759 research outputs found
Spin-stiffness of anisotropic Heisenberg model on square lattice and possible mechanism for pinning of the electronic liquid crystal direction in YBCO
Using series expansions and spin-wave theory we calculate the spin-stiffness
anisotropy in Heisenberg models on the square lattice
with anisotropic couplings . We find that for the weakly anisotropic
spin-half model (), deviates
substantially from the naive estimate . We
argue that this deviation can be responsible for pinning the electronic liquid
crystal direction, a novel effect recently discovered in YBCO. For
completeness, we also study the spin-stiffness for arbitrary anisotropy
for spin-half and spin-one models. In the limit of ,
when the model reduces to weakly coupled chains, the two show dramatically
different behavior. In the spin-one model, the stiffness along the chains goes
to zero, implying the onset of Haldane-gap phase, whereas for spin-half the
stiffness along the chains increases monotonically from a value of
for towards for . Spin-wave theory is
extremely accurate for spin-one but breaks down for spin-half presumably due to
the onset of topological terms.Comment: 6 pages, 3 figure
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Local entanglement and confinement transitions in the random transverse-field Ising model on the pyrochlore lattice
We use numerical linked cluster expansions (NLC) and exact diagonalization to study confinement transitions out of the quantum spin liquid phase in the pyrochlore-lattice Ising antiferromagnet with random transverse fields. We calculate entanglement entropies associated with local regions defined by single tetrahedron to observe these transitions. The randomness-induced confinement transition is marked by a sharp reduction in the local entanglement and a concomitant increase in Ising correlations. In NLC, it is studied through the destruction of loop resonances due to random transverse-fields. The confining phase is characterized by a distribution of local entanglement entropies, which persists to large random fields
Variation of Area-to-Mass-Ratio of HAMR Space Debris Objects
An unexpected space debris population has been detected in 2004 Schildknecht
et al. (2003, 2004) with the unique properties of a very high area-to-mass
ratio (HAMR) Schildknecht et al. (2005a). Ever since it has been tried to
investigate the dynamical properties of those objects further. The orbits of
those objects are heavily perturbed by the effect of direct radiation pressure.
Unknown attitude motion complicates orbit prediction. The area-to-mass ratio of
the objects seems to be not stable over time. Only sparse optical data is
available for those objects in drift orbits. The current work uses optical
observations of five HAMR objects, observed over several years and investigates
the variation of their area-to-mass ratio and orbital parameters. A normalized
orbit determination setup has been established and validated with two low and
two of the high ratio objects, to ensure, that comparable orbits over longer
time spans are determined even with sparse optical data.Comment: 10 pages, accepted Monthly Notices of Royal Astronomical Society,
MN-11-1785-MJ.R1, The definitive version is available at
www.blackwell-synergy.co
Magnetic excitations and electronic interactions in SrCuTeO: a spin-1/2 square lattice Heisenberg antiferromagnet
SrCuTeO presents an opportunity for exploring low-dimensional
magnetism on a square lattice of Cu ions. We employ ab initio
multi-reference configuration interaction calculations to unravel the Cu
electronic structure and to evaluate exchange interactions in SrCuTeO.
The latter results are validated by inelastic neutron scattering using linear
spin-wave theory and series-expansion corrections for quantum effects to
extract true coupling parameters. Using this methodology, which is quite
general, we demonstrate that SrCuTeO is an almost realization of a
nearest-neighbor Heisenberg antiferromagnet but with relatively weak coupling
of 7.18(5) meV.Comment: 10 pages, 7 figure
Environmental correlates of hunting and bushmeat consumption in the Amazonian agricultural frontier.
