279 research outputs found
Deterministic and Stochastic Spin Diffusion in Classical Heisenberg Magnets
This computer simulation study provides further evidence that spin diffusion in the oneâdimensional classical Heisenberg model at T=â is anomalous: ăS j ( t )â
S j ă âŒt âα 1 withα1 âł1/2. However, the exponential instability of the numerically integrated phaseâspace trajectories transforms the deterministic transport of spin fluctuations into a computationally generated stochastic process in which the global conservation laws are still satisfied to high precision. This may cause a crossover in ăS j ( t )â
S j ă from anomalous spin diffusion (α1 âł 1/2) to normal spin diffusion (α1 = 1/2) at some characteristic time lag that depends on the precision of the numerical integration
Spin Diffusion in Classical Heisenberg Magnets with Uniform, Alternating, and Random Exchange
We have carried out an extensive simulation study for the spin autocorrelation function at T=â of the oneâdimensional classical Heisenberg model with four different types of isotropic bilinear nearestâneighbor coupling: uniform exchange, alternating exchange, and two kinds of random exchange. For the longâtime tails of all but one case, the simulation data seem incompatible with the simple âŒt â1/2 leading term predicted by spin diffusion phenomenology
Reconstruction of a first-order phase transition from computer simulations of individual phases and subphases
We present a new method for investigating first-order phase transitions using
Monte Carlo simulations. It relies on the multiple-histogram method and uses
solely histograms of individual phases. In addition, we extend the method to
include histograms of subphases. The free energy difference between phases,
necessary for attributing the correct statistical weights to the histograms, is
determined by a detour in control parameter space via auxiliary systems with
short relaxation times. We apply this method to a recently introduced model for
structure formation in polypeptides for which other methods fail.Comment: 13 pages in preprint mode, REVTeX, 2 Figures available from the
authors ([email protected], [email protected]
Spin dynamics simulations of the magnetic dynamics of RbMnF and direct comparison with experiment
Spin-dynamics techniques have been used to perform large-scale simulations of
the dynamic behavior of the classical Heisenberg antiferromagnet in simple
cubic lattices with linear sizes . This system is widely recognized
as an appropriate model for the magnetic properties of RbMnF.
Time-evolutions of spin configurations were determined numerically from coupled
equations of motion for individual spins using a new algorithm implemented by
Krech {\it etal}, which is based on fourth-order Suzuki-Trotter decompositions
of exponential operators. The dynamic structure factor was calculated from the
space- and time-displaced spin-spin correlation function. The crossover from
hydrodynamic to critical behavior of the dispersion curve and spin-wave
half-width was studied as the temperature was increased towards the critical
temperature. The dynamic critical exponent was estimated to be , which is slightly lower than the dynamic scaling prediction, but in
good agreement with a recent experimental value. Direct, quantitative
comparisons of both the dispersion curve and the lineshapes obtained from our
simulations with very recent experimental results for RbMnF are presented.Comment: 30 pages, RevTex, 9 figures, to appear in PR
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
Diet and subsistence in Bronze Age pastoral communities from the southern Russian steppes and the North Caucasus
The flanks of the Caucasus Mountains and the steppe landscape to their north offered highly productive grasslands for Bronze Age herders and their flocks of sheep, goat, and cattle. While the archaeological evidence points to a largely pastoral lifestyle, knowledge regarding the general composition of human diets and their variation across landscapes and during the different phases of the Bronze Age is still restricted. Human and animal skeletal remains from the burial mounds that dominate the archaeological landscape and their stable isotope compositions are major sources of dietary information. Here, we present stable carbon and nitrogen isotope data of bone collagen of 105 human and 50 animal individuals from the 5th millennium BC to the Sarmatian period, with a strong focus on the Bronze Age and its cultural units including Maykop, Yamnaya, Novotitorovskaya, North Caucasian, Catacomb, post-Catacomb and late Bronze Age groups. The samples comprise all inhumations with sufficient bone preservation from five burial mound sites and a flat grave cemetery as well as subsamples from three further sites. They represent the Caucasus Mountains in the south, the piedmont zone and Kuban steppe with humid steppe and forest vegetation to its north, and more arid regions in the Caspian steppe. The stable isotope compositions of the bone collagen of humans and animals varied across the study area and reflect regional diversity in environmental conditions and diets. The data agree with meat, milk, and/or dairy products from domesticated herbivores, especially from sheep and goats having contributed substantially to human diets, as it is common for a largely pastoral economy. This observation is also in correspondence with the faunal remains observed in the graves and offerings of animals in the mound shells. In addition, foodstuffs with elevated carbon and nitrogen isotope values, such as meat of unweaned animals, fish, or plants, also contributed to human diets, especially among communities living in the more arid landscapes. The regional distinction of the animal and human data with few outliers points to mobility radii that were largely concentrated within the environmental zones in which the respective sites are located. In general, dietary variation among the cultural entities as well as regarding age, sex and archaeologically indicated social status is only weakly reflected. There is, however, some indication for a dietary shift during the Early Bronze Age Maykop period
Inductively coupled plasma mass spectrometric detection for multielement flow injection analysis and elemental speciation by reversed-phase liquid chromatography
The feasibility of using an inductively coupled plasma mass spectrometer as a muitieiement detector for flow injection analysis (FIA) and ion-pair reversed-phase liquid chromatography was investigated. Sample introduction was by uitrasonk nebulization with aerosol desolvation. Absolute detecton limits for FIA ranged from 0.01 to 0.1 ng for most elements using 10-pL injections. Over 30 elements were surveyed for their response to both anionic and cationic ion pairing reagents. The separation and selective detection of various As and Se species were demonstrated, yielding detection limits near 0.1 ng (as element) for ail six species present. Determination of 15 elements in a single injection with multiple ion monitoring produced shniiar detection limits. Isotope ratios were measured with sufficient precision (better than 2%) and accuracy (about 1 %) on eluting peaks of Cd and Pb to demonstrate that liquid chromatographyhductively coupled plasma mass spectrometry should make speciation studies with stable tracer isotopes feasible
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