19,907 research outputs found
Quantum Monte Carlo Impurity Solver for Cluster DMFT and Electronic Structure Calculations in Adjustable Base
We generalized the recently introduced new impurity solver based on the
diagrammatic expansion around the atomic limit and Quantum Monte Carlo
summation of the diagrams. We present generalization to the cluster of
impurities, which is at the heart of the cluster Dynamical Mean-Field methods,
and to realistic multiplet structure of a correlated atom, which will allow a
high precision study of actinide and lanthanide based compounds with the
combination of the Dynamical Mean-Field theory and band structure methods. The
approach is applied to both, the two dimensional Hubbard and t-J model within
Cellular Dynamical Mean Field method. The efficient implementation of the new
algorithm, which we describe in detail, allows us to study coherence of the
system at low temperature from the underdoped to overdoped regime. We show that
the point of maximal superconducting transition temperature coincides with the
point of maximum scattering rate although this optimal doped point appears at
different electron densities in the two models. The power of the method is
further demonstrated on the example of the Kondo volume collapse transition in
Cerium. The valence histogram of the DMFT solution is presented showing the
importance of the multiplet splitting of the atomic states.Comment: 12 pages, 4 figure
Seasonal and inter-annual patterns of sediment-water nutrient and oxygen fluxes in Mobile Bay, Alabama (USA): Regulating factors and ecological significance.
Sediment oxygen and nutrient fluxes were measured monthly for 2 yr in Mobile Bay, Alabama, USA. Rates of sediment oxygen consumption (0.1 to 1.25 gO2 m-2 d-1), ammonium flux (-22 to 181 µmol m-2 h-1), nitrate flux (-14 to 67 µmol m-2 h-1), phosphate flux (-2 to 20.4 µmol m-2 h-1), and dissolved silicate flux (-15 to 342 µmol m-2 h-1) were moderate to high compared to values for other estuaries. A step-wise regression analysis revealed that dissolved oxygen concentration and temperature in bottom-waters explained much of the variance in fluxes. This is presumably because of their influence on rates of microbial and physico-chemical processes. Organic matter availability was not found to be an important factor in regulating temporal (month to month) variability of fluxes, possibly because frequent resuspension of the sediments in this shallow system rendered indices of sediment organic matter nearly constant with time. However, warm season-averaged sediment nutrient releases were correlated with sediment chlorophyll a. This relationship in Mobile Bay is in strong agreement with similar relationships found in other estuarine systems, and suggests that the availability of labile organic matter ultimately regulates the maximum rate of nutrient release by the sediments. Annually averaged sediment fluxes supplied 36% of the nitrogen (N) and 25% of the phosphorus (P) required by phytoplankton in Mobile Bay. While this is not particularly high compared to other estuaries, monthly estimates show that the sediments can supply from 0 to 94% of the N, and 0 to 83% of the P required by phytoplankton. In addition, flux ratios show that N and P are released from sediments at N:P ratios that rapidly switch from above (maximum 98) to below (minimum 1.2) that required for phytoplankton growth. This pattern is different from cooler temperate systems, where such switching is seasonally base
X-ray absorption spectra at the Ca-L-edge calculated within multi-channel multiple scattering theory
We report a new theoretical method for X-ray absorption spectroscopy (XAS) in
condensed matter which is based on the multi-channel multiple scattering theory
of Natoli et al. and the eigen-channel R-matrix method. While the highly
flexible real-space multiple scattering (RSMS) method guarantees a precise
description of the single-electron part of the problem, multiplet-like electron
correlation effects between the photo-electron and localized electrons can be
taken account for in a configuration interaction scheme. For the case where
correlation effects are limited to the absorber atom, a technique for the
solution of the equations is devised, which requires only little more
computation time than the normal RSMS method for XAS. The new method is
described and an application to XAS at the Ca -edge in bulk Ca, CaO
and CaF is presented.Comment: 10 pages, 4 figures, submitted to Phys. Rev.
Increasing Returns to Scale in Heterogeneous Populations
This is very much a working paper. It presents some preliminary results having to do with the long run dynamics of certain types of demographic processes. A population is heterogeneous with regard to its preferences for two alternatives A and B. If the choice of alternatives displays increasing returns, i.e. the more one of the alternatives is chosen the more attractive it becomes, the long run properties of the system are, in general, not predictable. It may, however. have fixed points to one of which the system will converge. The stability of the fixed points depends very much on the correlation of the distribution of original preferences. As this is work in progress, suggested directions for future research are presented
Notes on the Effects of Cohort Size on Intergenerational Transfer
In this paper I will describe a framework designed to answer certain questions about how cohort size affects total lifetime consumption. I will use an overlapping generations model in which intergenerational transfers are the mechanism through which changes in cohort size affect consumption levels. The model will seem rather contrived with respect to many economies, in that the state will intermediate all transfers. This assumption is not crucial to the results, as I will argue later, but it does facilitate discussion
Multiplicities in ultrarelativistic proton-(anti)proton collisions and negative binomial distribution fits
Likelihood ratio tests are performed for the hypothesis that charged-particle
multiplicities measured in proton-(anti)proton collisions at = 0.9
and 2.36 TeV are distributed according to the negative binomial form. Results
indicate that the hypothesis should be rejected in the all cases of ALICE-LHC
measurements in the limited pseudo-rapidity windows, whereas should be accepted
in the corresponding cases of UA5 data. Possible explanations of that and of
the disagreement with the least-squares fitting method are given.Comment: 14 pages, clarified version, reference added. To appear in
International Journal of Modern Physics
Number Skills in Primary School Development and Individual Differences
From the start of universal schooling there has been dissatisfaction with the outcomes of primary education and disagreement about why some children make much slower progress
A comparison between detailed and configuration-averaged collisional-radiative codes applied to non-local thermal equilibrium plasma
A collisional-radiative model describing nonlocal-thermodynamic-equilibrium
plasmas is developed. It is based on the HULLAC (Hebrew University Lawrence
Livermore Atomic Code) suite for the transitions rates, in the zero-temperature
radiation field hypothesis. Two variants of the model are presented: the first
one is configuration averaged, while the second one is a detailed level
version. Comparisons are made between them in the case of a carbon plasma; they
show that the configuration-averaged code gives correct results for an
electronic temperature Te=10 eV (or higher) but fails at lower temperatures
such as Te=1 eV. The validity of the configuration-averaged approximation is
discussed: the intuitive criterion requiring that the average
configuration-energy dispersion must be less than the electron thermal energy
turns out to be a necessary but far from sufficient condition. Another
condition based on the resolution of a modified rate-equation system is
proposed. Its efficiency is emphasized in the case of low-temperature plasmas.
Finally, it is shown that near-threshold autoionization cascade processes may
induce a severe failure of the configuration-average formalism.Comment: 9
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