13,527 research outputs found
Tri-layer superlattices: A route to magnetoelectric multiferroics?
We explore computationally the formation of tri-layer superlattices as an
alternative approach for combining ferroelectricity with magnetism to form
magnetoelectric multiferroics. We find that the contribution to the
superlattice polarization from tri-layering is small compared to typical
polarizations in conventionalferroelectrics, and the switchable ferroelectric
component is negligible. In contrast, we show that epitaxial strain and
``negative pressure'' can yield large, switchable polarizations that are
compatible with the coexistence of magnetism, even in materials with no active
ferroelectric ions.Comment: 10 pages, 3 figures; references added, and minor editorial changes
mad
Variational approach to transport in quantum dots
We have derived a variational principle that defines the nonequilibrium
steady-state transport across a correlated impurity mimicking, e.g., a quantum
dot coupled to biased leads. This variational principle has been specialized to
a Gutzwiller's variational space, and applied to the study of the simple
single-orbital Anderson impurity model at half filling, finding a good
qualitative accord with the observed behavior in quantum dots for the expected
regime of values of the bias. Beyond the purely theoretical interest in the
formal definition of a variational principle in a nonequilibrium problem, the
particular methods proposed have the important advantage to be simple and
flexible enough to deal with more complicated systems and variational spaces.Comment: 15 pages, 4 figure
X-Ray Evolution of Active Galactic Nuclei and Hierarchical Galaxy Formation
We have incorporated the description of the X-ray properties of Active
Galactic Nuclei (AGNs) into a semi-analytic model of galaxy formation, adopting
physically motivated scaling laws for accretion triggered by galaxy encounters.
Our model reproduces the level of the cosmic X-ray background at 30 keV; we
predict that the largest contribution (around 2/3) comes from sources with
intermediate X-ray luminosity 10^{43.5}< L_X/erg/s <10^{44.5}, with 50 % of the
total specific intensity produced at z<2. The predicted number density of
luminous X-ray AGNs (L_X>10^{44.5} erg/s in the 2-10 keV band) peaks at z
around 2 with a decline of around 3 dex to z=0; for the low luminosity sources
(10^{43}<L_X/erg/s <10^{44}) it has a broaderand less pronounced maximum around
z 1.5. The comparison with the data shows a generally good agreement. The model
predictions slightly exceed the observed number of low-luminosity AGNs at z
around 1.5, with the discrepancy progressively extending to
intermediate-luminosity objects at higher redshifts; we discuss possible
origins for the mismatch. Finally, we predict the source counts and the flux
distribution at different redshifts in the hard (20-100 keV) X-ray band for the
sources contributing to the X- ray background.Comment: 27 pages, accepted for publication in Ap
Neutral pion decay in dense skyrmion matter
We study the density dependence of the decay using
the Skyrme Lagrangian to describe simultaneously both the matter background and
mesonic fluctuations. Pion properties such as mass and decay constant are
modified by the medium. This leads to large suppression at high density of both
photo-production from the neutral pion and the reverse process. The in-medium
effective charge of are also discussed in the same framework.Comment: 8 pages, 4 figures. Corrections in light of referee comment
Regional diversity in the murine cortical vascular network is revealed by synchrotron X-ray tomography and is amplified with age
Cortical bone is permeated by a system of pores, occupied by the blood supply and osteocytes. With ageing, bone mass reduction and disruption of the microstructure are associated with reduced vascular supply. Insight into the regulation of the blood supply to the bone could enhance the understanding of bone strength determinants and fracture healing. Using synchrotron radiation-based computed tomography, the distribution of vascular canals and osteocyte lacunae was assessed in murine cortical bone and the influence of age on these parameters was investigated. The tibiofibular junction from 15-week- and 10-month-old female C57BL/6J mice were imaged post-mortem. Vascular canals and three-dimensional spatial relationships between osteocyte lacunae and bone surfaces were computed for both age groups. At 15 weeks, the posterior region of the tibiofibular junction had a higher vascular canal volume density than the anterior, lateral and medial regions. Intracortical vascular networks in anterior and posterior regions were also different, with connectedness in the posterior higher than the anterior at 15 weeks. By 10 months, cortices were thinner, with cortical area fraction and vascular density reduced, but only in the posterior cortex. This provided the first evidence of age-related effects on murine bone porosity due to the location of the intracortical vasculature. Targeting the vasculature to modulate bone porosity could provide an effective way to treat degenerative bone diseases, such as osteoporosis
How large are present-day heat flux variations across the surface of Mars?
