27,883 research outputs found
The Vector-Tensor nature of Bekenstein's relativistic theory of Modified Gravity
Bekenstein's theory of relativistic gravity is conventionally written as a
bi-metric theory. The two metrics are related by a disformal transformation
defined by a dynamical vector field and a scalar field. In this comment we show
that the theory can be re-written as Vector-Tensor theory akin to
Einstein-Aether theories with non-canonical kinetic terms. We discuss some of
the implications of this equivalence.Comment: Updated version: Notation cleaned up and some typos corrected-TG
On the growth of structure in theories with a dynamical preferred frame
We study the cosmological stability of a class of theories with a dynamical
preferred frame. For a range of actions, we find cosmological solutions which
are compatible with observations of the recent history of the Universe: a
matter dominated era followed by accelerated expansion. We then study the
evolution of linear perturbations on these backgrounds and find conditions on
the parameters of the theory which allow for the growth of structure sourced by
the new degrees of freedom
Nanogold-based materials in medicine: from their origins to their future.
The properties of gold-based materials have been explored for centuries in several research fields, including medicine. Multiple published production methods for gold nanoparticles (AuNPs) have shown that the physicochemical and optical properties of AuNPs depend on the production method used. These different AuNP properties have allowed exploration of their usefulness in countless distinct biomedical applications over the last few years. Here we present an extensive overview of the most commonly used AuNP production methods, the resulting distinct properties of the AuNPs and the potential application of these AuNPs in diagnostic and therapeutic approaches in biomedicine
Vector field models of modified gravity and the dark sector
We present a comprehensive investigation of cosmological constraints on the
class of vector field formulations of modified gravity called Generalized
Einstein-Aether models. Using linear perturbation theory we generate cosmic
microwave background and large-scale structure spectra for general parameters
of the theory, and then constrain them in various ways. We investigate two
parameter regimes: a dark-matter candidate where the vector field sources
structure formation, and a dark-energy candidate where it causes late-time
acceleration. We find that the dark matter candidate does not fit the data, and
identify five physical problems that can restrict this and other theories of
dark matter. The dark energy candidate does fit the data, and we constrain its
fundamental parameters; most notably we find that the theory's kinetic index
parameter can differ significantly from its CDM
value.Comment: 16 pages, 11 figure
Modelling of epitaxial film growth with a Ehrlich-Schwoebel barrier dependent on the step height
The formation of mounded surfaces in epitaxial growth is attributed to the
presence of barriers against interlayer diffusion in the terrace edges, known
as Ehrlich-Schwoebel (ES) barriers. We investigate a model for epitaxial growth
using a ES barrier explicitly dependent on the step height. Our model has an
intrinsic topological step barrier even in the absence of an explicit ES
barrier. We show that mounded morphologies can be obtained even for a small
barrier while a self-affine growth, consistent with the Villain-Lai-Das Sarma
equation, is observed in absence of an explicit step barrier. The mounded
surfaces are described by a super-roughness dynamical scaling characterized by
locally smooth (faceted) surfaces and a global roughness exponent .
The thin film limit is featured by surfaces with self-assembled
three-dimensional structures having an aspect ratio (height/width) that may
increase or decrease with temperature depending on the strength of step
barrier.Comment: To appear in J. Phys. Cond. Matter; 3 movies as supplementary
materia
Kinetic modelling of epitaxial film growth with up- and downward step barriers
The formation of three-dimensional structures during the epitaxial growth of
films is associated to the reflection of diffusing particles in descending
terraces due to the presence of the so-called Ehrlich-Schwoebel (ES) barrier.
We generalize this concept in a solid-on-solid growth model, in which a barrier
dependent on the particle coordination (number of lateral bonds) exists
whenever the particle performs an interlayer diffusion. The rules do not
distinguish explicitly if the particle is executing a descending or an
ascending interlayer diffusion. We show that the usual model, with a step
barrier in descending steps, produces spurious, columnar, and highly unstable
morphologies if the growth temperature is varied in a usual range of mound
formation experiments. Our model generates well-behaved mounded morphologies
for the same ES barriers that produce anomalous morphologies in the standard
model. Moreover, mounds are also obtained when the step barrier has an equal
value for all particles independently if they are free or bonded. Kinetic
roughening is observed at long times, when the surface roughness w and the
characteristic length scale as and where
and , independently of the growth
temperature.Comment: 15 pages, 7 figure
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