121 research outputs found
Noise-induced switching between vortex states with different polarization in classical two-dimensional easy-plane magnets
In the 2-dimensional anisotropic Heisenberg model with XY-symmetry there are
non-planar vortices which exhibit a localized structure of the z-components of
the spins around the vortex center. We study how thermal noise induces a
transition of this structure from one polarization to the opposite one. We
describe the vortex core by a discrete Hamiltonian and consider a stationary
solution of the Fokker-Planck equation. We find a bimodal distribution function
and calculate the transition rate using Langer's instanton theory (1969). The
result is compared with Langevin dynamics simulations for the full many-spin
model.Comment: 15 pages, 4 figures, Phys. Rev. B., in pres
Heat in Germany: Health risks and preventive measures
BACKGROUND: Climate change has already led to a significant temperature increase in Germany. The average temperature in the past decade was approximately 2°C above the pre-industrial level and eight of the ten hottest summers since the beginning of systematic weather records in 1881 were recorded in the last 30 years. METHODS: Based on a selective literature search and authors' own results, the article summarises the current state of knowledge on heat and its health impacts for Germany, addresses adaptation measures, and gives an outlook on implementation and research questions. RESULTS: Heat can aggravate pre-existing conditions such as diseases of the cardiovascular system, the respiratory tract, or the kidneys and trigger potentially harmful side effects for numerous medications. A significant increase in mortality is regularly observed during heat events. Previous approaches to mitigate the health impact of high temperatures include, for example, the heat alerts of the German Meteorological Service and recommendations for the preparation of heat-health action plans. CONCLUSIONS: Evidence on health impacts of heat and awareness of the need for heat-related health protection have grown in recent years, but there is still a need for further action and research
Recent Developments of World-Line Monte Carlo Methods
World-line quantum Monte Carlo methods are reviewed with an emphasis on
breakthroughs made in recent years. In particular, three algorithms -- the loop
algorithm, the worm algorithm, and the directed-loop algorithm -- for updating
world-line configurations are presented in a unified perspective. Detailed
descriptions of the algorithms in specific cases are also given.Comment: To appear in Journal of Physical Society of Japa
Topological solitons in highly anisotropic two dimensional ferromagnets
e study the solitons, stabilized by spin precession in a classical
two--dimensional lattice model of Heisenberg ferromagnets with non-small
easy--axis anisotropy. The properties of such solitons are treated both
analytically using the continuous model including higher then second powers of
magnetization gradients, and numerically for a discrete set of the spins on a
square lattice. The dependence of the soliton energy on the number of spin
deviations (bound magnons) is calculated. We have shown that the
topological solitons are stable if the number exceeds some critical value
. For and the intermediate values of anisotropy
constant ( is an exchange constant), the soliton
properties are similar to those for continuous model; for example, soliton
energy is increasing and the precession frequency is decreasing
monotonously with growth. For high enough anisotropy we found some fundamentally new soliton features absent for continuous
models incorporating even the higher powers of magnetization gradients. For
high anisotropy, the dependence of soliton energy E(N) on the number of bound
magnons become non-monotonic, with the minima at some "magic" numbers of bound
magnons. Soliton frequency have quite irregular behavior with
step-like jumps and negative values of for some regions of . Near
these regions, stable static soliton states, stabilized by the lattice effects,
exist.Comment: 17 page
Switching between different vortex states in 2-dimensional easy-plane magnets due to an ac magnetic field
Using a discrete model of 2-dimensional easy-plane classical ferromagnets, we
propose that a rotating magnetic field in the easy plane can switch a vortex
from one polarization to the opposite one if the amplitude exceeds a threshold
value, but the backward process does not occur. Such switches are indeed
observed in computer simulations.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
The linker domain of basal transcription factor TFIIB controls distinct recruitment and transcription stimulation functions
RNA polymerases (RNAPs) require basal transcription factors to assist them during transcription initiation. One of these factors, TFIIB, combines promoter recognition, recruitment of RNAP, promoter melting, start site selection and various post-initiation functions. The ability of 381 site-directed mutants in the TFIIB âlinker domainâ to stimulate abortive transcription was systematically quantitated using promoter-independent dinucleotide extension assays. The results revealed two distinct clusters (mjTFIIB E78-R80 and mjTFIIB R90-G94, respectively) that were particularly sensitive to substitutions. In contrast, a short sequence (mjTFIIB A81-K89) between these two clusters tolerated radical single amino acid substitutions; short deletions in that region even caused a marked increase in the ability of TFIIB to stimulate abortive transcription (âsuperstimulationâ). The superstimulating activity did, however, not correlate with increased recruitment of the TFIIB/RNAP complex because substitutions in a particular residue (mjTFIIB K87) increased recruitment by more than 5-fold without affecting the rate of abortive transcript stimulation. Our work demonstrates that highly localized changes within the TFIIB linker have profound, yet surprisingly disconnected, effects on RNAP recruitment, TFIIB/RNAP complex stability and the rate of transcription initiation. The identification of superstimulating TFIIB variants reveals the existence of a previously unknown rate-limiting step acting on the earliest stages of gene expression
Critical dynamics in the 2d classical XY-model: a spin dynamics study
Using spin-dynamics techniques we have performed large-scale computer
simulations of the dynamic behavior of the classical three component XY-model
(i.e. the anisotropic limit of an easy-plane Heisenberg ferromagnet), on square
lattices of size up to 192^2, for several temperatures below, at, and above
T_KT. The temporal evolution of spin configurations was determined numerically
from coupled equations of motion for individual spins by a fourth order
predictor-corrector method, with initial spin configurations generated by a
hybrid Monte Carlo algorithm. The neutron scattering function S(q,omega) was
calculated from the resultant space-time displaced spin-spin correlation
function. Pronounced spin-wave peaks were found both in the in-plane and the
out-of-plane scattering function over a wide range of temperatures. The
in-plane scattering function S^xx also has a large number of clear but weak
additional peaks, which we interpret to come from two-spin-wave scattering. In
addition, we observed a small central peak in S^xx, even at temperatures well
below the phase transition. We used dynamic finite size scaling theory to
extract the dynamic critical exponent z. We find z=1.00(4) for all T <= T_KT,
in excellent agreement with theoretical predictions, although the shape of
S(q,omega) is not well described by current theory.Comment: 31 pages, LaTex, 13 figures (38 subfigures) included as eps-files,
needs psfig, 260 K
Polarized Neutron Reflectometry of Nickel Corrosion Inhibitors.
