542 research outputs found
Geometric Finite Element Discretization of Maxwell Equations in Primal and Dual Spaces
Based on a geometric discretization scheme for Maxwell equations, we unveil a
mathematical\textit{\}transformation between the electric field intensity
and the magnetic field intensity , denoted as Galerkin duality. Using
Galerkin duality and discrete Hodge operators, we construct two system
matrices, (primal formulation) and (dual
formulation) respectively, that discretize the second-order vector wave
equations. We show that the primal formulation recovers the conventional
(edge-element) finite element method (FEM) and suggests a geometric foundation
for it. On the other hand, the dual formulation suggests a new (dual) type of
FEM. Although both formulations give identical dynamical physical solutions,
the dimensions of the null spaces are different.Comment: 22 pages and 4 figure
On the degrees of freedom of lattice electrodynamics
Using Euler's formula for a network of polygons for 2D case (or polyhedra for
3D case), we show that the number of dynamic\textit{\}degrees of freedom of the
electric field equals the number of dynamic degrees of freedom of the magnetic
field for electrodynamics formulated on a lattice. Instrumental to this
identity is the use (at least implicitly) of a dual lattice and of a (spatial)
geometric discretization scheme based on discrete differential forms. As a
by-product, this analysis also unveils a physical interpretation for Euler's
formula and a geometric interpretation for the Hodge decomposition.Comment: 14 pages, 6 figure
Fermionic anticommutators for open superstrings in the presence of antisymmetric tensor field
We build up the anticommutator algebra for the fermionic coordinates of open
superstrings attached to branes with antisymmetric tensor fields. We use both
Dirac quantization and the symplectic Faddeev Jackiw approach. In the
symplectic case we find a way of generating the boundary conditions as zero
modes of the symplectic matrix by taking a discretized form of the action and
adding terms that vanish in the continuous limit. This way boundary conditions
can be handled as constraints.Comment: Revision: passage from discrete to continuous clarified, comment on
previous results using Dirac quantization included, typos corrected. Version
to appear in Phys. Lett.
Reply to âComment on âMuon spin relaxation study of spin dynamics in poly(triarylamine)ââ
Unitarity bounds on low scale quantum gravity
We study the unitarity of models with low scale quantum gravity both in four
dimensions and in models with a large extra-dimensional volume. We find that
models with low scale quantum gravity have problems with unitarity below the
scale at which gravity becomes strong. An important consequence of our work is
that their first signal at the Large Hadron Collider would not be of a
gravitational nature such as graviton emission or small black holes, but rather
linked to the mechanism which fixes the unitarity problem. We also study models
with scalar fields with non minimal couplings to the Ricci scalar. We consider
the strength of gravity in these models and study the consequences for
inflation models with non-minimally coupled scalar fields. We show that a
single scalar field with a large non-minimal coupling can lower the Planck mass
in the TeV region. In that model, it is possible to lower the scale at which
gravity becomes strong down to 14 TeV without violating unitarity below that
scale.Comment: 15 page
Rings and rigidity transitions in network glasses
Three elastic phases of covalent networks, (I) floppy, (II) isostatically
rigid and (III) stressed-rigid have now been identified in glasses at specific
degrees of cross-linking (or chemical composition) both in theory and
experiments. Here we use size-increasing cluster combinatorics and constraint
counting algorithms to study analytically possible consequences of
self-organization. In the presence of small rings that can be locally I, II or
III, we obtain two transitions instead of the previously reported single
percolative transition at the mean coordination number , one from a
floppy to an isostatic rigid phase, and a second one from an isostatic to a
stressed rigid phase. The width of the intermediate phase and the
order of the phase transitions depend on the nature of medium range order
(relative ring fractions). We compare the results to the Group IV
chalcogenides, such as Ge-Se and Si-Se, for which evidence of an intermediate
phase has been obtained, and for which estimates of ring fractions can be made
from structures of high T crystalline phases.Comment: 29 pages, revtex, 7 eps figure
Thermal AdS(3), BTZ and competing winding modes condensation
We study the thermal physics of AdS(3) and the BTZ black hole when embedded
in String theory. The exact calculation of the Hagedorn temperature in TAdS(3)
is reinterpreted as the appearance of a winding tachyon both in AdS(3) and BTZ.
