464 research outputs found
Substitution effects on the temperature vs. magnetic-field phase diagrams of the quasi-1D effective Ising spin-1/2 chain system BaCoVO
BaCoVO is a one-dimensional antiferromagnetic spin-1/2 chain
system with pronounced Ising anisotropy of the magnetic exchange. Due to finite
interchain interactions long-range antiferromagnetic order develops below
K, which is accompanied by a structural distortion in
order to lift magnetic frustration effects. The corresponding temperature magnetic-field phase diagram is highly anisotropic with respect to the
magnetic-field direction and various details are still under vivid discussion.
Here, we report the influence of several substitutions on the magnetic
properties and the phase diagrams of BaCoVO. We investigate the
substitution series
BaSrCoVO
over the full range as well as the influence of a partial
substitution of the magnetic Co by small amounts of other magnetic
transition metals or by non-magnetic magnesium. In all cases, the phase
diagrams were obtained on single crystals from magnetization data and/or
high-resolution studies of the thermal expansion and magnetostriction.Comment: 10 pages, 10 figure
Competition between intermediate plaquette phases in SrCu(BO) under pressure
Building on the growing evidence based on NMR, magnetization, neutron
scattering, ESR, and specific heat that, under pressure, SrCu(BO)
has an intermediate phase between the dimer and the N\'eel phase, we study the
competition between two candidate phases in the context of a minimal model that
includes two types of intra- and inter-dimer interactions without enlarging the
unit cell. We show that the empty plaquette phase of the Shastry-Sutherland
model is quickly replaced by a quasi-1D full plaquette phase when intra- and/or
inter-dimer couplings take different values, and that this full plaquette phase
is in much better agreement with available experimental data than the empty
plaquette one.Comment: 19 page
Thermodynamic properties of the Shastry-Sutherland model from quantum Monte Carlo simulations
We investigate the minus-sign problem that afflicts quantum Monte Carlo (QMC)
simulations of frustrated quantum spin systems, focusing on spin S=1/2, two
spatial dimensions, and the extended Shastry-Sutherland model. We show that
formulating the Hamiltonian in the diagonal dimer basis leads to a sign problem
that becomes negligible at low temperatures for small and intermediate values
of the ratio of the inter- and intradimer couplings. This is a consequence of
the fact that the product state of dimer singlets is the exact ground state
both of the extended Shastry-Sutherland model and of a corresponding
"sign-problem-free" model, obtained by changing the signs of all positive
off-diagonal matrix elements in the dimer basis. By exploiting this insight, we
map the sign problem throughout the extended parameter space from the
Shastry-Sutherland to the fully frustrated bilayer model and compare it with
the phase diagram computed by tensor-network methods. We use QMC to compute
with high accuracy the temperature dependence of the magnetic specific heat and
susceptibility of the Shastry-Sutherland model for large systems up to a
coupling ratio of 0.526(1) and down to zero temperature. For larger coupling
ratios, our QMC results assist us in benchmarking the evolution of the
thermodynamic properties by systematic comparison with exact diagonalization
calculations and interpolated high-temperature series expansions.Comment: 13 pages including 10 figures; published version with minor changes
and correction
Quantum Criticality of an Ising-like Spin-1/2 Antiferromagnetic Chain in Transverse Magnetic Field
We report on magnetization, sound velocity, and magnetocaloric-effect
measurements of the Ising-like spin-1/2 antiferromagnetic chain system
BaCoVO as a function of temperature down to 1.3 K and applied
transverse magnetic field up to 60 T. While across the N\'{e}el temperature of
K anomalies in magnetization and sound velocity confirm the
antiferromagnetic ordering transition, at the lowest temperature the
field-dependent measurements reveal a sharp softening of sound velocity
and a clear minimum of temperature at T,
indicating the suppression of the antiferromagnetic order. At higher fields,
the curve shows a broad minimum at T, accompanied by a
broad minimum in the sound velocity and a saturation-like magnetization. These
features signal a quantum phase transition which is further characterized by
the divergent behavior of the Gr\"{u}neisen parameter . By contrast, around the critical field, the
Gr\"{u}neisen parameter converges as temperature decreases, pointing to a
quantum critical point of the one-dimensional transverse-field Ising model.Comment: Phys. Rev. Lett., to appea
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The value of adding regional to local stakeholder involvement in evaluating the acceptability of innovative technologies
