15,611 research outputs found
Branes are Waves and Monopoles
In a recent paper it was shown that fundamental strings are null waves in
Double Field Theory. Similarly, membranes are waves in exceptional extended
geometry. Here the story is continued by showing how various branes are
Kaluza-Klein monopoles of these higher dimensional theories. Examining the
specific case of the E7 exceptional extended geometry, we see that all branes
are both waves and monopoles. Along the way we discuss the O(d; d)
transformation of localized brane solutions not associated to an isometry and
how true T-duality emerges in Double Field Theory when the background possesses
isometries.Comment: 32 pages, Latex, v2, typos correcte
Quasilinear spin voltage profiles in spin thermoelectrics
Recent experiments show that spin thermoelectrics is a promising approach to
generate spin voltages. While spin chemical potentials are often limited to a
surface layer of the order of the spin diffusion length, we show that
thermoelectrically induced spin chemical potentials can extend much further in
itinerant ferromagnets with paramagnetic impurities. In some cases,
conservation laws, e.g., for a combination of spin and heat currents, give rise
to a linear spin voltage profile. More generally, we find quasilinear profiles
involving a spin thermoelectric length scale which far exceeds the spin
diffusion length.Comment: 4+ page
Structure and dynamics of binary liquid mixtures near their continuous demixing transitions
The dynamic and static critical behavior of five binary Lennard-Jones liquid
mixtures, close to their continuous demixing points (belonging to the so-called
model H' dynamic universality class), are studied computationally by combining
semi-grand canonical Monte Carlo simulations and large-scale molecular dynamics
(MD) simulations, accelerated by graphic processing units (GPU). The symmetric
binary liquid mixtures considered cover a variety of densities, a wide range of
compressibilities, and various interactions between the unlike particles. The
static quantities studied here encompass the bulk phase diagram (including both
the binodal and the -line), the correlation length, the concentration
susceptibility, the compressibility of the finite-sized systems at the bulk
critical temperature , and the pressure. Concerning the collective
transport properties, we focus on the Onsager coefficient and the shear
viscosity. The critical power-law singularities of these quantities are
analyzed in the mixed phase (above ) and non-universal critical amplitudes
are extracted. Two universal amplitude ratios are calculated. The first one
involves static amplitudes only and agrees well with the expectations for the
three-dimensional Ising universality class. The second ratio includes also
dynamic critical amplitudes and is related to the Einstein--Kawasaki relation
for the interdiffusion constant. Precise estimates of this amplitude ratio are
difficult to obtain from MD simulations, but within the error bars our results
are compatible with theoretical predictions and experimental values for model
H'. Evidence is reported for an inverse proportionality of the pressure and the
isothermal compressibility at the demixing transition, upon varying either the
number density or the repulsion strength between unlike particles.Comment: 15 pages, 12 figure
Graded Lie algebras with finite polydepth
If A is a graded connected algebra then we define a new invariant, polydepth
A, which is finite if for some A-module M of at most
polynomial growth. Theorem 1: If f : X \to Y is a continuous map of finite
category, and if the orbits of H_*(\Omega Y) acting in the homology of the
homotopy fibre grow at most polynomially, then H_*(\Omega Y) has finite
polydepth. Theorem 2: If L is a graded Lie algebra and polydepth UL is finite
then either L is solvable and UL grows at most polynomially or else for some
integer d and all r, , some
Sparsity-Sensitive Finite Abstraction
Abstraction of a continuous-space model into a finite state and input
dynamical model is a key step in formal controller synthesis tools. To date,
these software tools have been limited to systems of modest size (typically
6 dimensions) because the abstraction procedure suffers from an
exponential runtime with respect to the sum of state and input dimensions. We
present a simple modification to the abstraction algorithm that dramatically
reduces the computation time for systems exhibiting a sparse interconnection
structure. This modified procedure recovers the same abstraction as the one
computed by a brute force algorithm that disregards the sparsity. Examples
highlight speed-ups from existing benchmarks in the literature, synthesis of a
safety supervisory controller for a 12-dimensional and abstraction of a
51-dimensional vehicular traffic network
Time Dependent Modeling of the Markarian 501 X-ray and TeV Gamma-Ray Data Taken During March and April, 1997
If the high-energy emission from TeV blazars is produced by the Synchrotron
Self-Compton (SSC) mechanism, then simultaneous X-ray and Gamma-ray
observations of these objects are a powerful probe of the electron (and/or
positron) populations responsible for this emission. Understanding the emitting
particle distributions and their evolution in turn allow us to probe physical
conditions in the inner blazar jet and test, for example, various acceleration
scenarios. By constraining the SSC emission model parameters, such observations
also allow us to predict the intrinsic (unabsorbed) Gamma-ray spectra of these
sources, a major uncertainty in current attempts to use the observed Gamma-ray
spectra to constrain the intensity of the extragalactic background at
optical/infrared wavelengths. As a next step in testing the SSC model and as a
demonstration of the potential power of coordinated X-ray and Gamma-ray
observations, we attempt to model in detail the X-ray and Gamma-ray light
curves of the TeV Blazar Mrk 501 during its April-May 1997 outburst using a
time dependent SSC emission model. Extensive, quasi-simultaneous X-ray and
gamma-ray coverage exists for this period. We discuss and explore
quantitatively several of the flare scenarios presented in the literature. We
show that simple two-component models (with a soft, steady X-ray component plus
a variable SSC component) involving substantial pre-acceleration of electrons
to Lorentz factors on the order of 1E+5 describe the data train surprisingly
well. All considered models imply an emission region that is strongly out of
equipartition and low radiative efficiencies (ratio between kinetic jet
luminosity and comoving radiative luminosity) of 1 per-mill and less.Comment: 16 pages, Refereed Manuscript. Minor changes to previous versio
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