4,793 research outputs found
Double-exchange theory of ferroelectric polarization in orthorhombic manganites with twofold periodic magnetic texture
We argue that many aspects of improper ferroelectric activity in orthorhombic
manganites can be rationalized by considering the limit of infinite
intra-atomic splitting between the majority- and minority-spin states (or the
double exchange limit), which reduces the problem to the analysis of a spinless
double exchange (DE) Hamiltonian. We apply this strategy to the low-energy
model, derived from the first-principles calculations, and combine it with the
Berry-phase theory of electric polarization. We start with the simplest
two-orbital model, describing the behavior of the eg bands, and apply it to the
E-type antiferromagnetic (AFM) phase, which in the DE limit effectively breaks
up into one-dimensional zigzag chains. We derive an analytical expression for
the electronic polarization (Pel) and explain how it depends on the orbital
ordering and the energy splitting Delta between eg states. Then, we evaluate
parameters of this model, starting from a more general five-orbital model for
all Mn 3d bands and constructing a new downfolded model for the eg bands. From
the analysis of these parameters, we conclude that the behavior of Pel in
realistic manganites corresponds to the limit of large Delta. We further
utilize this property in order to derive an analytical expression for Pel in a
general two-fold periodic magnetic texture, based on the five-orbital model and
the perturbation-theory expansion for the Wannier functions in the first order
of 1/Delta. This expression explains the functional dependence of Pel on the
relative directions of spins. Furthermore, it suggests that Pel is related to
the asymmetry of the transfer integrals, which should simultaneously have
symmetric and antisymmetric components. Finally, we explain how the
polarization can be switched between orthorhombic directions a and c by
inverting the zigzag AFM texture in every second ab plane.Comment: 41 page, 10 figure
Unitarity cutting rules for the nucleus excitation and topological cross sections in hard production off nuclei from nonlinear k_t-factorization
At the partonic level, a typical final state in small-x deep inelastic
scattering off nuclei and hard proton-nucleus collisions can be characterized
by the multiplicity of color-excited nucleons. Within reggeon field theory,
each color-excited nucleon is associated with the unitarity cut of the pomeron
exchanged between the projectile and nucleus. In this communication we derive
the unitarity rules for the multiplicity of excited nucleons, alias cut
pomerons, alias topological cross sections, for typical hard dijet production
processes. We demonstrate how the coupled-channel non-Abelian intranuclear
evolution of color dipoles, inherent to pQCD, gives rise to the reggeon field
theory diagrams for final states in terms of the uncut, and two kinds of cut,
pomerons. Upon the proper identification of the uncut and cut pomeron
exchanges, the topological cross sections for dijet production follow in a
straightforward way from the earlier derived nonlinear k_t - factorization
quadratures for the inclusive dijet cross sections. The concept of a coherent
(collective) nuclear glue proves extremely useful for the formulation of
reggeon field theory vertices of multipomeron - cut and uncut - couplings to
particles and between themselves. A departure of our unitarity cutting rules
from the ones suggested by the pre-QCD Abramovsky-Kancheli-Gribov rules, stems
from the coupled-channel features of intranuclear pQCD. We propose a
multiplicity re-summation as a tool for the isolation of topological cross
sections for single-jet production.Comment: 53 pages, 16 eps-figures, to appear in Phys. Rev.
Kinetic Scalar Curvature Extended Gravity
In this work we study a modified version of vacuum gravity with a
kinetic term which consists of the first derivatives of the Ricci scalar. We
develop the general formalism of this kinetic Ricci modified gravity and
we emphasize on cosmological applications for a spatially flat cosmological
background. By using the formalism of this theory, we investigate how it is
possible to realize various cosmological scenarios. Also we demonstrate that
this theoretical framework can be treated as a reconstruction method, in the
context of which it is possible to realize various exotic cosmologies for
ordinary Einstein-Hilbert action. Finally, we derive the scalar-tensor
counterpart theory of this kinetic Ricci modified gravity, and we show
the mathematical equivalence of the two theories.Comment: NPB Accepte
- …