2,256 research outputs found
van der Waals density functionals built upon the electron-gas tradition: Facing the challenge of competing interactions
The theoretical description of sparse matter attracts much interest, in
particular for those ground-state properties that can be described by density
functional theory (DFT). One proposed approach, the van der Waals density
functional (vdW-DF) method, rests on strong physical foundations and offers
simple yet accurate and robust functionals. A very recent functional within
this method called vdW-DF-cx [K. Berland and P. Hyldgaard, Phys. Rev. B 89,
035412] stands out in its attempt to use an exchange energy derived from the
same plasmon-based theory from which the nonlocal correlation energy was
derived. Encouraged by its good performance for solids, layered materials, and
aromatic molecules, we apply it to several systems that are characterized by
competing interactions. These include the ferroelectric response in PbTiO,
the adsorption of small molecules within metal-organic frameworks (MOFs), the
graphite/diamond phase transition, and the adsorption of an aromatic-molecule
on the Ag(111) surface. Our results indicate that vdW-DF-cx is overall well
suited to tackle these challenging systems. In addition to being a competitive
density functional for sparse matter, the vdW-DF-cx construction presents a
more robust general purpose functional that could be applied to a range of
materials problems with a variety of competing interactions
From electronic structure to catalytic activity: A single descriptor for adsorption and reactivity on transition-metal carbides
Adsorption and catalytic properties of the polar (111) surface of
transition-metal carbides (TMC's) are investigated by density-functional
theory. Atomic and molecular adsorption are rationalized with the
concerted-coupling model, in which two types of TMC surface resonances (SR's)
play key roles. The transition-metal derived SR is found to be a single
measurable descriptor for the adsorption processes, implying that the
Br{\o}nsted-Evans-Polanyi relation and scaling relations apply. This gives a
picture with implications for ligand and vacancy effects and which has a
potential for a broad screening procedure for heterogeneous catalysts.Comment: 5 pages, 3 figure
Isospin phases of vertically coupled double quantum rings under the influence of perpendicular magnetic fields
Vertically coupled double quantum rings submitted to a perpendicular magnetic
field are addressed within the local spin-density functional theory. We
describe the structure of quantum ring molecules containing up to 40 electrons
considering different inter-ring distances and intensities of the applied
magnetic field. When the rings are quantum mechanically strongly coupled, only
bonding states are occupied and the addition spectrum of the artificial
molecules resembles that of a single quantum ring, with some small differences
appearing as an effect of the magnetic field. Despite the latter has the
tendency to flatten the spectra, in the strong coupling limit some clear peaks
are still found even when that can be interpretated from the
single-particle energy levels analogously as at zero applied field, namely in
terms of closed-shell and Hund's-rule configurations. Increasing the inter-ring
distance, the occupation of the first antibonding orbitals washes out such
structures and the addition spectra become flatter and irregular. In the weak
coupling regime, numerous isospin oscillations are found as a function of .Comment: 27 pages, 11 figures. To be published in Phys. Rev.
Context-based modelling of information demand: approaches from information logistics and decision support
Readily available information is a crucial basis for making decisions, solving problems, or performing knowledge intensive work. Providing such information meeting the needs of a user has to be based on an accurate, purpose-oriented and up-to-date representation of the demand in question. The paper is devoted to a study of different context-based models of user demand. The selected approaches from the fields of information logistics and decision support are based on enterprise models and objectoriented constraint networks (OOCN). Combining these approaches will allow for an orchestrated use of enterprise models and OOCN for decision support. Discussion and integration of these approaches is illustrated using an example enterprise model, a related information demand context and a corresponding decision support context
Variation in wood properties among five full-sib families of Norway spruce (Picea abies)
Genetic- and environmental variation and correlation patterns were
characterized for modulus of elasticity (MOE), modulus of rupture (MOR) and
related wood traits: latewood proportion, wood density, spiral grain,
microfibril angle and lignin content in five full-sib families of Norway
spruce. The families were evaluated on the basis of clearwood specimens from
the juvenile -mature wood transition zone of 93 sampled trees at age 30 year
from seed. Family-means varied significantly ( < 0.05) for all wood traits
studied except lignin content. MOE varied between 7.9–14.1 GPa among trees
and 9.4–11.0 GPa among families. MOR varied between 47–87 MPa among trees
and 61–71 MPa among families. Families remained significantly different in
an analysis of specific MOE (MOE/density) and MOR (MOR/density). Hence,
solely relying on wood density as a wood quality trait in tree breeding
would not fully yield the potential genetic gain for MOE and MOR.
