1,614 research outputs found
Modified Friedmann equations via conformal Bohm -- De Broglie gravity
We use an alternative interpretation of quantum mechanics, based on the
Bohmian trajectory approach, and show that the quantum effects can be included
in the classical equation of motion via a conformal transformation on the
background metric. We apply this method to the Robertson-Walker metric to
derive a modified version of Friedmann's equations for a Universe consisting of
scalar, spin-zero, massive particles. These modified equations include
additional terms that result from the non-local nature of matter and appear as
an acceleration in the expansion of the Universe. We see that the same effect
may also be present in the case of an inhomogeneous expansion.Comment: Accepted for publication in Ap
Axion-like-particle decay in strong electromagnetic backgrounds
The decay of a massive pseudoscalar, scalar and U(1) boson into an
electron-positron pair in the presence of strong electromagnetic backgrounds is
calculated. Of particular interest is the constant-crossed-field limit,
relevant for experiments that aim to measure high-energy axion-like-particle
conversion into electron-positron pairs in a magnetic field. The total
probability depends on the quantum nonlinearity parameter - a product of field
and lightfront momentum invariants. Depending on the seed particle mass,
different decay regimes are identified. In the below-threshold case, we find
the probability depends on a non-perturbative tunnelling exponent depending on
the quantum parameter and the particle mass. In the above-threshold case, we
find that when the quantum parameter is varied linearly, the probability
oscillates nonlinearly around the spontaneous decay probability. A strong-field
limit is identified in which the threshold is found to disappear. In modelling
the fall-off of a quasi-constant-crossed magnetic field, we calculate
probabilities beyond the constant limit and investigate when the decay
probability can be regarded as locally constant.Comment: 22 pages, 7 figure
R^2 Corrections and Non-perturbative Dualities of N=4 String ground states
We compute and analyse a variety of four-derivative gravitational terms in
the effective action of six- and four-dimensional type II string ground states
with N=4 supersymmetry. In six dimensions, we compute the relevant perturbative
corrections for the type II string compactified on K3. In four dimensions we do
analogous computations for several models with (4,0) and (2,2) supersymmetry.
Such ground states are related by heterotic-type II duality or type II-type II
U-duality. Perturbative computations in one member of a dual pair give a
non-perturbative result in the other member. In particular, the exact CP-even
R^2 coupling on the (2,2) side reproduces the tree-level term plus NS 5-brane
instanton contributions on the (4,0) side. On the other hand, the exact CP-odd
coupling yields the one-loop axionic interaction a.R\wedge R together with a
similar instanton sum. In a subset of models, the expected breaking of the
SL(2,Z)_S S-duality symmetry to a \Gamma(2)_S subgroup is observed on the
non-perturbative thresholds. Moreover, we present a duality chain that provides
evidence for the existence of heterotic N=4 models in which N=8 supersymmetry
appears at strong coupling.Comment: Latex2e, 51 pages, 1 figur
Fast nonadiabatic dynamics of many-body quantum systems
Modeling many-body quantum systems with strong interactions is one of the core challenges of modern physics. A range of methods has been developed to approach this task, each with its own idiosyncrasies, approximations, and realm of applicability. However, there remain many problems that are intractable for existing methods. In particular, many approaches face a huge computational barrier when modeling large numbers of coupled electrons and ions at finite temperature. Here, we address this shortfall with a new approach to modeling many-body quantum systems. On the basis of the Bohmian trajectory formalism, our new method treats the full particle dynamics with a considerable increase in computational speed. As a result, we are able to perform large-scale simulations of coupled electron-ion systems without using the adiabatic Born-Oppenheimer approximation
Digital Storytelling as a genre of mediatized self-representations: an introduction
Este artĂculo ofrece una revisiĂłn crĂtica de algunos de los estudios más relevantes sobre relato digital y propone una tipologĂa de gĂ©nero que permite una clasificaciĂłn inicial en dos grandes tipos: educativo y social. El argumento principal es que la diferenciaciĂłn entre lo social y educativo, aunque uno no excluye al otro, ofrece la posibilidad de clasificar la producciĂłn masiva de relatos digitales disponibles hoy en dĂa en Internet. Los artĂculos incluidos en este nĂşmero son, en su mayorĂa, educativos (RamĂrez-Verdugo y Grande Sotomayor, y Reyes, Pich & GarcĂa, Londoño-Godoy), aunque en todos ellos todos ellos incluyen algunos rasgo de lo social. Bou-Franch nos habla de las estrategias utilizadas por los estudiantes en la interpretaciĂłn de acontecimientos histĂłricos que tuvieron impacto social; mientras que el artĂculo de Westman conlleva la creaciĂłn de comunidades de práctica entre aquellos que comparten los mismos intereses. Por Ăşltimo, para Herreros Navarro el relato digital es un acto social en el que los estudiantes deben decidir como quieren que la sociedad les vea ya que describen y exploran su "o" a travĂ©s del relato digital. This article provides a critical review of some of the most relevant studies on digital storytelling and proposes a genre typology that allows an initial classification of digital storytelling into two main types: educational and social. Digital storytelling is a multimodal emergent genre characterised by its versatility and flexibility which has resulted in a series of subgenres. However, the main premise here is that differentiating between social and educational- although one does not exclude the other- and bearing in mind that most digital stories may lie at the intersection of both, is the most useful way to start labeling the massive production of digital stories available nowadays on the Internet. The articles included in this number are mostly educational (RamĂrez-Verdugo & Sotomayor Grande, and Reyes, Pich & GarcĂa, Londoño-Monroy) but they all include some traces of the social type. Thus, Bou-Franch is an example of how students interpret certain events that had social impact and that are part of history while Westman's article involves the creation of communities of practice among those who share the same interests. Finally, Herreros-Navarro, although educational in essence, describes a social act in which students intentionally choose a way to present their own identity to society using digital storytelling
