3,053 research outputs found
A Provably Stable Discontinuous Galerkin Spectral Element Approximation for Moving Hexahedral Meshes
We design a novel provably stable discontinuous Galerkin spectral element
(DGSEM) approximation to solve systems of conservation laws on moving domains.
To incorporate the motion of the domain, we use an arbitrary
Lagrangian-Eulerian formulation to map the governing equations to a fixed
reference domain. The approximation is made stable by a discretization of a
skew-symmetric formulation of the problem. We prove that the discrete
approximation is stable, conservative and, for constant coefficient problems,
maintains the free-stream preservation property. We also provide details on how
to add the new skew-symmetric ALE approximation to an existing discontinuous
Galerkin spectral element code. Lastly, we provide numerical support of the
theoretical results
Orbital magnetization in crystalline solids: Multi-band insulators, Chern insulators, and metals
We derive a multi-band formulation of the orbital magnetization in a normal
periodic insulator (i.e., one in which the Chern invariant, or in 2d the Chern
number, vanishes). Following the approach used recently to develop the
single-band formalism [T. Thonhauser, D. Ceresoli, D. Vanderbilt, and R. Resta,
Phys. Rev. Lett. {\bf 95}, 137205 (2005)], we work in the Wannier
representation and find that the magnetization is comprised of two
contributions, an obvious one associated with the internal circulation of
bulk-like Wannier functions in the interior and an unexpected one arising from
net currents carried by Wannier functions near the surface. Unlike the
single-band case, where each of these contributions is separately
gauge-invariant, in the multi-band formulation only the \emph{sum} of both
terms is gauge-invariant. Our final expression for the orbital magnetization
can be rewritten as a bulk property in terms of Bloch functions, making it
simple to implement in modern code packages. The reciprocal-space expression is
evaluated for 2d model systems and the results are verified by comparing to the
magnetization computed for finite samples cut from the bulk. Finally, while our
formal proof is limited to normal insulators, we also present a heuristic
extension to Chern insulators (having nonzero Chern invariant) and to metals.
The validity of this extension is again tested by comparing to the
magnetization of finite samples cut from the bulk for 2d model systems. We find
excellent agreement, thus providing strong empirical evidence in favor of the
validity of the heuristic formula.Comment: 14 pages, 8 figures. Fixed a typo in appendix
Experimental Demonstration of Greenberger-Horne-Zeilinger Correlations Using Nuclear Magnetic Resonance
The Greenberger-Horne-Zeilinger (GHZ) effect provides an example of quantum
correlations that cannot be explained by classical local hidden variables. This
paper reports on the experimental realization of GHZ correlations using nuclear
magnetic resonance (NMR). The NMR experiment differs from the originally
proposed GHZ experiment in several ways: it is performed on mixed states rather
than pure states; and instead of being widely separated, the spins on which it
is performed are all located in the same molecule. As a result, the NMR version
of the GHZ experiment cannot entirely rule out classical local hidden
variables. It nonetheless provides an unambiguous demonstration of the
"paradoxical" GHZ correlations, and shows that any classical hidden variables
must communicate by non-standard and previously undetected forces. The NMR
demonstration of GHZ correlations shows the power of NMR quantum information
processing techniques for demonstrating fundamental effects in quantum
mechanics.Comment: Latex2.09, 8 pages, 1 eps figur
Neutral Higgs-pair production at Linear Colliders within the general 2HDM: quantum effects and triple Higgs boson self-interactions
The pairwise production of neutral Higgs bosons is analyzed in the context of
the future linear colliders, such as the ILC and CLIC, within the general
Two-Higgs-Doublet Model (2HDM). The corresponding cross-sections are computed
at the one-loop level in full compliance with the current phenomenological
bounds and the stringent theoretical constraints inherent to the consistency of
the model. We uncover regions across the 2HDM parameter space, mainly for low
tan\beta near 1 and moderate values of the relevant lambda_5 parameter, wherein
the radiative corrections to the Higgs-pair production cross sections can
comfortably reach 50% This behavior can be traced back to the enhancement
capabilities of the trilinear Higgs self-interactions -- a trademark feature of
the 2HDM, with no counterpart in the Minimal Supersymmetric Standard Model.
