204 research outputs found
Modeling of complex oxide materials from the first principles: systematic applications to vanadates RVO3 with distorted perovskite structure
"Realistic modeling" is a new direction of electronic structure calculations,
where the main emphasis is made on the construction of some effective
low-energy model entirely within a first-principle framework. Ideally, it is a
model in form, but with all the parameters derived rigorously, on the basis of
first-principles electronic structure calculations. The method is especially
suit for transition-metal oxides and other strongly correlated systems, whose
electronic and magnetic properties are predetermined by the behavior of some
limited number of states located near the Fermi level. After reviewing general
ideas of realistic modeling, we will illustrate abilities of this approach on
the wide series of vanadates RVO3 (R= La, Ce, Pr, Nd, Sm, Gd, Tb, Yb, and Y)
with distorted perovskite structure. Particular attention will be paid to
computational tools, which can be used for microscopic analysis of different
spin and orbital states in the partially filled t2g-band. We will explicitly
show how the lifting of the orbital degeneracy by the monoclinic distortion
stabilizes C-type antiferromagnetic (AFM) state, which can be further
transformed to the G-type AFM state by changing the crystal distortion from
monoclinic to orthorhombic one. Two microscopic mechanisms of such a
stabilization, associated with the one-electron crystal field and electron
correlation interactions, are discussed. The flexibility of the orbital degrees
of freedom is analyzed in terms of the magnetic-state dependence of interatomic
magnetic interactions.Comment: 23 pages, 13 figure
Multidimensional rasch models for partial credit scoring
Rasch models for partial-credit scoring are discussed
and a multidimensional version of the model is formulated.
A model may be specified in which consecutive
item responses depend on an underlying latent trait. In
the multidimensional partial-credit model, different responses
may be explained by different latent traits. Data
from van Kuyk’s (1988) size concept test and the Raven
Progressive Matrices test were analyzed. Maximum
likelihood estimation and goodness-of-fit testing are discussed
and applied to these datasets. Goodness-of-fit
statistics show that for both tests, multidimensional partial-credit models were more appropriate than the unidimensional
partial-credit model. Index terms: X2 testing,
exponential family model, multidimensional item response
theory, multidimensional Rasch model, partial-credit
models, Progressive Matrices test, Rasch model
Maximally-localized Wannier functions for entangled energy bands
We present a method for obtaining well-localized Wannier-like functions (WFs)
for energy bands that are attached to or mixed with other bands. The present
scheme removes the limitation of the usual maximally-localized WFs method (N.
Marzari and D. Vanderbilt, Phys. Rev. B 56, 12847 (1997)) that the bands of
interest should form an isolated group, separated by gaps from higher and lower
bands everywhere in the Brillouin zone. An energy window encompassing N bands
of interest is specified by the user, and the algorithm then proceeds to
disentangle these from the remaining bands inside the window by filtering out
an optimally connected N-dimensional subspace. This is achieved by minimizing a
functional that measures the subspace dispersion across the Brillouin zone. The
maximally-localized WFs for the optimal subspace are then obtained via the
algorithm of Marzari and Vanderbilt. The method, which functions as a
postprocessing step using the output of conventional electronic-structure
codes, is applied to the s and d bands of copper, and to the valence and
low-lying conduction bands of silicon. For the low-lying nearly-free-electron
bands of copper we find WFs which are centered at the tetrahedral interstitial
sites, suggesting an alternative tight-binding parametrization.Comment: 13 pages, with 9 postscript figures embedded. Uses REVTEX and epsf
macro
Hubbard-U calculations for Cu from first-principles Wannier functions
We present first-principles calculations of optimally localized Wannier
functions for Cu and use these for an ab-initio determination of Hubbard
(Coulomb) matrix elements. We use a standard linearized muffin-tin orbital
calculation in the atomic-sphere approximation (LMTO-ASA) to calculate Bloch
functions, and from these determine maximally localized Wannier functions using
a method proposed by Marzari and Vanderbilt. The resulting functions were
highly localized, with greater than 89% of the norm of the function within the