We construct a dual framework for analyzing the phases of the system. In this
dual framework, tachyon condensation and geometric capping appear on the same
footing, bridging the usual gap of connecting tachyon condensation to
modifications of geometry. This allows us to construct in a natural way a
candidate for the unstable phase, analogous to a small black hole in higher
dimensions. Additional peculiar effects associated with the Hagedorn
temperature and the Hawking-Page transition, some to do with the asymptotic
structure of AdS(3) and some with strong curvature effects, are analyzed and
explained.Comment: 40 pages, 5 figures, JHEP3 format. v2: added references, minor
corrections and clarification
Stochastic excitation of acoustic modes in stars
For more than ten years, solar-like oscillations have been detected and
frequencies measured for a growing number of stars with various characteristics
(e.g. different evolutionary stages, effective temperatures, gravities, metal
abundances ...).
Excitation of such oscillations is attributed to turbulent convection and
takes place in the uppermost part of the convective envelope. Since the
pioneering work of Goldreich & Keely (1977), more sophisticated theoretical
models of stochastic excitation were developed, which differ from each other
both by the way turbulent convection is modeled and by the assumed sources of
excitation. We review here these different models and their underlying
approximations and assumptions.
We emphasize how the computed mode excitation rates crucially depend on the
way turbulent convection is described but also on the stratification and the
metal abundance of the upper layers of the star. In turn we will show how the
seismic measurements collected so far allow us to infer properties of turbulent
convection in stars.Comment: Notes associated with a lecture given during the fall school
organized by the CNRS and held in St-Flour (France) 20-24 October 2008 ; 39
pages ; 11 figure
The indispensable role of the transversal spin fluctuations mechanism in laser-induced demagnetization of Co/Pt multilayers with nanoscale magnetic domains
The switching of magnetic domains induced by an ultrashort laser pulse has been demonstrated in nanostructured ferromagnetic films. This leads to the dawn of a new era in breaking the ultimate physical limit for the speed of magnetic switching and manipulation, which is relevant to current and future information storage. However, our understanding of the interactions between light and spins in magnetic heterostructures with nanoscale domain structures is still lacking. Here, both time-resolved magneto-optical Kerr effect experiments and atomistic simulations are carried out to investigate the dominant mechanism of laser-induced ultrafast demagnetization in [Co/Pt]20 multilayers with nanoscale magnetic domains. It is found that the ultrafast demagnetization time remains constant with various magnetic configurations, indicating that the domain structures play a minor role in laser-induced ultrafast demagnetization. In addition, both in experiment and atomistic simulations, we find a dependence of ultrafast demagnetization time Ï M on the laser fluence, which is in contrast to the observations of spin transport within magnetic domains. The remarkable agreement between experiment and atomistic simulations indicates that the local dissipation of spin angular momentum is the dominant demagnetization mechanism in this system. More interestingly, we made a comparison between the atomistic spin dynamic simulation and the longitudinal spin flip model, highlighting that the transversal spin fluctuations mechanism is responsible for the ultrafast demagnetization in the case of inhomogeneous magnetic structures. This is a significant advance in clarifying the microscopic mechanism underlying the process of ultrafast demagnetization in inhomogeneous magnetic structures
Grain Surface Models and Data for Astrochemistry
AbstractThe cross-disciplinary field of astrochemistry exists to understand the formation, destruction, and survival of molecules in astrophysical environments. Molecules in space are synthesized via a large variety of gas-phase reactions, and reactions on dust-grain surfaces, where the surface acts as a catalyst. A broad consensus has been reached in the astrochemistry community on how to suitably treat gas-phase processes in models, and also on how to present the necessary reaction data in databases; however, no such consensus has yet been reached for grain-surface processes. A team of âŒ25 experts covering observational, laboratory and theoretical (astro)chemistry met in summer of 2014 at the Lorentz Center in Leiden with the aim to provide solutions for this problem and to review the current state-of-the-art of grain surface models, both in terms of technical implementation into models as well as the most up-to-date information available from experiments and chemical computations. This review builds on the results of this workshop and gives an outlook for future directions
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