3,815 research outputs found
On the role of physisorption states in molecular scattering: A semi-local density-functional theory study on O2/Ag(111)
We simulate the scattering of O from Ag(111) with classical dynamics
simulations performed on a six-dimensional potential energy surface calculated
within semi-local density-functional theory (DFT). The enigmatic experimental
trends that originally required the conjecture of two types of repulsive walls,
arising from a physisorption and chemisorption part of the interaction
potential, are fully reproduced. Given the inadequate description of the
physisorption properties in semi-local DFT, our work casts severe doubts on the
prevalent notion to use molecular scattering data as indirect evidence for the
existence of such states.Comment: 5 pages, 4 figure
On the Possibility of Quantum Gravity Effects at Astrophysical Scales
The nonperturbative renormalization group flow of Quantum Einstein Gravity
(QEG) is reviewed. It is argued that at large distances there could be strong
renormalization effects, including a scale dependence of Newton's constant,
which mimic the presence of dark matter at galactic and cosmological scales.Comment: LaTeX, 18 pages, 4 figures. Invited contribution to the Int. J. Mod.
Phys. D special issue on dark matter and dark energ
First-principles kinetic Monte Carlo simulations for heterogeneous catalysis, applied to the CO oxidation at RuO2(110)
We describe a first-principles statistical mechanics approach enabling us to
simulate the steady-state situation of heterogeneous catalysis. In a first step
density-functional theory together with transition-state theory is employed to
obtain the energetics of all relevant elementary processes. Subsequently the
statistical mechanics problem is solved by the kinetic Monte Carlo method,
which fully accounts for the correlations, fluctuations, and spatial
distributions of the chemicals at the surface of the catalyst under
steady-state conditions. Applying this approach to the catalytic oxidation of
CO at RuO2(110), we determine the surface atomic structure and composition in
reactive environments ranging from ultra-high vacuum (UHV) to technologically
relevant conditions, i.e. up to pressures of several atmospheres and elevated
temperatures. We also compute the CO2 formation rates (turnover frequencies).
The results are in quantitative agreement with all existing experimental data.
We find that the high catalytic activity of this system is intimately connected
with a disordered, dynamic surface ``phase'' with significant compositional
fluctuations. In this active state the catalytic function results from a
self-regulating interplay of several elementary processes.Comment: 18 pages including 9 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Magnetic field dependence of hole levels in self-assembled InAs quantum dots
Recent magneto-transport experiments of holes in InGaAs quantum dots [D.
Reuter, P. Kailuweit, A.D. Wieck, U. Zeitler, O. Wibbelhoff, C. Meier, A.
Lorke, and J.C. Maan, Phys. Rev. Lett. 94, 026808 (2005)] are interpreted by
employing a multi-band kp Hamiltonian, which considers the interaction between
heavy hole and light hole subbands explicitely. No need of invoking an
incomplete energy shell filling is required within this model. The crucial role
we ascribe to the heavy hole-light hole interaction is further supported by
one-band local-spin-density functional calculations, which show that Coulomb
interactions do not induce any incomplete hole shell filling and therefore
cannot account for the experimental magnetic field dispersion.Comment: 5 pages with 3 figures and one table. The paper has been submitted to
Phys.Rev.
Nonperturbative renomalization group for Einstein gravity with matter
we investigate the exact renormalization group (RG) in Einstein gravity
coupled to N-component scalar field, working in the effective average action
formalism and background field method. The truncated evolution equation is
obtained for the Newtonian and cosmological constants. We have shown that
screening or antiscreening behaviour of the gravitational coupling depends
cricially on the choice of scalar-gravitational and the number of scalar
fields.Comment: 7 pages, LaTeX, a few typos correcte
Neutrality of a magnetized two-flavor quark superconductor
We investigate the effect of electric and color charge neutrality on the
two-flavor color superconducting (2SC) phase of cold and dense quark matter in
presence of constant external magnetic fields and at moderate baryon densities.
