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 La2−xSrxCuO4

In this work, we used a periodic lattice potential in order to model the infrared optical data of the high-temperature superconductor La2−xSrxCuO4. 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 fit 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 fits of the reflectivity. By refitting available reflectance data, we deduced a simple law for the doping dependence of the optical gap in La2−xSrxCuO4. In the present study, we argue that the optical gap is distinct from the pseudogap or the two-magnon gap, because it characterizes La2−xSrxCuO4 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 La2−xSrxCuO4. 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 fit 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 fits of the reflectivity. By refitting available reflectance data, we deduced a simple law for the doping dependence of the optical gap in La2−xSrxCuO4. In the present study, we argue that the optical gap is distinct from the pseudogap or the two-magnon gap, because it characterizes La2−xSrxCuO4 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, $J_\perp$, in the ladder. It is found that the gap is linear in $J_\perp$ for $J_\perp\gg 1$ and goes to zero for $J_\perp\to 0$. 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 $x$ and $y$ directions ($J_x \neq J_y$) 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 $20 \times 20$ 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