central site for the occupied Wannier states. Two methods for calculating
Coulomb matrix elements from Wannier functions are presented and applied to fcc
Cu. For the unscreened on-site Hubbard for the Cu 3d-bands we have obtained
about 25eV. These results are also compared with results obtained from a
constrained local-density approximation (LDA) calculation.Comment: 13 pages, 8 figures, 5 table
Space-time Phase Transitions in Driven Kinetically Constrained Lattice Models
Kinetically constrained models (KCMs) have been used to study and understand
the origin of glassy dynamics. Despite having trivial thermodynamic properties,
their dynamics slows down dramatically at low temperatures while displaying
dynamical heterogeneity as seen in glass forming supercooled liquids. This
dynamics has its origin in an ergodic-nonergodic first-order phase transition
between phases of distinct dynamical "activity". This is a "space-time"
transition as it corresponds to a singular change in ensembles of trajectories
of the dynamics rather than ensembles of configurations. Here we extend these
ideas to driven glassy systems by considering KCMs driven into non-equilibrium
steady states through non-conservative forces. By classifying trajectories
through their entropy production we prove that driven KCMs also display an
analogous first-order space-time transition between dynamical phases of finite
and vanishing entropy production. We also discuss how trajectories with rare
values of entropy production can be realized as typical trajectories of a
mapped system with modified forces
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The EU societal awareness of landscape indicator: a review of its meaning, utility and performance across different scales
There is increasing recognition that agricultural landscapes meet multiple societal needs and demands beyond provision of economic and environmental goods and services. Accordingly, there have been significant calls for the inclusion of societal, amenity and cultural values in agri-environmental landscape indicators to assist policy makers in monitoring the wider impacts of land-based policies. However, capturing the amenity and cultural values that rural agrarian areas provide, by use of such indicators, presents significant challenges. The EU social awareness of landscape indicator represents a new class of generalized social indicator using a top-down methodology to capture the social dimensions of landscape without reference to the specific structural and cultural characteristics of individual landscapes. This paper reviews this indicator in the context of existing agri-environmental indicators and their differing design concepts. Using a stakeholder consultation approach in five case study regions, the potential and limitations of the indicator are evaluated, with a particular focus on its perceived meaning, utility and performance in the context of different user groups and at different geographical scales. This analysis supplements previous EU-wide assessments, through regional scale assessment of the limitations and potentialities of the indicator and the need for further data collection. The evaluation finds that the perceived meaning of the indicator does not vary with scale, but in common with all mapped indicators, the usefulness of the indicator, to different user groups, does change with scale of presentation. This indicator is viewed as most useful when presented at the scale of governance at which end users operate. The relevance of the different sub-components of the indicator are also found to vary across regions
Genome-Wide Association Study in BRCA1 Mutation Carriers Identifies Novel Loci Associated with Breast and Ovarian Cancer Risk
BRCA1-associated breast and ovarian cancer risks can be modified by common genetic variants. To identify further cancer risk-modifying loci, we performed a multi-stage GWAS of 11,705 BRCA1 carriers (of whom 5,920 were diagnosed with breast and 1,839 were diagnosed with ovarian cancer), with a further replication in an additional sample of 2,646 BRCA1 carriers. We identified a novel breast cancer risk modifier locus at 1q32 for BRCA1 carriers (rs2290854, P = 2.7Ă—10-8, HR = 1.14, 95% CI: 1.09-1.20). In addition, we identified two novel ovarian cancer risk modifier loci: 17q21.31 (rs17631303, P = 1.4Ă—10-8, HR = 1.27, 95% CI: 1.17-1.38) and 4q32.3 (rs4691139, P = 3.4Ă—10-8, HR = 1.20, 95% CI: 1.17-1.38). The 4q32.3 locus was not associated with ovarian cancer risk in the general population or BRCA2 carriers, suggesting a BRCA1-specific associat
Metanálise para caracterĂsticas de carcaça de bovinos de diferentes grupos genĂ©ticos
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