4,345 research outputs found
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
String Nature of Confinement in (Non-)Abelian Gauge Theories
Recent progress achieved in the solution of the problem of confinement in
various (non-)Abelian gauge theories by virtue of a derivation of their string
representation is reviewed. The theories under study include QCD within the
so-called Method of Field Correlators, QCD-inspired Abelian-projected theories,
and compact QED in three and four space-time dimensions. Various
nonperturbative properties of the vacua of the above mentioned theories are
discussed. The relevance of the Method of Field Correlators to the study of
confinement in Abelian models, allowing for an analytical description of this
phenomenon, is illustrated by an evaluation of field correlators in these
models.Comment: 100 pages, LaTeX2e, no figures, 1 table, based on the Ph.D. thesises
at the Humboldt University of Berlin (1999) (available under
http://dochost.rz.hu-berlin.de) and the Institute of Theoretical and
Experimental Physics, Moscow (2000), new results are included, extended with
respect to the journal versio
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
Hydrogen on graphene: Electronic structure, total energy, structural distortions, and magnetism from first-principles calculations
Density functional calculations of electronic structure, total energy,
structural distortions, and magnetism for hydrogenated single-layer, bilayer,
and multi-layer graphene are performed. It is found that hydrogen-induced
magnetism can survives only at very low concentrations of hydrogen (single-atom
regime) whereas hydrogen pairs with optimized structure are usually
nonmagnetic. Chemisorption energy as a function of hydrogen concentration is
calculated, as well as energy barriers for hydrogen binding and release. The
results confirm that graphene can be perspective material for hydrogen storage.
Difference between hydrogenation of graphene, nanotubes, and bulk graphite is
discussed.Comment: 8 pages 8 figures (accepted to Phys. Rev. B
Manifestation of anisotropy persistence in the hierarchies of MHD scaling exponents
The first example of a turbulent system where the failure of the hypothesis
of small-scale isotropy restoration is detectable both in the `flattening' of
the inertial-range scaling exponent hierarchy, and in the behavior of odd-order
dimensionless ratios, e.g., skewness and hyperskewness, is presented.
Specifically, within the kinematic approximation in magnetohydrodynamical
turbulence, we show that for compressible flows, the isotropic contribution to
the scaling of magnetic correlation functions and the first anisotropic ones
may become practically indistinguishable. Moreover, skewness factor now
diverges as the P\'eclet number goes to infinity, a further indication of
small-scale anisotropy.Comment: 4 pages Latex, 1 figur
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
Effect of angular momentum distribution on gravitational loss-cone instability in stellar clusters around massive BH
Small perturbations in spherical and thin disk stellar clusters surrounding
massive a black hole are studied. Due to the black hole, stars with
sufficiently low angular momentum escape from the system through the loss cone.
We show that stability properties of spherical clusters crucially depend on
whether the distribution of stars is monotonic or non-monotonic in angular
momentum. It turns out that only non-monotonic distributions can be unstable.
At the same time the instability in disk clusters is possible for both types of
distributions.Comment: 14 pages, 7 figures, submitted to MNRA
Sensitivity of depth of maximum and absorption depth of EAS to hadron production mechanism
Comparison of experimental data on depth of extensive air showers (EAS) development maximum in the atmosphere, T sub M and path of absorption, lambda, in the lower atmosphere of EAS with fixed particle number in the energy region eV with the results of calculation show that these parameters are sensitive mainly to the inelastic interaction cross section and scaling violation in the fragmentation and pionization region. The data are explained in a unified manner within the framework of a model in which scaling is violated slightly in the fragmentation region and strongly in the pionization region at primary cosmic rays composition close to the normal one and a permanent increase of inelastic interaction cross section. It is shown that, while interpreting the experimental data, disregard of two methodical points causes a systematic shift in T sub M: (1) shower selection system; and (2) EAS electron lateral distribution when performing the calculations on basis of which the transfer is made from the Cerenkov pulse FWHM to the depth of shower maximum, T sub M
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