59,326 research outputs found
Derivation of effective spin models from a three band model for CuO_2-planes
The derivation of effective spin models describing the low energy magnetic
properties of undoped CuO_2-planes is reinvestigated. Our study aims at a
quantitative determination of the parameters of effective spin models from
those of a multi-band model and is supposed to be relevant to the analysis of
recent improved experimental data on the spin wave spectrum of La_2CuO_4.
Starting from a conventional three-band model we determine the exchange
couplings for the nearest and next-nearest neighbor Heisenberg exchange as well
as for 4- and 6-spin exchange terms via a direct perturbation expansion up to
12th (14th for the 4-spin term) order with respect to the copper-oxygen hopping
t_pd. Our results demonstrate that this perturbation expansion does not
converge for hopping parameters of the relevant size. Well behaved
extrapolations of the couplings are derived, however, in terms of Pade
approximants. In order to check the significance of these results from the
direct perturbation expansion we employ the Zhang-Rice reformulation of the
three band model in terms of hybridizing oxygen Wannier orbitals centered at
copper ion sites. In the Wannier notation the perturbation expansion is
reorganized by an exact treatment of the strong site-diagonal hybridization.
The perturbation expansion with respect to the weak intersite hybridizations is
calculated up to 4th order for the Heisenberg coupling and up to 6th order for
the 4-spin coupling. It shows excellent convergence and the results are in
agreement with the Pade approximants of the direct expansion. The relevance of
the 4-spin coupling as the leading correction to the nearest neighbor
Heisenberg model is emphasized.Comment: 27 pages, 10 figures. Changed from particle to hole notation, right
value for the charge transfer gap used; this results in some changes in the
figures and a higher value of the ring exchang
On the Origin of the Non-Fermi Liquid Behavior of SrRuO_{3}
Motivated by the unusual features observed in the transport properties of the
ferromagnetic "bad metal" , we construct a model incorporating
essential features of the realistic structure of this nearly cubic material. In
particular, we show how the orbital {\it orientation} in the perfectly
cubic structure determines the peculiar structure of the hybridization matrix,
and demonstrate how the local non-Fermi liquid features arise when interactions
are switched on. we discuss the effect of the slight deviation from the cubic
structure (at low-) qualitatively. The model provides a consistent
explanation of the features observed recently in the optical response of
.Comment: 4 pages. Submitted to Physical Review Letter
Some Exact Solutions For The Classical Hall Effect In Inhomogeneous Magnetic Field
The classical Hall effect in inhomogeneous systems is considered for the case
of one-dimensional inhomogeneity. For a certain geometry of the problem and for
the magnetic field linearly depending on the coordinate the density of current
distribution corresponds to the skin-effect.Comment: 5 pages, LaTe
Rotating massive O stars with non-spherical 2D winds
We present solutions for the velocity field and mass-loss rates for 2D
axisymmetric outflows, as well as for the case of mass accretion through the
use of the Lambert W-function. For the case of a rotating radiation-driven wind
the velocity field is obtained analytically using a parameterised description
of the line acceleration that only depends on radius r at any given latitude
. The line acceleration g(r) is obtained from Monte-Carlo multi-line
radiative transfer calculations. The critical/sonic point of our equation of
motion varies with latitude . Furthermore, an approximate analytical
solution for the supersonic flow of a rotating wind is derived, which is found
to closely resemble the exact solution. For the simultaneous solution of the
mass-loss rate and velocity field, we use the iterative method of our 1D method
extended to the non-spherical 2D case. We apply the new theoretical expressions
with our iterative method to the stellar wind from a differentially rotating 40
O5-V main sequence star as well as to a 60 O-giant star,
and we compare our results to previous studies that are extensions of the
Castor et al. (1975, ApJ, 195, 157) CAK formalism. Next, we account for the
effects of oblateness and gravity darkening. Our numerical results predict an
equatorial decrease of the mass-loss rate, which would imply that
(surface-averaged) total mass-loss rates are lower than for the spherical 1D
case, in contradiction to the Maeder & Meynet (2000, A&A, 361, 159) formalism
that is oftentimes employed in stellar evolution calculations for rotating
massive stars. To clarify the situation in nature we discuss observational
tests to constrain the shapes of large-scale 2D stellar winds.Comment: 20 pages, 4 figures, 7 tables, accepted for publication in A&A, (one
corrected sentence in sect. 4.1.), a generalization of arXiv paper:
arXiv:0810.190
Interfaces between highly incompatible polymers of different stiffness: Monte Carlo simulations and self-consistent field calculations
We investigate interfacial properties between two highly incompatible
polymers of different stiffness. The extensive Monte Carlo simulations of the
binary polymer melt yield detailed interfacial profiles and the interfacial
tension via an analysis of capillary fluctuations. We extract an effective
Flory-Huggins parameter from the simulations, which is used in self-consistent
field calculations. These take due account of the chain architecture via a
partial enumeration of the single chain partition function, using chain
conformations obtained by Monte Carlo simulations of the pure phases. The
agreement between the simulations and self-consistent field calculations is
almost quantitative, however we find deviations from the predictions of the
Gaussian chain model for high incompatibilities or large stiffness. The
interfacial width at very high incompatibilities is smaller than the prediction
of the Gaussian chain model, and decreases upon increasing the statistical
segment length of the semi-flexible component.Comment: to appear in J.Chem.Phy
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