408 research outputs found
Modeling the mid-infrared optical gap in La2âxSrxCuO4
In this work, we used a periodic lattice potential in order to model the infrared optical data of the high-temperature superconductor La2âxSrxCuO4. This potential consists of a two-dimensional array of double-well potentials, which simulate the CuO2 layers. It is obtained by assembling Cu-O-Cu units rather than Cu and O single atoms in the tight-binding approach. A gap separating two energy bands can be obtained and is used to ïŹt the infrared (IR) optical gap of this cuprate. We derived the dielectric function and showed that in the classical limit it reduces to the one consisting of a Drude term plus a number of lorentz components, equivalent to the dielectric function used empirically by several authors in their ïŹts of the reïŹectivity. By reïŹtting available reïŹectance data, we deduced a simple law for the doping dependence of the optical gap in La2âxSrxCuO4. In the present study, we argue that the optical gap is distinct from the pseudogap or the two-magnon gap, because it characterizes La2âxSrxCuO4 for all doping regimes.In this work, we used a periodic lattice potential in order to model the infrared optical data of the high-temperature superconductor La2âxSrxCuO4. This potential consists of a two-dimensional array of double-well potentials, which simulate the CuO2 layers. It is obtained by assembling Cu-O-Cu units rather than Cu and O single atoms in the tight-binding approach. A gap separating two energy bands can be obtained and is used to ïŹt the infrared (IR) optical gap of this cuprate. We derived the dielectric function and showed that in the classical limit it reduces to the one consisting of a Drude term plus a number of lorentz components, equivalent to the dielectric function used empirically by several authors in their ïŹts of the reïŹectivity. By reïŹtting available reïŹectance data, we deduced a simple law for the doping dependence of the optical gap in La2âxSrxCuO4. In the present study, we argue that the optical gap is distinct from the pseudogap or the two-magnon gap, because it characterizes La2âxSrxCuO4 for all doping regimes
Calculation of the singlet-triplet gap of the antiferromagnetic Heisenberg Model on the ladder
The ground state energy and the singlet-triplet energy gap of the
antiferromagnetic Heisenberg model on a ladder is investigated using a mean
field theory and the density matrix renormalization group. Spin wave theory
shows that the corrections to the local magnetization are infinite. This
indicates that no long range order occurs in this system. A flux-phase state is
used to calculate the energy gap as a function of the transverse coupling,
, in the ladder. It is found that the gap is linear in for
and goes to zero for . The mean field theory
agrees well with the numerical results.Comment: 11pages,6 figures (upon request) Revtex 3.0, Report#CRPS-94-0
First report of Barley Yellow Dwarf Viruses (BYDVs) on dicotyledonous weed hosts in Turkey
Yellow dwarf viruses (YDVs) are economically destructive viral diseases of cereal crops, which cause the reduction of harvested yield and quality of grains. Up to now the identification of such viruses was limited to monocotyledonous Poaceae weed hosts, and was not investigated in dicotyledons. In this study, using DAS-ELISA and RT-PCR methods, 6 dicotyledonous weed species, collected from Trakya, Turkey, were examined for the presence of the YDVs pathogens BYDV-PAV, BYDV-MAV, BYDV-RMV, BYDV-SGV and CYDV-RPV. The screening tests revealed certain samples of Geranium dissectum L. and Juncus compressus Jacq. were infected with BYDV-PAV, while other samples of the same species were positive for BYDV-MAV. Additionally, RT-PCR tests of both weed species revealed cases of mixed infection by BYDV-PAV and BYDV-MAV. Transmission experiments using the aphid species Rhopalosiphum padi L. showed that BYDV-PAV was transmitted persistently from Geranium dissectum to barley cv. Barbaros seedlings. To our knowledge, this is the first report of Geranium dissectum and Juncus compressus as possible plant hosts of BYDV-PAV and BYDV-MAV in Turkey
A New Economic Dispatch for Coupled Transmission and Active Distribution Networks Via Hierarchical Communication Structure
Traditionally, the economic dispatch problem (EDP) of the bulk generators connected to transmission networks (TNs) is solved in a centralized dispatching center (CDC) while modeling distribution networks as passive loads. With the increasing penetration levels of distributed generation, coordinating the economic dispatch between TNs and active distribution networks (ADNs) became vital to maximizing system efficiency. This article proposes a hierarchical communication structure, which requires minimal upgrades to the CDC, for solving the EDP of coupled TNs and ADNs. Based on the minimal data transfer between the CDC and distribution network operators, the problem is formulated and solved while considering the network losses in both TNs and ADNs. Furthermore, a sensitivity analysis is conducted to assess the effect of the ratio of the distribution lines on the economic dispatch solution and the operational cost of the system. The numerical results demonstrate the effectiveness of the proposed centralized scheme and highlight the significance of considering the network losses of both TNs and ADNs when solving the EDP. The results show that the proposed framework can achieve savings of up to 17.98% by taking into account the network losses of TNs and ADNs
A Microstructure Sensitive Approach for the Prediction of the Creep Behaviour and Life under Complex Loading Paths
The prediction of the creep behaviour and life of components of aeronautic engines like high pressure turbine blades is still a challenging issue due to non-isothermal loadings. Indeed, certification procedures of turboshaft engines for helicopters consist of complex thermomechanical histories, sometimes including short and very high temperature excursions close to the Îłâ-solvus (T~1200°C) of the blade alloy. A better design of those components could be gained using a model that takes into account non-isothermal loadings inducing microstructural changes.