Polarized neutron reflectometry has been used to investigate the detailed adsorption behavior and corrosion inhibition mechanism of two surfactants on a nickel surface under acidic conditions. Both the corrosion of the nickel surface and the structure of the adsorbed surfactant layer could be monitored in situ by the use of different solvent contrasts. Layer thicknesses and roughnesses were evaluated over a range of pH values, showing distinctly the superior corrosion inhibition of one negatively charged surfactant (sodium dodecyl sulfate) compared to a positively charged example (dodecyl trimethylammonium bromide) due to its stronger binding interaction with the surface. It was found that adequate corrosion inhibition occurs at significantly less than full surface coverage.X-ray photoelectron spectra were obtained at the National Engineering and Physical Sciences Research Council (EPSRC) XPS Userâs Service (NEXUS) at Newcastle University, an EPSRC midrange facility. NR data were obtained on the D17 instrument, and samples were treated in the laboratories of the Partnership for Soft Condensed Matter (PSCM) at the Institut Laue-Langevin. M.H.W. is grateful for funding from the Oppenheimer Trust.This is the final version of the article. It first appeared from the American Chemical Society via http://dx.doi.org/10.1021/acs.langmuir.5b0171
The nucleotide addition cycle of RNA polymerase is controlled by two molecular hinges in the Bridge Helix domain
Abstract Background Cellular RNA polymerases (RNAPs) are complex molecular machines that combine catalysis with concerted conformational changes in the active center. Previous work showed that kinking of a hinge region near the C-terminus of the Bridge Helix (BH-HC) plays a critical role in controlling the catalytic rate. Results Here, new evidence for the existence of an additional hinge region in the amino-terminal portion of the Bridge Helix domain (BH-HN) is presented. The nanomechanical properties of BH-HN emerge as a direct consequence of the highly conserved primary amino acid sequence. Mutations that are predicted to influence its flexibility cause corresponding changes in the rate of the nucleotide addition cycle (NAC). BH-HN displays functional properties that are distinct from BH-HC, suggesting that conformational changes in the Bridge Helix control the NAC via two independent mechanisms. Conclusions The properties of two distinct molecular hinges in the Bridge Helix of RNAP determine the functional contribution of this domain to key stages of the NAC by coordinating conformational changes in surrounding domains.</p
Inclusive production of charged hadrons and mesons in photon-photon collisions
The production of charged hadrons and K_s mesons in the collisions of quasi-real photons has been measured using the OPAL detector at LEP. The data were taken at e+e- centre-of-mass energies of 161 and 172 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the charged hadrons and K_s mesons have been compared to the leading order Monte Carlo simulations of PHOJET and PYTHIA and to perturbative next-to-leading order (NLO) QCD calculations. The distributions have been measured in the range 10-125 GeV of the hadronic invariant mass W. By comparing the transverse momentum distribution of charged hadrons measured in gamma-gamma interactions with gamma-proton and meson-proton data we find evidence for hard photon interactions in addition to the purely hadronic photon interactions.The production of charged hadrons and K_s mesons in the collisions of quasi-real photons has been measured using the OPAL detector at LEP. The data were taken at e+e- centre-of-mass energies of 161 and 172 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the charged hadrons and K_s mesons have been compared to the leading order Monte Carlo simulations of PHOJET and PYTHIA and to perturbative next-to-leading order (NLO) QCD calculations. The distributions have been measured in the range 10-125 GeV of the hadronic invariant mass W. By comparing the transverse momentum distribution of charged hadrons measured in gamma-gamma interactions with gamma-proton and meson-proton data we find evidence for hard photon interactions in addition to the purely hadronic photon interactions
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