Magnetic order in coupled spin-half and spin-one Heisenberg chains in anisotropic triangular-lattice geometry
We study spin-half and spin-one Heisenberg models in the limit where one
dimensional (1-D) linear chains, with exchange constant J1, are weakly coupled
in an anisotropic triangular lattice geometry. Results are obtained by means of
linked-cluster series expansions at zero temperature around different
magnetically ordered phases. We study the non-colinear spiral phases that arise
classically in the model and the colinear antiferromagnet that has been
recently proposed for the spin-half model by Starykh and Balents using a
Renormalization Group approach. We find that such phases can be stabilized in
the spin-half model for arbitrarily small coupling between the chains. For
vanishing coupling between the chains the energy of each phase must approach
that of decoupled linear chains. With increasing inter-chain coupling, the
non-colinear phase appears to have a lower energy in our calculations. For the
spin-one chain, we find that there is a critical interchain coupling needed to
overcome the Haldane gap. When spin-one chains are coupled in an unfrustrated
manner, the critical coupling is very small (~0.01J1) and agrees well with
previous chain mean-field studies. When they are coupled in the frustrated
triangular-lattice geometry, the critical coupling required to develop magnetic
order is substantially larger (> 0.3J1). The colinear phase is not obtained for
the spin-one Heisenberg model.Comment: 7 pages, 8 figure
Hunting and bushmeat consumption in post-frontier landscapes in eastern Amazonia: The importance of large-scale environmental driver.
Defining responders to therapies by a statistical modeling approach applied to randomized clinical trial data
Background: Personalized medicine is the tailoring of treatment to the individual characteristics of patients. Once a treatment has been tested in a clinical trial and its effect overall quantified, it would be of great value to be able to use the baseline patients' characteristics to identify patients with larger/lower benefits from treatment, for a more personalized approach to therapy. Methods: We show here a previously published statistical method, aimed at identifying patients' profiles associated to larger treatment benefits applied to three identical randomized clinical trials in multiple sclerosis, testing laquinimod vs placebo (ALLEGRO, BRAVO, and CONCERTO). We identified on the ALLEGRO patients' specific linear combinations of baseline variables, predicting heterogeneous response to treatment on disability progression. We choose the best score on the BRAVO, based on its ability to identify responders to treatment in this dataset. We finally got an external validation on the CONCERTO, testing on this new dataset the performance of the score in defining responders and non-responders. Results: The best response score defined on the ALLEGRO and the BRAVO was a linear combination of age, sex, previous relapses, brain volume, and MRI lesion activity. Splitting patients into responders and non-responders according to the score distribution, in the ALLEGRO, the hazard ratio (HR) for disability progression of laquinimod vs placebo was 0.38 for responders, HR = 1.31 for non-responders (interaction p = 0.0007). In the BRAVO, we had similar results: HR = 0.40 for responders and HR = 1.24 for non-responders (interaction p = 0.006). These findings were successfully replicated in the CONCERTO study, with HR = 0.44 for responders and HR=1.08 for non-responders (interaction p = 0.033). Conclusions: This study demonstrates the possibility to refine and personalize the treatment effect estimated in randomized studies by using the baseline demographic and clinical characteristics of the included patients. The method can be applied to any randomized trial in any medical condition to create a treatment-specific score associated to different levels of response to the treatment tested in the trial. This is an easy and affordable method toward therapy personalization, indicating patient profiles related to a larger benefit from a specific drug, which may have implications for taking clinical decisions in everyday clinical practice
Characteristics of drug-resistant tuberculosis in Abkhazia (Georgia), a high-prevalence area in Eastern Europe
Although multidrug-resistant (MDR) tuberculosis (TB) is a major public health problem in Eastern Europe, the factors contributing to emergence, spread and containment of MDR-TB are not well defined. Here, we analysed the characteristics of drug-resistant TB in a cross-sectional study in Abkhazia (Georgia) between 2003 and 2005, where standard short-course chemotherapy is supplemented with individualized drug-resistance therapy. Drug susceptibility testing (DST) and molecular typing were carried out for Mycobacterium tuberculosis complex strains from consecutive smear-positive TB patients. Out of 366 patients, 60.4% were resistant to any first-line drugs and 21% had MDR-TB. Overall, 25% of all strains belong to the Beijing genotype, which was found to be strongly associated with the risk of MDR-TB (OR 25.9, 95% CI 10.2-66.0) and transmission (OR 2.8, 95% CI 1.6-5.0). One dominant MDR Beijing clone represents 23% of all MDR-TB cases. The level of MDR-TB did not decline during the study period, coinciding with increasing levels of MDR Beijing strains among previously treated cases. Standard chemotherapy plus individualized drug-resistance therapy, guided by conventional DST, might be not sufficient to control MDR-TB in Eastern Europe in light of the spread of "highly transmissible" MDR Beijing strains circulating in the community
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