Within the framework of the Nambu-Jona-Lasinio (NJL) model, we study the
inter-dependent evolution of the quark's BCS gap and constituent mass with
increasing density and magnetic field. While confirming previous results
derived for the highly magnetized 2SC phase with color neutrality alone, we
obtain new results as a consequence of imposing charge neutrality. In the
charge neutral gapless 2SC phase (g2SC), a large magnetic field drives the
color superconducting phase transition to a crossover, while the chiral phase
transition is first order. At larger diquark-to-scalar coupling ratio
, where the 2SC phase is preferred, we see hints of the
Clogston-Chandrasekhar limit at a very large value of the magnetic field
(G), but this limit is strongly affected by Shubnikov de
Haas-van Alphen oscillations of the gap, indicating the transition to a
domain-like state.Comment: 19 pages, 7 figures, Matches with the published versio
Estimation of the Measurement Uncertainty of Ambient Air Pollution Datasets Using Geostatistical Analysis
We developed a methodology able to automatically estimate of measurement uncertainty in the air pollution data sets of AIRBase. The figures produced with this method were consistent with expectations from laboratory and field estimation of uncertainty and with the Data Quality Objectives of the European Directives. The proposed method based on geostatistical analysis is not able to estimate directly the measurement uncertainty. It estimates the nugget effect together with a micro-scale variability that must be minimized by accurate selection of the type of station. Based on the results obtained so far, it is likely that measurement uncertainty is best estimated using all background stations of whatever area type. So far the methodology has been used to estimate uncertainty in 4 different countries independently. This work should be continued for the whole Europe or for background station without national borders. The method has been shown to be also useful to compare the spatial continuity of air pollution in different countries that seems to be influenced by the topography of each country.
Moreover, it may be used to quantify the trend of measurement uncertainty over long periods like decade with the possibility to evidence improvement in the data quality of AIRBase datasets.
Thanks to the implemented outlier detection module that would also be of interest as the warning system when Member States report they measurement to the European Environment Agency, we have proposed an easy solution to investigate wrong classified stations in AIRBase.JRC.DDG.H.4-Transport and air qualit
Electrical read-out of the local nuclear polarization in the quantum Hall effect
It is demonstrated that the now well-established `flip-flop' mechanism of
spin exchange between electrons and nuclei in the quantum Hall effect can be
reversed. We use a sample geometry which utilizes separately contacted edge
states to establish a local nuclear spin polarization --close to the maximum
value achievable-- by driving a current between electron states of different
spin orientation. When the externally applied current is switched off, the
sample exhibits an output voltage of up to a few tenths of a meV, which decays
with a time constant typical for the nuclear spin relaxation. The surprizing
fact that a sample with a local nuclear spin polarization can act as a source
of energy and that this energy is well above the nuclear Zeeman splitting is
explained by a simple model which takes into account the effect of a local
Overhauser shift on the edge state reconstruction.Comment: Submitted to Phys. Rev. Let
Screening tools for data quality and outlier detection applied to the Airbase ambient air pollution database
In order to provide scientifically sound information for regulatory purposes and environmental impact assessment, long term meso- to large-scale datasets of ambient air quality provide an indispensible means for model calibration, evaluation and validation. However, the collection of high quality datasets with suitable spatial coverage for air pollution management and decision support poses many challenges. It is thus critical to establish expedient tools for the efficient assessment and data quality control of air pollution measurements in large scale national and international monitoring networks.
The European Environmental Agency collects, in the Air Quality Database named AirBase, measurements of ambient air pollution at more than 6000 monitoring stations from over 30 countries. The quality of these data depends on the chosen method of measurements and QA/QC procedures applied by each country. We present a methodology to automatically screen the AirBase records for internal consistency and to detect spatio-temporal outliers nested in the data.
We implemented a spatial-set outlier detection method, which considers both attribute values and spatial relationships. Specifically, we adapted the “Smooth Spatial Attribute method” that was developed for the identification of outliers in traffic sensors. The method relies on the definition of a neighbourhood for each air pollutant measurement, corresponding to a spatio-temporal domain limited in time (+/- 1 day) and distance (+/- 1 degree) around location x. It is assumed that within a given spatio-temporal domain in which the attribute values of neighbours have a relationship due to the emission, transport and reaction of air pollutants, outliers will be detected by extreme values of their attributes compared to the attribute values of their neighbours.
The implemented method can be of interest as a data quality screening system when countries report their measurements to the European Environment Agency. Beyond this, it could also provide a simple solution to investigate the accuracy of station classification in AirBase.JRC.H.2-Air and Climat
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