Most of the commonly used models consider only a nearly constant (or slowly evolving) microstructure, i.e. far from the rapid microstructure evolutions encountered during close Îłâ-solvus overheatings where a rapid dissolution/precipitation of the Îłâ-phase and fast recovery mechanisms were observed by Cormier et al. (2007b). A new constitutive modelling approach was hence recently proposed in a crystal viscoplasticity framework to capture the transient effects of such rapid microstructure evolutions on the creep behaviour and life (Cormier and Cailletaud (2010a)).
In this article, an updated version of this model is detailed. Special attention will be paid to (i) the effect of the accumulated plastic strain on the microstructure evolution, (ii) the introduction of an additional damage formulation, and (iii) the creep strain at failure. The performances of the model are illustrated on the basis of isothermal or complex non-isothermal creep experiments performed on nearly [001] oriented samples
Haldane gap in the quasi one-dimensional nonlinear -model
This work studies the appearance of a Haldane gap in quasi one-dimensional
antiferromagnets in the long wavelength limit, via the nonlinear
-model. The mapping from the three-dimensional, integer spin Heisenberg
model to the nonlinear -model is explained, taking into account two
antiferromagnetic couplings: one along the chain axis () and one along the
perpendicular planes () of a cubic lattice. An implicit equation for
the Haldane gap is derived, as a function of temperature and coupling ratio
. Solutions to these equations show the existence of a critical
coupling ratio beyond which a gap exists only above a transition temperature
. The cut-off dependence of these results is discussed.Comment: 14 pages (RevTeX 3.0), 3 PostScript figures appended (printing
instructions included
Semiclassical description of spin ladders
The Heisenberg spin ladder is studied in the semiclassical limit, via a
mapping to the nonlinear model. Different treatments are needed if the
inter-chain coupling is small, intermediate or large. For intermediate
coupling a single nonlinear model is used for the ladder. Its predicts
a spin gap for all nonzero values of if the sum of the spins
of the two chains is an integer, and no gap otherwise. For small , a better
treatment proceeds by coupling two nonlinear sigma models, one for each chain.
For integer , the saddle-point approximation predicts a sharp drop
in the gap as increases from zero. A Monte-Carlo simulation of a spin 1
ladder is presented which supports the analytical results.Comment: 8 pages, RevTeX 3.0, 4 PostScript figure
Fermionic description of spin-gap states of antiferromagnetic Heisenberg ladders in a magnetic field
Employing the Jordan-Wigner transformation on a unique path and then making a
mean-field treatment of the fermionic Hamiltonian, we semiquantitatively
describe the spin-gap states of Heisenberg ladders in a field. The appearance
of magnetization plateaux is clarified as a function of the number of legs.Comment: 2 pages, 3 figures embedded, J. Phys. Soc. Jpn. Vol. 71, No. 6, 1607
(2002
Density Matrix Renormalization Group Study of the Spin 1/2 Heisenberg Ladder with Antiferromagnetic Legs and Ferromagnetic Rungs
The ground state and low lying excitation of the spin 1/2 Heisenberg ladder
with antiferromagnetic leg () and ferromagnetic rung () interaction is studied by means of the density matrix renormalization
group method. It is found that the state remains in the Haldane phase even for
small suggesting the continuous transition to the gapless
phase at . The critical behavior for small is studied by
the finite size scaling analysis. The result is consistent with the recent
field theoretical prediction.Comment: 11 pages, revtex, figures upon reques
Variational states for the spin-Peierls system
We introduce a family of Jastrow pair product states for quasi
one-dimensional spin systems. Depending on a parameter they interpolate between
the resonating valence bond ground state of the Haldane-Shastry model
describing a spin liquid and the (dimerized) valence bond solid ground states
of the Majumdar-Ghosh spin chain. These states are found to form an excellent
basis for variational studies of Heisenberg chains with next nearest neighbour
interaction and bond alternation as realized in the spin-Peierls system
CuGeO_3.Comment: RevTeX+epsf macros, 24 pp. incl. figures, some references adde
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