22,004 research outputs found
New insights on the matter-gravity coupling paradigm
The coupling between matter and gravity in General Relativity is given by a
proportionality relation between the stress tensor and the geometry. This is an
oriented assumption driven by the fact that both the stress tensor and the
Einstein tensor are divergenceless. However, General Relativity is in essence a
nonlinear theory, so there is no obvious reason why the coupling to matter
should be linear. On another hand, modified theories of gravity usually affect
the vacuum dynamics, yet keep the coupling to matter linear. In this Letter we
address the implications of consistent nonlinear gravity/matter coupling. The
Eddington inspired Born-Infeld theory recently introduced by Banados and
Ferreira provides an enlightening realization of such coupling modifications.
We find that this theory coupled to a perfect fluid reduces to General
Relativity coupled to a nonlinearly modified perfect fluid, leading to an
ambiguity between modified coupling and modified equation of state. We discuss
observational consequences of this degeneracy and argue that such a completion
of General Relativity is viable from both an experimental and theoretical point
of view through energy conditions, consistency, and singularity-avoidance
perspectives. We use these results to discuss the impact of changing the
coupling paradigm.Comment: 6 pages, 2 figures, v2: revised version, v3: published versio
The Importance of Material Flow Analysis for Commodity Transport Demand and Modelling
It can be shown that generated and attracted transport volumes, measured in tons, are closely related to direct material input (DMI). However, structural changes and new logistics concepts still lead to an increase of transportation performance. Therefore, the paper at hand aims to explain the scales of freight transport volumes (measured in tons) and performance (measured in ton-kilometers) from material flow analysis by additionally taking into account information from physical input-output tables. In so doing, effects of changing final demand on transport indicators can be identified. But while input-output tables give a good idea about technological processes, important information on the transport chain is missing. For this reason, the macroscopic approach of input-output analysis is supported by a microscopic analysis on freight transport markets and modern logistic concepts.
Uniqueness of infrared asymptotics in Landau gauge Yang-Mills theory
We uniquely determine the infrared asymptotics of Green functions in Landau
gauge Yang-Mills theory. They have to satisfy both,
Dyson-Schwinger equations and functional renormalisation group equations.
Then, consistency fixes the relation between the infrared power laws of these
Green functions. We discuss consequences for the interpretation of recent
results from lattice QCD.Comment: 24 pages, 8 figure
Quantitative Study of Magnetotransport through a (Ga,Mn)As Single Ferromagnetic Domain
We have performed a systematic investigation of the longitudinal and
transverse magnetoresistance of a single ferromagnetic domain in (Ga,Mn)As. We
find that, by taking into account the intrinsic dependence of the resistivity
on the magnetic induction, an excellent agreement between experimental results
and theoretical expectations is obtained. Our findings provide a detailed and
fully quantitative validation of the theoretical description of
magnetotransport through a single ferromagnetic domain. Our analysis
furthermore indicates the relevance of magneto-impurity scattering as a
mechanism for magnetoresistance in (Ga,Mn)As.Comment: 5 pages, 4 figures; v2: missing references included, figures
recompressed to improve readabilit
Microstructural enrichment functions based on stochastic Wang tilings
This paper presents an approach to constructing microstructural enrichment
functions to local fields in non-periodic heterogeneous materials with
applications in Partition of Unity and Hybrid Finite Element schemes. It is
based on a concept of aperiodic tilings by the Wang tiles, designed to produce
microstructures morphologically similar to original media and enrichment
functions that satisfy the underlying governing equations. An appealing feature
of this approach is that the enrichment functions are defined only on a small
set of square tiles and extended to larger domains by an inexpensive stochastic
tiling algorithm in a non-periodic manner. Feasibility of the proposed
methodology is demonstrated on constructions of stress enrichment functions for
two-dimensional mono-disperse particulate media.Comment: 27 pages, 12 figures; v2: completely re-written after the first
revie
Journal Staff
The aluminum–zinc-vacancy (Al Zn −V Zn ) complex is identified as one of the dominant defects in Al-containing n -type ZnO after electron irradiation at room temperature with energies above 0.8 MeV. The complex is energetically favorable over the isolated V Zn , binding more than 90% of the stable V Zn ’s generated by the irradiation. It acts as a deep acceptor with the (0/− ) energy level located at approximately 1 eV above the valence band. Such a complex is concluded to be a defect of crucial and general importance that limits the n -type doping efficiency by complex formation with donors, thereby literally removing the donors, as well as by charge compensation
Structure and elastic properties of Mg(OH) from density functional theory
The structure, lattice dynamics and mechanical properties of the magnesium
hydroxide have been investigated with static density functional theory
calculations as well as \it {ab initio} molecular dynamics. The hypothesis of a
superstructure existing in the lattice formed by the hydrogen atoms has been
tested. The elastic constants of the material have been calculated with static
deformations approach and are in fair agreement with the experimental data. The
hydrogen subsystem structure exhibits signs of disordered behaviour while
maintaining correlations between angular positions of neighbouring atoms. We
establish that the essential angular correlations between hydrogen positions
are maintained to the temperature of at least 150 K and show that they are well
described by a physically motivated probabilistic model. The rotational degree
of freedom appears to be decoupled from the lattice directions above 30K
Time-dependent density functional theory: Past, present, and future
Time-dependent density functional theory (TDDFT) is presently enjoying
enormous popularity in quantum chemistry, as a useful tool for extracting
electronic excited state energies. This article discusses how TDDFT is much
broader in scope, and yields predictions for many more properties. We discuss
some of the challenges involved in making accurate predictions for these
properties.Comment: 12 pages, 4 figure
AVIRIS ground data-processing system
The Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) has been under development at JPL for the past four years. During this time, a dedicated ground data-processing system has been designed and implemented to store and process the large amounts of data expected. This paper reviews the objectives of this ground data-processing system and describes the hardware. An outline of the data flow through the system is given, and the software and incorporated algorithms developed specifically for the systematic processing of AVIRIS data are described
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