Correlations between wood structural traits and specific MOE and MOR are
presented and discussed.Variabilité des propriétés du bois pour
cinq familles de pleins-frères d'épicéa commun
(Picea abies). La variabilité
génétique et environnementale pour le module d'élasticité
(MOE), le module de rupture (MOR) et certaines autres propriétés de
base du bois (proportion de bois final, densité du bois, angle du fil,
angle des microfibrilles et teneur en lignine) ont été
étudiées au sein de cinq familles de pleins-frères
d'épicéa commun ainsi que la liaison entre caractères. L'analyse
a été réalisée à partir d'échantillons sans
défaut issus de la zone de transition entre bois juvenile et bois mature
de 93 arbres âgés de 30 ans (depuis la graine). Des différences
significatives ( < 0.05) entre familles ont été observées pour
tous les caractères sauf pour la teneur en lignine. Les valeurs de MOE
variaient entre 7.9–14.1 GPa entre arbres et entre 9.4–11.0 GPa entre
familles. Pour MOR, ces valeurs s'échelonnaient entre 47–87 MPa entre
arbres et entre 61–71 MPa entre familles. Les différences entre familles
pour MOE et MOR sont restées significatives après normalisation pour
la densité du bois. Il est noté que l'utilisation seule de la
densité du bois comme critère de sélection ne permettrait pas
d'obtenir les gains génétiques potentiels escomptés d'une
sélection directe pour MOE et MOR. Les corrélations entre
propriétés de base du bois et les valeurs normalisées de MOE et
MOR sont présentées et discutées
Temperature dependent effective mass renormalization in a Coulomb Fermi liquid
We calculate numerically the quasiparticle effective mass (m*)
renormalization as a function of temperature and electron density in two- and
three-dimensional electron systems with long-range Coulomb interaction. In two
dimensions, the leading temperature correction is linear and positive with the
slope being a universal density independent number in the high density limit.
We predict an enhancement of the effective mass at low temperatures and a
non-monotonic temperature dependence at higher temperatures (T/T_F ~ 0.1) with
the peak shifting toward higher temperatures as density decreases. In three
dimensions, we find that the effective mass temperature dependence is nonlinear
and non-universal, and depends on the electron density in a complicated way. At
very high densities, the leading correction is positive, while at lower
densities it changes sign and the effective mass decreases monotonically from
its zero temperature value with increasing temperature
Modeling the Radio and X-ray Emission of SN 1993J and SN 2002ap
Modeling of radio and X-ray observations of supernovae interacting with their
circumstellar media are discussed, with special application to SN 1993J and SN
2002ap. We emphasize the importance of including all relevant physical
mechanisms, especially for the modeling of the radio light curves. The
different conclusions for the absorption mechanism (free-free or synchrotron
self-absorption), as well as departures from an CSM, as
inferred by some authors, are discussed in detail. We conclude that the
evidence for a variation in the mass loss rate with time is very weak. The
results regarding the efficiencies of magnetic field generation and
relativistic particle acceleration are summarized.Comment: 10 pages, 2 figures. Uses svmult.cls. To appear in proceedings of IAU
Colloquium 192 "Supernovae (10 years of SN 1993J)", April 2003, Valencia,
Spain, eds. J. M. Marcaide and K. W. Weile
Vertically coupled double quantum rings at zero magnetic field
Within local-spin-density functional theory, we have investigated the
`dissociation' of few-electron circular vertical semiconductor double quantum
ring artificial molecules at zero magnetic field as a function of inter-ring
distance. In a first step, the molecules are constituted by two identical
quantum rings. When the rings are quantum mechanically strongly coupled, the
electronic states are substantially delocalized, and the addition energy
spectra of the artificial molecule resemble those of a single quantum ring in
the few-electron limit. When the rings are quantum mechanically weakly coupled,
the electronic states in the molecule are substantially localized in one ring
or the other, although the rings can be electrostatically coupled. The effect
of a slight mismatch introduced in the molecules from nominally identical
quantum wells, or from changes in the inner radius of the constituent rings,
induces localization by offsetting the energy levels in the quantum rings. This
plays a crucial role in the appearance of the addition spectra as a function of
coupling strength particularly in the weak coupling limit.Comment: 18 pages, 8 figures, submitted to Physical Review
Adaptive Optics Discovery of Supernova 2004ip in the Nuclear Regions of the Luminous Infrared Galaxy IRAS 18293-3413
We report a supernova discovery in Ks-band images from the NAOS CONICA
adaptive optics (AO) system on the ESO Very Large Telescope (VLT). The images
were obtained as part of a near-infrared search for highly-obscured supernovae
in the nuclear regions of luminous and ultraluminous infrared galaxies. SN
2004ip is located within a circumnuclear starburst at 1.4 arcsec (or 500 pc)
projected distance from the K-band nucleus of the luminous infrared galaxy IRAS
18293-3413. The supernova luminosity and light curve are consistent with a
core-collapse event suffering from a host galaxy extinction of up to about 40
magnitudes in V-band which is as expected for a circumnuclear starburst
environment. This is the first supernova to be discovered making use of AO
correction and demonstrates the potential of the current 8-meter class
telescopes equipped with AO in discovering supernovae from the innermost
nuclear regions of luminous and ultraluminous infrared galaxies.Comment: 4 pages, 3 figures, ApJ Letters (accepted
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