1,489 research outputs found
Photoemission Spectra from Reduced Density Matrices: the Band Gap in Strongly Correlated Systems
We present a method for the calculation of photoemission spectra in terms of
reduced density matrices. We start from the spectral representation of the
one-body Green's function G, whose imaginary part is related to photoemission
spectra, and we introduce a frequency-dependent effective energy that accounts
for all the poles of G. Simple approximations to this effective energy give
accurate spectra in model systems in the weak as well as strong correlation
regime. In real systems reduced density matrices can be obtained from reduced
density-matrix functional theory. Here we use this approach to calculate the
photoemission spectrum of bulk NiO: our method yields a qualitatively correct
picture both in the antiferromagnetic and paramagnetic phases, contrary to
mean-field methods, in which the paramagnet is a metal
Reduced Density-Matrix Functional Theory: correlation and spectroscopy
In this work we explore the performance of approximations to electron
correlation in reduced density-matrix functional theory (RDMFT) and of
approximations to the observables calculated within this theory. Our analysis
focuses on the calculation of total energies, occupation numbers,
removal/addition energies, and spectral functions. We use the exactly solvable
Hubbard molecule at 1/4 and 1/2 filling as test systems. This allows us to
analyze the underlying physics and to elucidate the origin of the observed
trends. For comparison we also report the results of the approximation,
where the self-energy functional is approximated, but no further hypothesis are
made concerning the approximations of the observables. In particular we focus
on the atomic limit, where the two sites of the molecule are pulled apart and
electrons localize on either site with equal probability, unless a small
perturbation is present: this is the regime of strong electron correlation. In
this limit, using the Hubbard molecule at 1/2 filling with or without a
spin-symmetry-broken ground state, allows us to explore how degeneracies and
spin-symmetry breaking are treated in RDMFT. We find that, within the used
approximations, neither in RDMFT nor in the signature of strong
correlation are present in the spin-singlet ground state, whereas both give the
exact result for the spin-symmetry broken case. Moreover we show how the
spectroscopic properties change from one spin structure to the other. Our
findings can be generalized to other situations, which allows us to make
connections to real materials and experiment
Reliability of third-order moment parameterization for models of turbulent boundary layer over gentle topography
An analysis is made of the transport equation of Reynolds shear stress, written in a streamline coordinate system, starting from the fields of first- and secondorder
moments of wind velocity, measured in a terrain-following system over gentle topography, in order to verify the usual parameterizations of third-order moments. The equation is split into two parts: the first contains the terms which can be calculated directly from measurements, the second involves the pressure-velocity correlation considering the terms of rapid distortion, curvature and return to isotropy and the transport of triple velocity-correlation modelled assuming a flux-gradient approximation. Moreover, the error estimates associated with both parts have been computed
using a Monte Carlo technique which takes into account the experimental errors. This analysis is performed on wind tunnel data over a gently shaped two-dimensional valley
and hill. The comparison between the measured and modelled parts is good near the surface, whereas, at higher levels, where the pertubations induced by the topography are significant, there are large zones generally characterized by streamlines with concave curvature in which the
flux-gradient approximation used to compute the triple
product correlation cannot be applied
Chronic pelvic pain: comorbidity between chronic musculoskeletal pain and vulvodynia.
