693 research outputs found
Incommensurate nodes in the energy spectrum of weakly coupled antiferromagnetic Heisenberg ladders
Heisenberg ladders are investigated using the bond-mean-field theory
[M.Azzouz, Phys. Rev. B 48, 6136 (1993)]. The zero inter-ladder coupling energy
gap, the uniform spin susceptibility and the nuclear magnetic resonance
spin-relaxation rate are calculated as a function of temperature and magnetic
field. For weakly coupled ladders, the energy spectrum vanishes at
incommensurate wavevectors giving rise to nodes. As a consequence, the spin
susceptibility becomes linear at low temperature. Our results for the single
ladder successfully compare to experiments on SrCu_2O_3 and (VO)_2P_2O_7
materials and new predictions concerning the coupling to the magnetic field are
made.Comment: 4 revtex pages, 3 figures available upon reques
Quantum and classical criticalities in the frustrated two-leg Heisenberg ladder
This talk was about the frustration-induced criticality in the
antiferromagnetic Heisenberg model on the two-leg ladder with exchange
interactions along the chains, rungs, and diagonals, and also about the effect
of thermal fluctuations on this criticlity. The method used is the bond
mean-field theory, which is based on the Jordan-Wigner transformation in
dimensions higher than one. In this paper, we will summarize the main results
presented in this talk, and report on new results about the couplings and
temperature dependences of the spin susceptibility.Comment: 6 pages, 4 figures, talk presented at the Theory Canada 3 conference
in 2007, submitted to the Canadian Journal of Physic
Interplay between field-induced and frustration-induced quantum criticalities in the frustrated two-leg Heisenberg ladder
The antiferromagnetic Heisenberg two-leg ladder in the presence of
frustration and an external magnetic field is a system that is characterized by
two sorts of quantum criticalities, not only one. One criticality is the
consequence of intrinsic frustration, and the other one is a result of the
external magnetic field. So the behaviour of each of them in the presence of
the other deserves to be studied. Using the Jordan-Wigner transformation in
dimensions higher than one and bond-mean-field theory we examine the interplay
between the field-induced and frustration-induced quantum criticalities in this
system. The present work could constitute a prototype for those systems showing
multiple, perhaps sometimes competing, quantum criticalities. We calculate
several physical quantities like the magnetization and spin susceptibility as
functions of field and temperature.Comment: 9 pages, 8 figures, submitted to the Canadian Journal of Physic
Mean-field theory of the spin-Peierls systems: Application to CuGeO3
A mean-field theory of the spin Peierls systems based on the two dimensional
dimerized Heisenberg model is proposed by introducing an alternating bond order
parameter. Improvements with respect to previous mean-field results are found
in the one-dimensional limit for the ground state and the gap energies. In two
dimensions, the analysis of the competition between antiferromagnetic long
range order and the spin-Peierls ordering is given as a function of the
coupling constants. We show that the lowest energy gap to be observed does not
have a singlet-triplet character in agreement with the low temperature
thermodynamic properties of CuGeO3.Comment: 3 Revtex pages. Submitted to Rapid Comm. Figures available upon
reques
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
Viral delivery of antioxidant genes as a therapeutic strategy in experimental models of amyotrophic lateral sclerosis.
Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder with no effective treatment to date. Despite its multi-factorial aetiology, oxidative stress is hypothesized to be one of the key pathogenic mechanisms. It is thus proposed that manipulation of the expression of antioxidant genes that are downregulated in the presence of mutant SOD1 may serve as a therapeutic strategy for motor neuronal protection. Lentiviral vectors expressing either PRDX3 or NRF2 genes were tested in the motor neuronal-like NSC34 cell line, and in the ALS tissue culture model, NSC34 cells expressing the human SOD1(G93A) mutation. The NSC34 SOD1(G93A) cells overexpressing either PRDX3 or NRF2 showed a significant decrease in endogenous oxidation stress levels by 40 and 50% respectively compared with controls, whereas cell survival was increased by 30% in both cases. The neuroprotective potential of those two genes was further investigated in vivo in the SOD1(G93A) ALS mouse model, by administering intramuscular injections of adenoassociated virus serotype 6 (AAV6) expressing either of the target genes at a presymptomatic stage. Despite the absence of a significant effect in survival, disease onset or progression, which can be explained by the inefficient viral delivery, the promising in vitro data suggest that a more widespread CNS delivery is needed
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
Anisotropic two-dimensional Heisenberg model by Schwinger-boson Gutzwiller projected method
Two-dimensional Heisenberg model with anisotropic couplings in the and
directions () is considered. The model is first solved in the
Schwinger-boson mean-field approximation. Then the solution is Gutzwiller
projected to satisfy the local constraint that there is only one boson at each
site. The energy and spin-spin correlation of the obtained wavefunction are
calculated for systems with up to sites by means of the
variational Monte Carlo simulation. It is shown that the antiferromagnetic
long-range order remains down to the one-dimensional limit.Comment: 15 pages RevTex3.0, 4 figures, available upon request, GWRVB8-9
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