4,837 research outputs found
Effect of Adiabatic Phonons on Striped and Homogeneous Ground States
The effects of adiabatic phonons on a spin-fermion model for high T_c
cuprates are studied using numerical simulations. In the absence of
electron-phonon interactions (EPI), stripes in the ground state are observed
for certain dopings while homogeneous states are stabilized in other regions of
parameter space. Different modes of adiabatic phonons are added to the
Hamiltonian:breathing, shear and half-breathing modes. Diagonal and
off-diagonal electron-phonon couplings are considered. It is observed that
strong diagonal EPI generate stripes in previously homogeneous states, while in
striped ground states an increase in the diagonal couplings tends to stabilize
the stripes, inducing a gap in the density of states (DOS) and rendering the
ground state insulating. The off-diagonal terms, on the other hand, destabilize
the stripes creating inhomogeneous ground states with a pseudogap at the
chemical potential in the DOS. The breathing mode stabilizes static diagonal
stripes; while the half-breathing (shear) modes stabilize dynamical (localized)
vertical and horizontal stripes. The EPI induces decoherence of the
quasi-particle peaks in the spectral functions.Comment: latex, 9 pages,13 figure
Metal nanoring and tube formation on carbon nanotubes
The structural and electronic properties of aluminum covered single wall
carbon nanotubes (SWNT) are studied from first-principles for a large number of
coverage. Aluminum-aluminum interaction that is stronger than aluminum-tube
interaction, prevents uniform metal coverage, and hence gives rise to the
clustering. However, a stable aluminum ring and aluminum nanotube with well
defined patterns can also form around the semiconducting SWNT and lead to
metallization. The persistent current in the Al nanoring is discussed to show
that a high magnetic field can be induced at the center of SWNT.Comment: Submitted to Physical Review
Joint gateway selection, transmission slot assignment, routing and power control for wireless mesh networks
Cataloged from PDF version of article.Wireless mesh networks (WMNs) provide cost effective solutions for setting up a communications network over a certain geographic area. In this paper, we study strategic problems of WMNs such as selecting the gateway nodes along with several operational problems such as routing, power control, and transmission slot assignment. Under the assumptions of the physical interference model and the tree-based routing restriction for traffic flow, a mixed integer linear programming (MILP) formulation is presented, in which the objective is to maximize the minimum service level provided at the nodes. A set of valid inequalities is derived and added to the model in an attempt to improve the solution quality. Since the MILP formulation becomes computationally infeasible for larger instances, we propose a heuristic method that is aimed at solving the problem in two stages. In the first stage, we devise a simple MILP problem that is concerned only with the selection of gateway nodes. In the second stage, the MILP problem in the original formulation is solved by fixing the gateway nodes from the first stage. Computational experiments are provided to evaluate the proposed models and the heuristic method. (C) 2013 Elsevier Ltd. All rights reserved
External magnetic field effects on a distorted kagome antiferromagnet
We report bulk magnetization, and elastic and inelastic neutron scattering
measurements under an external magnetic field, , on the weakly coupled
distorted kagome system, Cu_{2}(OD)_3Cl. Our results show that the ordered
state below 6.7 K is a canted antiferromagnet and consists of large
antiferromagnetic -components and smaller ferromagnetic -components. By
first-principle calculations and linear spin wave analysis, we present a simple
spin hamiltonian with non-uniform nearest neighbor exchange interactions
resulting in a system of coupled spin trimers with a single-ion anisotropy that
can qualitatively reproduce the spin dynamics of Cu_{2}(OD)_3Cl.Comment: 5 figure
Exo-hydrogenated Single Wall Carbon Nanotubes
An extensive first-principles study of fully exo-hydrogenated zigzag (n,0)
and armchair (n,n) single wall carbon nanotubes (CH), polyhedral
molecules including cubane, dodecahedrane, and CH points to
crucial differences in the electronic and atomic structures relevant to
hydrogen storage and device applications. CH's are estimated to be
stable up to the radius of a (8,8) nanotube, with binding energies proportional
to 1/R. Attaching a single hydrogen to any nanotube is always exothermic.
Hydrogenation of zigzag nanotubes is found to be more likely than armchair
nanotubes with similar radius. Our findings may have important implications for
selective functionalization and finding a way of separating similar radius
nanotubes from each other.Comment: 5 pages, 4 postscript figures, Revtex file, To be appear in Physical
Review
Polarized neutron scattering studies of the kagome lattice antiferromagnet KFe3(OH)6(SO4)2
We report polarized neutron scattering studies of spin-wave excitations and
spin fluctuations in the S=5/2 kagome lattice antiferromagnet KFe3(OH)6(SO4)2
(jarosite). Inelastic polarized neutron scattering measurements at 10 K on a
single crystal sample reveal two spin gaps, associated with in-plane and
out-of-plane excitations. The polarization analysis of quasi-elastic scattering
at 67 K shows in-plane spin fluctuations with XY symmetry, consistent with the
disappearance of the in-plane gap above the Neel temperature T_N = 65 K. Our
results suggest that jarosite is a promising candidate for studying the 2D XY
universality class in magnetic systems.Comment: 3 pages, 3 figures, Proceeding to the 7th International Workshop on
Polarized Neutrons for Condensed Matter Investigations and 2nd International
Symposium of Quantum Beam Science Directorat
Multi-phonon scattering and Ti-induced hydrogen dynamics in sodium alanate
We use ab initio methods and neutron inelastic scattering (NIS) to study the
structure, energetics, and dynamics of pure and Ti-doped sodium alanate
(NaAlH_4), focusing on the possibility of substitutional Ti doping. The NIS
spectrum is found to exhibit surprisingly strong and sharp two-phonon features.
The calculations reveal that substitutional Ti doping is energetically
possible. Ti prefers to substitute for Na and is a powerful hydrogen attractor
that facilitates multiple Al--H bond breaking. Our results hint at new ways of
improving the hydrogen dynamics and storage capacity of the alanates.Comment: 5 pages, with 4 postscript figures embedded. Uses REVTEX4 and
graphicx macro
Hidden Symmetries and their Consequences in Cubic Perovskites
The five-band Hubbard model for a band with one electron per site is a
model which has very interesting properties when the relevant ions are located
at sites with high (e. g. cubic) symmetry. In that case, if the crystal field
splitting is large one may consider excitations confined to the lowest
threefold degenerate orbital states. When the electron hopping matrix
element () is much smaller than the on-site Coulomb interaction energy
(), the Hubbard model can be mapped onto the well-known effective
Hamiltonian (at order ) derived by Kugel and Khomskii (KK). Recently
we have shown that the KK Hamiltonian does not support long range spin order at
any nonzero temperature due to several novel hidden symmetries that it
possesses. Here we extend our theory to show that these symmetries also apply
to the underlying three-band Hubbard model. Using these symmetries we develop a
rigorous Mermin-Wagner construction, which shows that the three-band Hubbard
model does not support spontaneous long-range spin order at any nonzero
temperature and at any order in -- despite the three-dimensional lattice
structure. Introduction of spin-orbit coupling does allow spin ordering, but
even then the excitation spectrum is gapless due to a subtle continuous
symmetry. Finally we showed that these hidden symmetries dramatically simplify
the numerical exact diagonalization studies of finite clusters.Comment: 26 pages, 3 figures, 520 KB, submitted Phys. Rev.
Use of an antibiotic growth promoter and two herbal natural feed additives with and without exogenous enzymes in wheat based broiler diets
No Abstract. South African Journal of Animal Science Vol. 35 (1) 2005: pp.61-7
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