Chronic pelvic pain (CPP) is a common condition that has a major impact on the quality of life of both men and women. Male CPP is usually attributable to well-defined urogenital conditions (most frequently infectious/non infectious prostatic diseases) or musculoskeletal or bowel diseases, whereas the features of female CPP are much more complex and are of particular clinical and epidemiological importance. It is a multifactorial syndrome that can be due to diseases of the urogenital, gastrointestinal, or musculoskeletal systems, or to neurological or neuropsychiatric disorders. It is not always easy to identify its predominant pathogenesis, although it often occurs as a central sensitization syndrome triggered by an initial stimulus which is no longer detectable and only manifests itself clinically through pain. In this respect, there are some very interesting relationships between vulvodynia and fibromyalgic syndrome, as identified in a preliminary study of women with chronic musculoskeletal pain in which it was demonstrated that vulvar pain plays an important role, although it is often overlooked and undiagnosed
Screened extended Koopmans' theorem: photoemission at weak and strong correlation
By introducing electron screening in the extended Koopmans' theorem we
correctly describe the band gap opening in weakly as well as strongly
correlated systems. We show this by applying our method to bulk LiH, Si, and
paramagnetic as well as antiferromagnetic NiO. Although incorrect features
remain in the full photoemission spectra, this is a remarkable result for an
ab-initio electronic structure method and it opens the way to a unified
description of photoemission spectra at weak and strong correlation
Evaluation of three new surface irrigation parameterizations in the WRF-ARW v3.8.1 model: the Po Valley (Italy) case study
Abstract. Irrigation is a method of land management that can affect the local climate. Recent literature
shows that it affects mostly the near-surface variables and it is associated with an irrigation
cooling effect. However, there is no common parameterization that also accounts for a realistic
water amount, and this factor could ascribe one cause to the different impacts found in previous
studies. This work aims to introduce three new surface irrigation parameterizations within the
WRF-ARW model (v3.8.1) that consider different evaporative processes. The parameterizations are
tested on one of the regions where global studies disagree on the signal of irrigation: the
Mediterranean area and in particular the Po Valley. Three sets of experiments are performed using
the same irrigation water amount of 5.7âmmâdâ1, derived from Eurostat data. Two complementary
validations are performed for July 2015: monthly mean, minimum, and maximum temperature with ground
stations and potential evapotranspiration with the MODIS product. All tests show that for both
mean and maximum temperature, as well as potential evapotranspiration simulated fields
approximate observation-based values better when using the irrigation parameterizations. This
study addresses the sensitivity of the results to human-decision
assumptions of the parameterizations: start time, length, and frequency. The main impact of irrigation on surface variables
such as soil moisture is due to the parameterization choice itself affecting evaporation, rather
than the timing. Moreover, on average, the atmosphere and soil variables are not very sensitive to
the parameterization assumptions for realistic timing and length
Structure of Turbulence in Katabatic Flows below and above the Wind-Speed Maximum
Measurements of small-scale turbulence made over the complex-terrain
atmospheric boundary layer during the MATERHORN Program are used to describe
the structure of turbulence in katabatic flows. Turbulent and mean
meteorological data were continuously measured at multiple levels at four
towers deployed along the East lower slope (2-4 deg) of Granite Mountain. The
multi-level observations made during a 30-day long MATERHORN-Fall field
campaign in September-October 2012 allowed studying of temporal and spatial
structure of katabatic flows in detail, and herein we report turbulence and
their variations in katabatic winds. Observed vertical profiles show steep
gradients near the surface, but in the layer above the slope jet the vertical
variability is smaller. It is found that the vertical (normal to the slope)
momentum flux and horizontal (along the slope) heat flux in a slope-following
coordinate system change their sign below and above the wind maximum of a
katabatic flow. The vertical momentum flux is directed downward (upward)
whereas the horizontal heat flux is downslope (upslope) below (above) the wind
maximum. Our study therefore suggests that the position of the jet-speed
maximum can be obtained by linear interpolation between positive and negative
values of the momentum flux (or the horizontal heat flux) to derive the height
where flux becomes zero. It is shown that the standard deviations of all wind
speed components (therefore the turbulent kinetic energy) and the dissipation
rate of turbulent kinetic energy have a local minimum, whereas the standard
deviation of air temperature has an absolute maximum at the height of
wind-speed maximum. We report several cases where the vertical and horizontal
heat fluxes are compensated. Turbulence above the wind-speed maximum is
decoupled from the surface, and follows the classical local z-less predictions
for stably stratified boundary layer.Comment: Manuscript submitted to Boundary-Layer Meteorology (05 December 2014
Evaporating waterbody effects in a simplified urban neighbourhood: A RANS analysis
The incorporation of nature-based solutions comprising green and blue infrastructure is often touted as a way to cool cities and enhance pollutant removal. However, there is little agreement between different methodologies to measure the effect of any single intervention. Here, we present 3D steady RANS simulations to investigate the influence of waterbody on in-canyon flow structure, temperature (T*) and water vapour (!*) distribution in a simplified urban neighbourhood. A novel solver that captures evaporation effects is developed and validated against wind tunnel experiments. Simulations are performed under neutral atmospheric conditions for forced -and mixed-convection cases and different air-water temperature differences, indicative of either daytime or night-time conditions. Results under forced convection show minimal impact on the flow structure, whilst T* and !* effects are distributed primarily over and around the water surface. However, the mixed-convection case shows that a cooler waterbody weakens the principal vortex in the open square, whilst T* and !* effects reach further upwind and are more widely distributed in the spanwise direction. A warmer waterbody is shown to disrupt the skimming flow structure, indicating a possible heat and pollutant removal mechanism from around the waterbody and also downwind canyons
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