3,377 research outputs found
Separatrix Reconnections in Chaotic Regimes
In this paper we extend the concept of separatrix reconnection into chaotic
regimes. We show that even under chaotic conditions one can still understand
abrupt jumps of diffusive-like processes in the relevant phase-space in terms
of relatively smooth realignments of stable and unstable manifolds of unstable
fixed points.Comment: 4 pages, 5 figures, submitted do Phys. Rev. E (1998
Climate drives long-term change in Antarctic Silverfish along the western Antarctic Peninsula
Over the last half of the 20th century, the western Antarctic Peninsula has been one of the most rapidly warming regions on Earth, leading to substantial reductions in regional sea ice coverage. These changes are modulated by atmospheric forcing, including the Amundsen Sea Low (ASL) pressure system. We utilized a novel 25-year (1993–2017) time series to model the effects of environmental variability on larvae of a keystone species, the Antarctic Silverfish (Pleuragramma antarctica). Antarctic Silverfish use sea ice as spawning habitat and are important prey for penguins and other predators. We show that warmer sea surface temperature and decreased sea ice are associated with reduced larval abundance. Variability in the ASL modulates both sea surface temperature and sea ice; a strong ASL is associated with reduced larvae. These findings support a narrow sea ice and temperature tolerance for adult and larval fish. Further regional warming predicted to occur during the 21st century could displace populations of Antarctic Silverfish, altering this pelagic ecosystem
A First-Principles Approach to Insulators in Finite Electric Fields
We describe a method for computing the response of an insulator to a static,
homogeneous electric field. It consists of iteratively minimizing an electric
enthalpy functional expressed in terms of occupied Bloch-like states on a
uniform grid of k points. The functional has equivalent local minima below a
critical field E_c that depends inversely on the density of k points; the
disappearance of the minima at E_c signals the onset of Zener breakdown. We
illustrate the procedure by computing the piezoelectric and nonlinear
dielectric susceptibility tensors of III-V semiconductors.Comment: 4 pages, with 1 postscript figure embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/is_ef/index.htm
Serre's "formule de masse" in prime degree
For a local field F with finite residue field of characteristic p, we
describe completely the structure of the filtered F_p[G]-module K^*/K^*p in
characteristic 0 and $K^+/\wp(K^+) in characteristic p, where K=F(\root{p-1}\of
F^*) and G=\Gal(K|F). As an application, we give an elementary proof of Serre's
mass formula in degree p. We also determine the compositum C of all degree p
separable extensions with solvable galoisian closure over an arbitrary base
field, and show that C is K(\root p\of K^*) or K(\wp^{-1}(K)) respectively, in
the case of the local field F. Our method allows us to compute the contribution
of each character G\to\F_p^* to the degree p mass formula, and, for any given
group \Gamma, the contribution of those degree p separable extensions of F
whose galoisian closure has group \Gamma.Comment: 36 pages; most of the new material has been moved to the new Section
A data-driven energy platform: from energy performance certificates to human-readable knowledge through dynamic high-resolution geospatial maps
The energy performance certificate (EPC) is a document that certifies the average annual energy consumption of a building in standard conditions and allows it to be classified within a so-called energy class. In a period such as this, when greenhouse gas emissions are of considerable importance and where the objective is to improve energy security and reduce energy costs in our cities, energy certification has a key role to play. The proposed work aims to model and characterize residential buildings’ energy efficiency by exploring heterogeneous, geo-referenced data with different spatial and temporal granularity. The paper presents TUCANA (TUrin Certificates ANAlysis), an innovative data mining engine able to cover the whole analytics workflow for the analysis of the energy performance certificates, including cluster analysis and a model generalization step based on a novel spatial constrained K-NN, able to automatically characterize a broad set of buildings distributed across a major city and predict different energy-related features for new unseen buildings. The energy certificates analyzed in this work have been issued by the Piedmont Region (a northwest region of Italy) through open data. The results obtained on a large dataset are displayed in novel, dynamic, and interactive geospatial maps that can be consulted on a web application integrated into the system. The visualization tool provides transparent and human-readable knowledge to various stakeholders, thus supporting the decision-making process
Enhancement of piezoelectricity in a mixed ferroelectric
We use first-principles density-functional total energy and polarization
calculations to calculate the piezoelectric tensor at zero temperature for both
cubic and simple tetragonal ordered supercells of Pb_3GeTe_4. The largest
piezoelectric coefficient for the tetragonal configuration is enhanced by a
factor of about three with respect to that of the cubic configuration. This can
be attributed to both the larger strain-induced motion of cations relative to
anions and higher Born effective charges in the tetragonal case. A normal mode