Technology is urgently needed to clean up contamination by volatile organic compounds at United States Department of Energy (DOE) sites. In many cases, however, existing technology is too slow, inefficient, or expensive. The record of technology development is, in some cases, similarly disappointing. Remediation technologies developed at great expense and evaluated piecemeal over long periods have not been deployed because, in the end, the public judged them ineffective or unacceptable. The need for successful methods of remediation is too great and resources too limited to continue with ineffective technology evaluation. In order to make good decisions about which technologies to deploy, remedial project managers need to know stakeholders` requirements for the performance of proposed technologies. Expanding stakeholder involvement regionally identifies the concerns of a broad range of stakeholders at and DOE sites throughout the West -- issues that must be taken into account if technologies are to be accepted for wide deployment
Structure activity related, mechanistic, and modeling studies of gallotannins containing a glucitol-core and α-glucosidase
Gallotannins containing a glucitol core, which are only produced by members of the maple (Acer) genus, are more potent α-glucosidase inhibitors than the clinical drug, acarbose. While this activity is influenced by the number of substituents on the glucitol core (e.g. more galloyl groups leads to increased activity), the mechanisms of inhibitory action are not known. Herein, we investigated ligand–enzyme interactions and binding mechanisms of a series of ‘glucitol-core containing gallotannins (GCGs)’ against the α-glucosidase enzyme. The GCGs included ginnalins A, B and C (containing two, one, and one galloyl/s, respectively), maplexin F (containing 3 galloyls) and maplexin J (containing 4 galloyls). All of the GCGs were noncompetitive inhibitors of α-glucosidase and their interactions with the enzyme were further explored using biophysical and spectroscopic measurements. Thermodynamic parameters (by isothermal titration calorimetry) revealed a 1 : 1 binding ratio between GCGs and α-glucosidase. The binding regions between the GCGs and α-glucosidase, probed by a fluorescent tag, 1,1′-bis(4-anilino-5-naphthalenesulfonic acid), revealed that the GCGs decreased the hydrophobic surface of the enzyme. In addition, circular dichroism analyses showed that the GCGs bind to α-glucosidase and lead to loss of the secondary α-helix structure of the protein. Also, molecular modeling was used to predict the binding site between the GCGs and the α-glucosidase enzyme. This is the first study to evaluate the mechanisms of inhibitory activities of gallotannins containing a glucitol core on α-glucosidase
Water Dynamics Around Proteins: T- and R-States of Hemoglobin and Melittin
The water dynamics, as characterized by the local hydrophobicity (LH), is
investigated for tetrameric hemoglobin and dimeric melittin. For the T0 to R0
transition in Hb it is found that LH provides additional molecular-level
insight into the Perutz mechanism, i.e., the breaking and formation of salt
bridges at the alpha1 / beta2 and alpha2 / beta1 interface is accompanied by
changes in LH. For Hb in cubic water boxes with 90 Aengstroem and 120
Aengstroem edge length it is observed that following a decrease in LH as a
consequence of reduced water density or change of water orientation at the
protein/water interface the alpha / beta interfaces are destabilized; this is a
hallmark of the Perutz stereochemical model for the T to R transition in Hb.
The present work thus provides a dynamical view of the classical structural
model relevant to the molecular foundations of Hb function. For dimeric
melittin, earlier results by Cheng and Rossky (Nature, 1998, 392, 696-699) are
confirmed and interpreted on the basis of LH from simulations in which the
protein structure is frozen. For the flexible melittin dimer the changes in the
local hydration can be as much as 30 % than for the rigid dimer, reflecting the
fact that protein and water dynamics are coupled
Pretty Picky for a Generalist: Impacts of Toxicity and Nutritional Quality on Mantid Prey Processing
Prey have evolved a number of defenses against predation, and predators have developed means of countering these protective measures. Although caterpillars of the monarch butterfly, Danaus plexippus L., are defended by cardenolides sequestered from their host plants, the Chinese mantid Tenodera sinensis Saussure guts the caterpillar before consuming the rest of the body. We hypothesized that this gutting behavior might be driven by the heterogeneous quality of prey tissue with respect to toxicity and/or nutrients. We conducted behavioral trials in which mantids were offered cardenolide-containing and cardenolide-free D. plexippus caterpillars and butterflies. In addition, we fed mantids starved and unstarved D. plexippus caterpillars from each cardenolide treatment and nontoxic Ostrinia nubilalis Hübner caterpillars. These trials were coupled with elemental analysis of the gut and body tissues of both D. plexippus caterpillars and corn borers. Cardenolides did not affect mantid behavior: mantids gutted both cardenolide-containing and cardenolide-free caterpillars. In contrast, mantids consumed both O. nubilalis and starved D. plexippus caterpillars entirely. Danaus plexippus body tissue has a lower C:N ratio than their gut contents, while O. nubilalis have similar ratios; gutting may reflect the mantid’s ability to regulate nutrient uptake. Our results suggest that post-capture prey processing by mantids is likely driven by a sophisticated assessment of resource quality
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