450 research outputs found
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
Current developments in gene therapy for amyotrophic lateral sclerosis.
INTRODUCTION: Amyotrophic lateral sclerosis (ALS) is a devastating adult neurodegenerative disorder characterized by motor neuron degeneration and death around 3 years from onset. So far, riluzole is the only treatment available, although it only offers a slight increase in survival. The complex etiology of ALS, with several genes able to trigger the disease, makes its study difficult. Areas covered: RNA-mediated or protein-mediated toxic gain-of-function leading to motor neuron degeneration appears to be likely common pathogenic mechanisms in ALS. Consequently, gene therapy technologies to reduce toxic RNA and/or proteins and to protect motor neurons by modulating gene expression are at the forefront of the field. Here, we review the most promising scientific advances, paying special attention to the successful treatments tested in animal models as well as analyzing relevant gene therapy clinical trials. Expert opinion: Despite broad advances in target gene identification in ALS and advances in gene therapy technologies, a successful gene therapy for ALS continues to elude researchers. Multiple hurdles encompassing technical, biological, economical and clinical challenges must be overcome before a therapy for patients becomes available. Optimism remains due to positive results obtained in several in vivo studies demonstrating significant disease amelioration in animal models of ALS
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
Spin-Peierls transition in an anisotropic two-dimensional XY model
The two-dimensional Jordan-Wigner transformation is used to investigate the
zero temperature spin-Peierls transition for an anisotropic two-dimensional XY
model in adiabatic limit. The phase diagram between the dimerized (D) state and
uniform (U) state is shown in the parameter space of dimensionless interchain
coupling and spin-lattice coupling . It is found
that the spin-lattice coupling must exceed some critical value
in order to reach the D phase for any finite . The dependence of on
is given by for and the transition between U and D
phase is of first-order for at least .Comment: 2 eps figures, considerable revisions were mad
Endoscopic Evaluation and Management of Gastrointestinal Bleeding in Patients with Ventricular Assist Devices
The optimal diagnostic approach and yield for gastrointestinal bleeding (GIB) in patients with ventricular assist devices (VAD) are unknown. We explored the etiology of bleeding and yield of upper and lower endoscopy, balloon-assisted enteroscopy, and video capsule endoscopy in the evaluation of GIB in patients with VADs. Methods. All VAD patients with overt gastrointestinal bleeding and drop in hematocrit from April 1, 2000 to July 31, 2008 were retrospectively reviewed. The endoscopic evaluation of each episode was recorded. Overall yield of EGD, colonoscopy, balloon-assisted, and video capsule endoscopy were evaluated. Results. Thirty-six bleeding episodes occurred involving 20 patients. The site of GIB was identified in 32/36 episodes (88.9%), and the etiology of bleeding was determined in 30/36 cases (83.3%). Five VAD patients underwent VCE. The VCE exams demonstrated a high yield with 80% of exams identifying the etiology of GIB. Endoscopic intervention was successful in 8/9 attempts. No adverse events were recorded. Two patients required surgical intervention for GIB. Conclusion. Upper, lower, video capsule, and balloon-assisted enteroscopies are safe and demonstrate a high yield in the investigation of gastrointestinal bleeding in VAD patients. Medical centers caring for VAD patients should employ a standardized protocol to optimize endoscopic evaluation and intervention
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
A Renormalization Group Method for Quasi One-dimensional Quantum Hamiltonians
A density-matrix renormalization group (DMRG) method for highly anisotropic
two-dimensional systems is presented. The method consists in applying the usual
DMRG in two steps. In the first step, a pure one dimensional calculation along
the longitudinal direction is made in order to generate a low energy
Hamiltonian. In the second step, the anisotropic 2D lattice is obtained by
coupling in the transverse direction the 1D Hamiltonians. The method is applied
to the anisotropic quantum spin half Heisenberg model on a square lattice.Comment: 4 pages, 4 figure
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
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