronal loops : numerical simulations with predicted observational signatures
We present numerical studies of the nonlinear, resistive magnetohydrodynamic
(MHD) evolution of coronal loops. For these simulations we assume that the
loops carry no net current, as might be expected if the loop had evolved due to
vortex flows. Furthermore the initial equilibrium is taken to be a cylindrical
flux tube with line-tied ends. For a given amount of twist in the magnetic
field it is well known that once such a loop exceeds a critical length it
becomes unstableto ideal MHD instabilities. The early evolution of these
instabilities generates large current concentrations. Firstly we show that
these current concentrations are consistent with the formation of a current
sheet. Magnetic reconnection can only occur in the vicinity of these current
concentrations and we therefore couple the resistivity to the local current
density. This has the advantage of avoiding resistive diffusion in regions
where it should be negligible. We demonstrate the importance of this procedure
by comparison with simulations based on a uniform resistivity. From our
numerical experiments we are able to estimate some observational signatures for
unstable coronal loops. These signatures include: the timescale of the loop
brightening; the temperature increase; the energy released and the predicted
observable flow speeds. Finally we discuss to what extent these observational
signatures are consistent with the properties of transient brightening loops.Comment: 13 pages, 9 figure
Measuring Polynomial Invariants of Multi-Party Quantum States
We present networks for directly estimating the polynomial invariants of
multi-party quantum states under local transformations. The structure of these
networks is closely related to the structure of the invariants themselves and
this lends a physical interpretation to these otherwise abstract mathematical
quantities. Specifically, our networks estimate the invariants under local
unitary (LU) transformations and under stochastic local operations and
classical communication (SLOCC). Our networks can estimate the LU invariants
for multi-party states, where each party can have a Hilbert space of arbitrary
dimension and the SLOCC invariants for multi-qubit states. We analyze the
statistical efficiency of our networks compared to methods based on estimating
the state coefficients and calculating the invariants.Comment: 8 pages, 4 figures, RevTex4, v2 references update
Redox reactions with empirical potentials: Atomistic battery discharge simulations
Batteries are pivotal components in overcoming some of today's greatest
technological challenges. Yet to date there is no self-consistent atomistic
description of a complete battery. We take first steps toward modeling of a
battery as a whole microscopically. Our focus lies on phenomena occurring at
the electrode-electrolyte interface which are not easily studied with other
methods. We use the redox split-charge equilibration (redoxSQE) method that
assigns a discrete ionization state to each atom. Along with exchanging partial
charges across bonds, atoms can swap integer charges. With redoxSQE we study
the discharge behavior of a nano-battery, and demonstrate that this reproduces
the generic properties of a macroscopic battery qualitatively. Examples are the
dependence of the battery's capacity on temperature and discharge rate, as well
as performance degradation upon recharge.Comment: 14 pages, 10 figure
Reconstruction of the phase of matter-wave fields using a momentum resolved cross-correlation technique
We investigate the potential of the so-called XFROG cross-correlation
technique originally developed for ultrashort laser pulses for the recovery of
the amplitude and phase of the condensate wave function of a Bose-Einstein
condensate. Key features of the XFROG method are its high resolution,
versatility and stability against noise and some sources of systematic errors.
After showing how an analogue of XFROG can be realized for Bose-Einstein
condensates, we illustrate its effectiveness in determining the amplitude and
phase of the wave function of a vortex state. The impact of a reduction of the
number of measurements and of typical sources of noise on the field
reconstruction are also analyzed.Comment: 7 pages; 9 figures; article with higher resolution figures available
from author
Temporary withdrawal of immunosuppression for life-threatening infections after liver transplantation
Photoemission spectra of many-polaron systems
The cross over from low to high carrier densities in a many-polaron system is
studied in the framework of the one-dimensional spinless Holstein model, using
unbiased numerical methods. Combining a novel quantum Monte Carlo approach and
exact diagonalization, accurate results for the single-particle spectrum and
the electronic kinetic energy on fairly large systems are obtained. A detailed
investigation of the quality of the Monte Carlo data is presented. In the
physically most important adiabatic intermediate electron-phonon coupling
regime, for which no analytical results are available, we observe a
dissociation of polarons with increasing band filling, leading to normal
metallic behavior, while for parameters favoring small polarons, no such
density-driven changes occur. The present work points towards the inadequacy of
single-polaron theories for a number of polaronic materials such as the
manganites.Comment: 15 pages, 13 figures; final version, accepted for publication in
Phys. Rev.
Cosmology and astrophysics from relaxed galaxy clusters - IV: Robustly calibrating hydrostatic masses with weak lensing
This is the fourth in a series of papers studying the astrophysics and
cosmology of massive, dynamically relaxed galaxy clusters. Here, we use
measurements of weak gravitational lensing from the Weighing the Giants project
to calibrate Chandra X-ray measurements of total mass that rely on the
assumption of hydrostatic equilibrium. This comparison of X-ray and lensing
masses provides a measurement of the combined bias of X-ray hydrostatic masses
due to both astrophysical and instrumental sources. Assuming a fixed cosmology,
and within a characteristic radius (r_2500) determined from the X-ray data, we
measure a lensing to X-ray mass ratio of 0.96 +/- 9% (stat) +/- 9% (sys). We
find no significant trends of this ratio with mass, redshift or the
morphological indicators used to select the sample. In accordance with
predictions from hydro simulations for the most massive, relaxed clusters, our
results disfavor strong, tens-of-percent departures from hydrostatic
equilibrium at these radii. In addition, we find a mean concentration of the
sample measured from lensing data of c_200 = . Anticipated
short-term improvements in lensing systematics, and a modest expansion of the
relaxed lensing sample, can easily increase the measurement precision by
30--50%, leading to similar improvements in cosmological constraints that
employ X-ray hydrostatic mass estimates, such as on Omega_m from the cluster
gas mass fraction.Comment: 13 pages. Submitted to MNRAS. Comments welcom
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