decomposition shows that both cation ordering and local relaxation weaken the
ferroelectric instability, enhancing piezoelectricity.Comment: 5 pages, revtex, 2 eps figure
Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
Quantum ESPRESSO is an integrated suite of computer codes for
electronic-structure calculations and materials modeling, based on
density-functional theory, plane waves, and pseudopotentials (norm-conserving,
ultrasoft, and projector-augmented wave). Quantum ESPRESSO stands for "opEn
Source Package for Research in Electronic Structure, Simulation, and
Optimization". It is freely available to researchers around the world under the
terms of the GNU General Public License. Quantum ESPRESSO builds upon
newly-restructured electronic-structure codes that have been developed and
tested by some of the original authors of novel electronic-structure algorithms
and applied in the last twenty years by some of the leading materials modeling
groups worldwide. Innovation and efficiency are still its main focus, with
special attention paid to massively-parallel architectures, and a great effort
being devoted to user friendliness. Quantum ESPRESSO is evolving towards a
distribution of independent and inter-operable codes in the spirit of an
open-source project, where researchers active in the field of
electronic-structure calculations are encouraged to participate in the project
by contributing their own codes or by implementing their own ideas into
existing codes.Comment: 36 pages, 5 figures, resubmitted to J.Phys.: Condens. Matte
A Geometric Formulation of Quantum Stress Fields
We present a derivation of the stress field for an interacting quantum system
within the framework of local density functional theory. The formulation is
geometric in nature and exploits the relationship between the strain tensor
field and Riemannian metric tensor field. Within this formulation, we
demonstrate that the stress field is unique up to a single ambiguous parameter.
The ambiguity is due to the non-unique dependence of the kinetic energy on the
metric tensor. To illustrate this formalism, we compute the pressure field for
two phases of solid molecular hydrogen. Furthermore, we demonstrate that
qualitative results obtained by interpreting the hydrogen pressure field are
not influenced by the presence of the kinetic ambiguity.Comment: 22 pages, 2 figures. Submitted to Physical Review B. This paper
supersedes cond-mat/000627
Density-Polarization Functional Theory of the response of a periodic insulating solid to an electric field.
The response of an infinite, periodic, insulating, solid to an
infinitesimally small electric field is investigated in the framework of
Density Functional Theory. We find that the applied perturbing potential is not
a unique functional of the periodic density change~: it depends also on the
change in the macroscopic {\em polarization}. Moreover, the dependence of the
exchange-correlation energy on polarization induces an exchange-correlation
electric field. These effects are exhibited for a model semiconductor. We also
show that the scissor-operator technique is an approximate way of bypassing
this polarization dependence.Comment: 11 pages, 1 Fig
Evaluating the use of amber in palaeoatmospheric reconstructions: The carbon-isotope variability of modern and Cretaceous conifer resins.
Stable carbon-isotope geochemistry of fossilized tree resin (amber) potentially could be a very useful tool to infer the composition of past atmospheres. To test the reliability of amber as a proxy for the atmosphere, we studied the variability of modern resin d13C at both local and global scales. An amber d13C curve was then built for the Cretaceous, a period of abundant resin production, and interpreted in light of data from modern resins. Our data show that hardening changes the pristine d13C value by causing a 13C-depletion in solid resin when compared to fresh liquid-viscous resin, probably due to the loss of 13C-enriched volatiles. Modern resin d13C values vary as a function of physiological and environmental parameters in ways that are similar to those described for leaves and wood. Resin d13C varies between plant species and localities, within the same tree and between different plant tissues by up to 6Âż, and in general increases with increasing altitudes of the plant-growing site. We show that, as is the case with modern resin, Cretaceous amber d13C has a high variability, generally higher than that of other fossil material. Despite the high natural variability, amber shows a negative 2.5-3Âż d13C trend from the middle Early Cretaceous to the Maastrichtian that parallels published terrestrial d13C records. This trend mirrors changes in the atmospheric d13C calculated from the d13C and d18O of benthic foraminiferal tests, although the magnitude of the shift is larger in plant material than in the atmosphere. Increasing mean annual precipitation and pO2 could have enhanced plant carbon-isotope fractionation during the Late Cretaceous, whereas changing pCO2 levels seem to have had no effect on plant carbon-isotope fractionation. The results of this study suggest that amber is a powerful fossil plant material for palaeoenvironmental and palaeoclimatic reconstructions. Improvement of the resolution of the existing data coupled with more detailed information about botanical source and environmental growing conditions of the fossil plant material will probably allow a more faithful interpretation of amber d13C records and a wider understanding of the composition of the past atmosphere
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