Interestingly enough, the quantum effects are positive for energies around 500
GeV, thereby producing a significant enhancement in the expected number of
events precisely around the fiducial startup energy of the ILC. The Higgs-pair
production rates can be substantial, typically amounting to a few thousand
events per 500 inverse femtobarn of integrated luminosity. In contrast, the
corrections are negative in the highest energy range (1 TeV). We also compare
the exclusive pairwise production of Higgs bosons with the inclusive gauge
boson fusion channels leading to 2H+X finals states, and also with the
exclusive triple Higgs boson production. We find that these multiparticle final
states can be highly complementary in the overall Higgs bosons search strategy.Comment: 42 pages, 23 figures, 10 tables. Accepted in Phys. Rev. D (the
published version is shorter
X-ray Emission from Young Stellar Objects in the \epsilon Chamaeleontis Group: the Herbig Ae Star HD 104237 and Associated Low-Mass Stars
We present Chandra-HETGS observations of the Herbig Ae star HD 104237 and the
associated young stars comprising lower mass stars, in the 0.15-1.75\msol mass
range, in their pre-main sequence phase. The brightest X-ray source in the
association is the central system harboring the Herbig Ae primary, and a K3
companion. Its X-ray variability indicates modulation possibly on time scales
of the rotation period of the Herbig Ae star, and this would imply that the
primary significantly contributes to the overall emission. The spectrum of the
Herbig Ae+K3 system shows a soft component significantly more pronounced than
in other K-type young stars. This soft emission is reminiscent of the unusually
soft spectra observed for the single Herbig Ae stars HD 163296 and AB Aur, and
therefore we tentatively attribute it to the Herbig Ae of the binary system.
The HETGS spectrum shows strong emission lines corresponding to a wide range of
plasma temperatures. The He-like triplet of MgXI and NeIX suggest the presence
of plasma at densities of about cm, possibly indicating
accretion related X-ray production mechanism.
The analysis of the zero-order spectra of the other sources indicates X-ray
emission characteristics typical of pre-main sequence stars of similar spectral
type, with the exception of the T Tauri HD104237-D, whose extremely soft
emission is very similar to the emission of the classical T Tauri star TW Hya,
and suggests X-ray production by shocked accreting plasma.Comment: accepted for publication on the Astrophysical Journa
Two-Dimensional Hydrodynamics of Pre-Core Collapse: Oxygen Shell Burning
By direct hydrodynamic simulation, using the Piecewise Parabolic Method (PPM)
code PROMETHEUS, we study the properties of a convective oxygen burning shell
in a SN 1987A progenitor star prior to collapse. The convection is too
heterogeneous and dynamic to be well approximated by one-dimensional
diffusion-like algorithms which have previously been used for this epoch.
Qualitatively new phenomena are seen.
The simulations are two-dimensional, with good resolution in radius and
angle, and use a large (90-degree) slice centered at the equator. The
microphysics and the initial model were carefully treated. Many of the
qualitative features of previous multi-dimensional simulations of convection
are seen, including large kinetic and acoustic energy fluxes, which are not
accounted for by mixing length theory. Small but significant amounts of
carbon-12 are mixed non-uniformly into the oxygen burning convection zone,
resulting in hot spots of nuclear energy production which are more than an
order of magnitude more energetic than the oxygen flame itself. Density
perturbations (up to 8%) occur at the `edges' of the convective zone and are
the result of gravity waves generated by interaction of penetrating flows into
the stable region. Perturbations of temperature and electron fraction at the
base of the convective zone are of sufficient magnitude to create angular
inhomogeneities in explosive nucleosynthesis products, and need to be included
in quantitative estimates of yields. Combined with the plume-like velocity
structure arising from convection, the perturbations will contribute to the
mixing of nickel-56 throughout supernovae envelopes. Runs of different
resolution, and angular extent, were performed to test the robustness of theseComment: For mpeg movies of these simulations, see
http://www.astrophysics.arizona.edu/movies.html Submitted to the
Astrophysical Journa
The Consistency of Causal Quantum Geometrodynamics and Quantum Field Theory
We consider quantum geometrodynamics and parametrized quantum field theories
in the framework of the Bohm-de Broglie interpretation. In the first case, and
following the lines of our previous work [1], where a hamiltonian formalism for
the bohmian trajectories was constructed, we show the consistency of the theory
for any quantum potential, completing the scenarios for canonical quantum
cosmology presented there. In the latter case, we prove the consistency of
scalar field theory in Minkowski spacetime for any quantum potential, and we
show, using this alternative hamiltonian method, a concrete example where
Lorentz invariance of individual events is broken.Comment: Final version. See also http://www.cosmologia.cbpf.b
A back-to-front derivation: the equal spacing of quantum levels is a proof of simple harmonic oscillator physics
The dynamical behaviour of simple harmonic motion can be found in numerous natural phenomena. Within the quantum realm of atomic, molecular and optical systems, two main features are associated with harmonic oscillations: a finite ground-state energy and equally spaced quantum energy levels. Here it is shown that there is in fact a one-to-one mapping between the provision of equally spaced quantum energy levels and simple harmonic motion. The analysis establishes that the Hamiltonian of any system featuring quantized energy levels in an evenly spaced, infinite set must have a quadratic dependence on a pair of canonically conjugate variables. Moreover, specific physical inferences can be drawn. For example, exploiting this 'back-to-front' derivation, and based on the known existence of photons, it can be proved that an electromagnetic energy density is quadratic in both the electric and magnetic fields
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