121 research outputs found
The reduction of nitric-oxide by hydrogen over Pt, Rh and Pt-Rh single crystal surfaces
R.A. van Santen: question, p. 35
Numerical study of the one-dimensional quantum compass model
The ground state magnetic phase diagram of the one-dimensional quantum
compass model (QCM) is studied using the numerical Lanczos method. A detailed
numerical analysis of the low energy excitation spectrum is presented. The
energy gap and the spin-spin correlation functions are calculated for finite
chains. Two kind of the magnetic long-range orders, the Neel and a type of the
stripe-antiferromagnet, in the ground state phase diagram are identified. Based
on the numerical analysis, the first and second order quantum phase transitions
in the ground state phase diagram are identified.Comment: 6 pages, 8 figures. arXiv admin note: text overlap with
arXiv:1105.211
Massive scalar states localized on a de Sitter brane
We consider a brane scenario with a massive scalar field in the
five-dimensional bulk. We study the scalar states that are localized on the
brane, which is assumed to be de Sitter. These localized scalar modes are
massive in general, their effective four-dimensional mass depending on the mass
of the five-dimensional scalar field, on the Hubble parameter in the brane and
on the coupling between the brane tension and the bulk scalar field. We then
introduce a purely four-dimensional approach based on an effective potential
for the projection of the scalar field in the brane, and discuss its regime of
validity. Finally, we explore the quasi-localized scalar states, which have a
non-zero width that quantifies their probability of tunneling from the brane
into the bulk.Comment: 14 pages; 5 figure
Brane Universes with Gauss-Bonnet-Induced-Gravity
The DGP brane world model allows us to get the observed late time
acceleration via modified gravity, without the need for a ``dark energy''
field. This can then be generalised by the inclusion of high energy terms, in
the form of a Gauss-Bonnet bulk. This is the basis of the
Gauss-Bonnet-Induced-Gravity (GBIG) model explored here with both early and
late time modifications to the cosmological evolution. Recently the simplest
GBIG models (Minkowski bulk and no brane tension) have been analysed. Two of
the three possible branches in these models start with a finite density
``Big-Bang'' and with late time acceleration. Here we present a comprehensive
analysis of more general models where we include a bulk cosmological constant
and brane tension. We show that by including these factors it is possible to
have late time phantom behaviour.Comment: 12 pages, 19 figures. Minor modifications to text, comments on
phantom behaviour added. References added. As submitted to JCA
Bulk scalar field in the braneworld can mimic the 4D inflaton dynamics
Based on the recently proposed scenario of inflation driven by a bulk scalar
field in the braneworld of the Randall-Sundrum (RS) type, we investigate the
dynamics of a bulk scalar field on the inflating braneworld. We derive the late
time behavior of the bulk scalar field by analyzing the property of the
retarded Green function. We find that the late time behavior is basically
dominated by a single (or a pair of) pole(s) in the Green function irrespective
of the initial condition and of the signature of , where
is the potential of the bulk scalar field. Including the lowest order
back-reaction to the geometry, this late time behavior can be well approximated
by an effective 4-dimensional scalar field with . The
mapping to the 4-dimensional effective theory is given by a simple scaling of
the potential with a redefinition of the field. Our result supports the picture
that the scenario of inflation driven by a bulk scalar field works in a quite
similar way to that in the standard 4-dimensional cosmology.Comment: 12 pages, no figures, final version to be published in PR
Braneworld dynamics with the BraneCode
We give a full nonlinear numerical treatment of time-dependent 5d braneworld
geometry, which is determined self-consistently by potentials for the scalar
field in the bulk and at two orbifold branes, supplemented by boundary
conditions at the branes. We describe the BraneCode, an algorithm which we
designed to solve the dynamical equations numerically. We applied the BraneCode
to braneworld models and found several novel phenomena of the brane dynamics.
Starting with static warped geometry with de Sitter branes, we found
numerically that this configuration is often unstable due to a tachyonic mass
of the radion during inflation. If the model admits other static configurations
with lower values of de Sitter curvature, this effect causes a violent
re-structuring towards them, flattening the branes, which appears as a lowering
of the 4d effective cosmological constant. Braneworld dynamics can often lead
to brane collisions. We found that in the presence of the bulk scalar field,
the 5d geometry between colliding branes approaches a universal, homogeneous,
anisotropic strong gravity Kasner-like asymptotic, irrespective of the
bulk/brane potentials. The Kasner indices of the brane directions are equal to
each other but different from that of the extra dimension.Comment: 38 pages, 10 figure
Bulk gravitational field and dark radiation on the brane in dilatonic brane world
We discuss the connection between the dark radiation on the brane and the
bulk gravitational field in a dilatonic brane world model proposed by Koyama
and Takahashi where the exact solutions for the five dimensional cosmological
perturbations can be obtained analytically. It is shown that the dark radiation
perturbation is related to the non-normalizable Kaluza-Klein (KK) mode of the
bulk perturbations. For the de Sitter brane in the anti-de Sitter bulk, the
squared mass of this KK mode is where is the Hubble parameter on
the brane. This mode is shown to be connected to the excitation of small black
hole in the bulk in the long wavelength limit. The exact solution for an
anisotropic stress on the brane induced by this KK mode is found, which plays
an important role in the calculation of cosmic microwave background radiation
anisotropies in the brane world.Comment: 11 page
Active Galactic Nuclei at the Crossroads of Astrophysics
Over the last five decades, AGN studies have produced a number of spectacular
examples of synergies and multifaceted approaches in astrophysics. The field of
AGN research now spans the entire spectral range and covers more than twelve
orders of magnitude in the spatial and temporal domains. The next generation of
astrophysical facilities will open up new possibilities for AGN studies,
especially in the areas of high-resolution and high-fidelity imaging and
spectroscopy of nuclear regions in the X-ray, optical, and radio bands. These
studies will address in detail a number of critical issues in AGN research such
as processes in the immediate vicinity of supermassive black holes, physical
conditions of broad-line and narrow-line regions, formation and evolution of
accretion disks and relativistic outflows, and the connection between nuclear
activity and galaxy evolution.Comment: 16 pages, 5 figures; review contribution; "Exploring the Cosmic
Frontier: Astrophysical Instruments for the 21st Century", ESO Astrophysical
Symposia Serie
Stable, Time-Dependent, Exact Solutions for Brane Models with a Bulk Scalar Field
We derive two classes of brane-world solutions arising in the presence of a
bulk scalar field. For static field configurations, we adopt a time-dependent,
factorizable metric ansatz that allows for radion stabilization. The solutions
are characterized by a non-trivial warping along the extra dimension, even in
the case of a vanishing bulk cosmological constant, and lead to a variety of
inflationary, time-dependent solutions of the 3D scale factor on the brane. We
also derive the constraints necessary for the stability of these solutions
under time-dependent perturbations of the radion field, and we demonstrate the
existence of phenomenologically interesting, stable solutions with a positive
cosmological constant on the brane.Comment: 24 pages, latex, 4 eps figur
- âŠ