Complexation of halide ions to tyrosine: role of non-covalent interactions evidenced by IRMPD spectroscopy
The binding motifs in the halide adducts with tyrosine ([Tyr + X]-, X = Cl, Br, I) have been investigated
and compared with the analogues with 3-nitrotyrosine (nitroTyr), a biomarker of protein nitration, in a
solvent-free environment by mass-selected infrared multiple photon dissociation (IRMPD) spectroscopy
over two IR frequency ranges, namely 950–1950 and 2800–3700 cm-1. Extensive quantum chemical
calculations at B3LYP, B3LYP-D3 and MP2 levels of theory have been performed using the 6-311++G(d,p)
basis set to determine the geometry, relative energy and vibrational properties of likely isomers and
interpret the measured spectra. A diagnostic carbonyl stretching band at B1720 cm-1 from the intact
carboxylic group characterizes the IRMPD spectra of both [Tyr + X]- and [nitroTyr + X]-, revealing that
the canonical isomers (maintaining intact amino and carboxylic functions) are the prevalent structures.
The spectroscopic evidence reveals the presence of multiple non-covalent forms. The halide complexes
of tyrosine conform to a mixture of plane and phenol isomers. The contribution of phenol-bound
isomers is sensitive to anion size, increasing from chloride to iodide, consistent with the decreasing
basicity of the halide, with relative amounts depending on the relative energies of the respective
structures. The stability of the most favorable phenol isomer with respect to the reference plane
geometry is in fact 1.3, -2.1, -6.8 kJ mol-1, for X = Cl, Br, I, respectively. The change in p-acidity by ring
nitration also stabilizes anion–p interactions yielding ring isomers for [nitroTyr + X]-, where the anion is
placed above the face of the aromatic ring
Axion particle production in a laser-induced dynamical spacetime
Abstract We consider the dynamics of a charged particle (e.g., an electron) oscillating in a laser field in flat spacetime and describe it in terms of the variable mass metric. By applying Einstein's equivalence principle, we show that, after representing the electron motion in a time-dependent manner, the variable mass metric takes the form of the Friedmann–Lemaître–Robertson–Walker metric. We quantize a pseudo-scalar field in this spacetime and derive the production rate of electrically neutral, spinless particles. We show that this approach can provide an alternative experimental method to axion searches
Robust numerical methods for boundary-layer equations for a model problem of flow over a symmetric curved surface
We investigate the model problem of flow of a viscous incompressible fluid past a symmetric curved surface when the flow is parallel to its axis. This problem is known to exhibit boundary layers. Also the problem does not have solutions in closed form, it is modelled by boundary-layer equations. Using a self-similar approach based on a Blasius series expansion (up to three terms), the boundary-layer equations can be reduced to a Blasius-type problem consisting of a system of eight third-order ordinary differential equations on a semi-infinite interval. Numerical methods need to be employed to attain the solutions of these equations and their derivatives, which are required for the computation of the velocity components, on a finite domain with accuracy independent of the viscosity, which can take arbitrary values from the interval (0, l]. To construct a robust numerical method we reduce the original problem on a semi-infinite axis to a problem on the finite interval [O,K], where K = K(N) = ln N. Employing numerical experiments we justify that the constructed numerical method is parameter robust
Plastic Deformation in Laser-Induced Shock Compression of Monocrystalline Copper
Copper monocrystals were subjected to shock compression at pressures of 10–60 GPa by a short (3 ns initial) duration laser pulse. Transmission electron microscopy revealed features consistent with previous observations of shock-compressed copper, albeit at pulse durations in the µs regime. The results suggest that the defect structure is generated at the shock front. A mechanism for dislocation generation is presented, providing a realistic prediction of dislocation density as a function of pressure. The threshold stress for deformation twinning in shock compression is calculated from the constitutive equations for slip, twinning, and the Swegle-Grady relationship
XUV Opacity of Aluminum between the Cold-Solid to Warm-Plasma Transition
We present calculations of the free-free XUV opacity of warm, solid-density
aluminum at photon energies between the plasma frequency at 15 eV and the
L-edge at 73 eV, using both density functional theory combined with molecular
dynamics and a semi-analytical model in the RPA framework with the inclusion of
local field corrections. As the temperature is increased from room temperature
to 10 eV, with the ion and electron temperatures equal, we calculate an
increase in the opacity in the range over which the degree of ionization is
constant. The effect is less pronounced if only the electron temperature is
allowed to increase. The physical significance of these increases is discussed
in terms of intense XUV-laser matter interactions on both femtosecond and
picosecond time-scales.Comment: 4 pages, 3 figure
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