©2016. American Geophysical UnionThe first in situ Martian heat flux measurement to be carried out by the InSight Discovery‐class mission will provide an important baseline to constrain the present‐day heat budget of the planet and, in turn, the thermochemical evolution of its interior. In this study, we estimate the magnitude of surface heat flux heterogeneities in order to assess how the heat flux at the InSight landing site relates to the average heat flux of Mars. To this end, we model the thermal evolution of Mars in a 3‐D spherical geometry and investigate the resulting surface spatial variations of heat flux at the present day. Our models assume a fixed crust with a variable thickness as inferred from gravity and topography data and with radiogenic heat sources as obtained from gamma ray measurements of the surface. We test several mantle parameters and show that the present‐day surface heat flux pattern is dominated by the imposed crustal structure. The largest surface heat flux peak‐to peak variations lie between 17.2 and 49.9 mW m−2, with the highest values being associated with the occurrence of prominent mantle plumes. However, strong spatial variations introduced by such plumes remain narrowly confined to a few geographical regions and are unlikely to bias the InSight heat flux measurement. We estimated that the average surface heat flux varies between 23.2 and 27.3 mW m−2, while at the InSight location it lies between 18.8 and 24.2 mW m−2. In most models, elastic lithosphere thickness values exceed 250 km at the north pole, while the south pole values lie well above 110 km
Bioaccumulation modelling and sensitivity analysis for discovering key players in contaminated food webs: the case study of PCBs in the Adriatic Sea
Modelling bioaccumulation processes at the food web level is the main step to analyse the effects of pollutants at the global
ecosystem level. A crucial question is understanding which species play a key role in the trophic transfer of contaminants to
disclose the contribution of feeding linkages and the importance of trophic dependencies in bioaccumulation dynamics. In this
work we present a computational framework to model the bioaccumulation of organic chemicals in aquatic food webs, and to
discover key species in polluted ecosystems. As a result, we reconstruct the first PCBs bioaccumulation model of the Adriatic food
web, estimated after an extensive review of published concentration data. We define a novel index aimed to identify the key species
in contaminated networks, Sensitivity Centrality, and based on sensitivity analysis. The index is computed from a dynamic ODE
model parametrised from the estimated PCBs bioaccumulation model and compared with a set of established trophic indices of
centrality. Results evidence the occurrence of PCBs biomagnification in the Adriatic food web, and highlight the dependence of
bioaccumulation on trophic dynamics and external factors like fishing activity. We demonstrate the effectiveness of the introduced
Sensitivity Centrality in identifying the set of species with the highest impact on the total contaminant flows and on the efficiency
of contaminant transport within the food web
Variational approach to gravitational theories with two independent connections
A new variational approach for general relativity and modified theories of
gravity is presented. In addition to the metric tensor, two independent affine
connections enter the action as dynamical variables. In the matter action the
dependence upon one of the connections is left completely unspecified. When the
variation is applied to the Einstein-Hilbert action the Einstein field
equations are recovered. However when applied to and Scalar-Tensor
theories, it yields gravitational field equations which differ from their
equivalents obtained with a metric or Palatini variation and reduce to the
former ones only when no connections appear in the matter action.Comment: 11 pages, no figure
Density functional theory in transition-metal chemistry: a self-consistent Hubbard U approach
Transition-metal centers are the active sites for many biological and
inorganic chemical reactions. Notwithstanding this central importance,
density-functional theory calculations based on generalized-gradient
approximations often fail to describe energetics, multiplet structures,
reaction barriers, and geometries around the active sites. We suggest here an
alternative approach, derived from the Hubbard U correction to solid-state
problems, that provides an excellent agreement with correlated-electron quantum
chemistry calculations in test cases that range from the ground state of Fe
and Fe to the addition-elimination of molecular hydrogen on FeO. The
Hubbard U is determined with a novel self-consistent procedure based on a
linear-response approach.Comment: 5 pages, 3 figures, Phys. Rev. Lett., in pres
- …