4,616 research outputs found
Effects of random environment on a self-organized critical system: Renormalization group analysis of a continuous model
We study effects of random fluid motion on a system in a self-organized
critical state. The latter is described by the continuous stochastic model,
proposed by Hwa and Kardar [{\it Phys. Rev. Lett.} {\bf 62}: 1813 (1989)]. The
advecting velocity field is Gaussian, not correlated in time, with the pair
correlation function of the form ,
where and is the component of the
wave vector, perpendicular to a certain preferred direction -- the
-dimensional generalization of the ensemble introduced by Avellaneda and
Majda [{\it Commun. Math. Phys.} {\bf 131}: 381 (1990)]. Using the field
theoretic renormalization group we show that, depending on the relation between
the exponent and the spatial dimension , the system reveals different
types of large-scale, long-time scaling behaviour, associated with the three
possible fixed points of the renormalization group equations. They correspond
to ordinary diffusion, to passively advected scalar field (the nonlinearity of
the Hwa--Kardar model is irrelevant) and to the "pure" Hwa--Kardar model (the
advection is irrelevant). For the special choice both the
nonlinearity and the advection are important. The corresponding critical
exponents are found exactly for all these cases
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
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
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
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