955 research outputs found
Hilbert's 16th Problem for Quadratic Systems. New Methods Based on a Transformation to the Lienard Equation
Fractionally-quadratic transformations which reduce any two-dimensional
quadratic system to the special Lienard equation are introduced. Existence
criteria of cycles are obtained
Metal-Insulator Transition and Lattice Instability of Paramagnetic V2O3
We determine the electronic structure and phase stability of paramagnetic
VO at the Mott-Hubbard metal-insulator phase transition, by employing a
combination of an ab initio method for calculating band structures with
dynamical mean-field theory. The structural transformation associated with the
metal-insulator transition is found to occur upon a slight expansion of the
lattice volume by %, in agreement with experiment. Our results show
that the structural change precedes the metal-insulator transition, implying a
complex interplay between electronic and lattice degrees of freedom at the
transition. Electronic correlations and full charge self-consistency are found
to be crucial for a correct description of the properties of VO.Comment: 5 pages, 4 figure
Electronic structure of charge-ordered Fe3O4 from calculated optical, megneto-optical Kerr effect, and O K-edge x-ray absorption spectra
The electronic structure of the low-temperature (LT) monoclinic magnetite,
Fe3O4, is investigated using the local spin density approximation (LSDA) and
the LSDA+U method. The self-consistent charge ordered LSDA+U solution has a
pronounced [001] charge density wave character. In addition, a minor [00{1/2}]
modulation in the phase of the charge order (CO) also occurs. While the
existence of CO is evidenced by the large difference between the occupancies of
the minority spin t_{2g} states of ``2+'' and ``3+'' Fe_B cations, the total 3d
charge disproportion is small, in accord with the valence-bond-sum analysis of
structural data. Weak Fe orbital moments of ~0.07 mB are obtained from
relativistic calculations for the CO phase which is in good agreement with
recent x-ray magnetic circular dichroism measurements. Optical, magneto-optical
Kerr effect, and O K-edge x-ray absorption spectra calculated for the charge
ordered LSDA+U solution are compared to corresponding LSDA spectra and to
available experimental data. Reasonably good agreement between the theoretical
and experimental spectra supports the relevance of the CO solution obtained for
the monoclinic LT phase. The results of calculations of effective exchange
coupling constants between Fe spin magnetic moments are also presented.Comment: 32 pages, 10 figure
Correlation strength, Lifshitz transition and the emergence of a two- to three-dimensional crossover in FeSe under pressure
We report a detailed theoretical study of the electronic structure, spectral
properties, and lattice parameters of bulk FeSe under pressure using a fully
charge self-consistent implementation of the density functional theory plus
dynamical mean-field theory method (DFT+DMFT). In particular, we perform a
structural optimization and compute the evolution of the lattice parameters
(volume, ratio, and the internal position of Se) and the electronic
structure of the tetragonal (space group ) paramagnetic FeSe. Our
results for the lattice parameters are in good quantitative agreement with
experiment. The ratio is slightly overestimated by about ~\%,
presumably due to the absence of the van der Waals interactions between the
FeSe layers in our calculations. The lattice parameters determined within DFT
are off the experimental values by a remarkable -~\%, implying a
crucial importance of electron correlations. Upon compression to ~GPa, the
ratio and the lattice volume show a decrease by and ~\%,
respectively, while the Se coordinate weakly increases by ~\%.
Most importantly, our results reveal a topological change of the Fermi surface
(Lifshitz transition) which is accompanied by a two- to three-dimensional
crossover. Our results indicate a small reduction of the quasiparticle mass
renormalization by about ~\% for the and less than ~\% for
the states, as compared to ambient pressure. The behavior of the
momentum-resolved magnetic susceptibility shows no topological
changes of magnetic correlations under pressure, but demonstrates a reduction
of the degree of the in-plane stripe-type nesting. Our results for
the electronic structure and lattice parameters of FeSe are in good qualitative
agreement with recent experiments on its isoelectronic counterpart
FeSeS.Comment: 10 pages, 6 figure
Charge order and spin-singlet pairs formation in Ti4O7
Charge ordering in the low-temperature triclinic structure of titanium oxide
(Ti4O7) is investigated using the local density approximation (LDA)+U method.
Although the total 3d charge separation is rather small, an orbital order
parameter defined as the difference between t2g occupancies of Ti and
Ti cations is large and gives direct evidence for charge ordering. Ti 4s
and 4p states make a large contribution to the static "screening" of the total
3d charge difference. This effective charge screening leads to complete loss of
the disproportionation between the charges at 3+ and 4+ Ti sites. The occupied
t2g states of Ti cations are predominantly of character and
form a spin-singlet molecular orbital via strong direct antiferromagnetic
exchange coupling between neighboring Ti(1) and Ti(3) sites, whereas the role
of superexchange is found to be negligible.Comment: 6 pages, 4 figure
Charge order in Fe2OBO3: An LSDA+U study
Charge ordering in the low-temperature monoclinic structure of iron oxoborate
(Fe2OBO3) is investigated using the local spin density approximation (LSDA)+U
method. While the difference between t_{2g} minority occupancies of Fe^{2+} and
Fe^{3+} cations is large and gives direct evidence for charge ordering, the
static "screening" is so effective that the total 3d charge separation is
rather small. The occupied Fe^{2+} and Fe^{3+} cations are ordered alternately
within the chain which is infinite along the a-direction. The charge order
obtained by LSDA+U is consistent with observed enlargement of the \beta angle.
An analysis of the exchange interaction parameters demonstrates the
predominance of the interribbon exchange interactions which determine the whole
L-type ferrimagnetic spin structure.Comment: 7 pages, 8 figure
Calculated phonon spectra of paramagnetic iron at the alpha-gamma phase transition
We compute lattice dynamical properties of iron at the bcc-fcc phase
transition using dynamical mean-field theory implemented with the frozen-phonon
method. Electronic correlations are found to have a strong effect on the
lattice stability of paramagnetic iron in the bcc phase. Our results for the
structural phase stability and lattice dynamical properties of iron are in good
agreement with experiment.Comment: 4 pages, 2 figure
Comparative Characteristics of Time Perspective of Professional Athletes and Drug Addicted People
AbstractThe present article analyses the comparative characteristics of time perspective of professional athletes and drug addicted people. Participants who are addicted to drugs and professional athletes pose significant difference in time perspective: in their perception of “Future”, “Negative Past” and Positive Past”. Characteristics of time perspective are dynamic and change in the context of meaning-life orientations and stressful events. Excessive concentration of participants on the past or on the future could be a predictor that contributes to dependent behavior. The necessity of individual profile consideration in psychological work with athletes and drug-addicted people was shown
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