539 research outputs found
Interaction effects between impurities in low dimensional spin-1/2 antiferromagnets
We are considering the interplay between several non-magnetic impurities in
the spin-1/2 Heisenberg antiferromagnet in chains, ladders and planes by
introducing static vacancies in numerical quantum Monte Carlo simulations. The
effective potential between two and more impurities is accurately determined,
which gives a direct measure of the quantum correlations in the systems. Large
effective interaction potentials are an indication of strong quantum
correlations in the system and reflect the detailed nature of the valence bond
ground states. In two-dimensions (2D) the interactions are smaller, but can
still be analyzed in terms of valence bonds.Comment: 8 pages, 6 figures, accepted by Europhys. Lett. The latest pdf file
is available at http://www.physik.uni-kl.de/eggert/papers/interact2d.pd
Is Neolithic land use correlated with demography? An evaluation of pollen-derived land cover and radiocarbon-inferred demographic change from Central Europe
The transformation of natural landscapes in Middle Europe began in the Neolithic as a result of the introduction of food-producing economies. This paper examines the relation between land-cover and demographic change in a regionally restricted case study. The study area is the Western Lake Constance area which has very detailed palynological as well as archaeological records. We compare land-cover change derived from nine pollen records using a pseudo-biomisation approach with 14C date probability density functions from archaeological sites which serve as a demographic proxy. We chose the Lake Constance area as a regional example where the pollen signal integrates a larger spatial pattern. The land-cover reconstructions for this region show first notable impacts at the Middle to Young Neolithic transition. The beginning of the Bronze Age is characterised by increases of arable land and pasture/meadow, whereas the deciduous woodland decreases dramatically. Changes in the land-cover classes show a correlation with the 14C density curve: the correlation is best with secondary woodland in the Young Neolithic which reflects the lake shore settlement dynamics. In the Early Bronze Age, the radiocarbon density correlates with open land-cover classes, such as pasture, meadow and arable land, reflecting a change in the land-use strategy. The close overall correspondence between the two archives implies that population dynamics and land-cover change were intrinsically linked. We therefore see human impact as a key driver for vegetation change in the Neolithic. Climate might have an influence on vegetation development, but the changes caused by human land use are clearly detectable from Neolithic times, at least in these densely settled, mid-altitude landscapes
Quantum Phase Transition of Randomly-Diluted Heisenberg Antiferromagnet on a Square Lattice
Ground-state magnetic properties of the diluted Heisenberg antiferromagnet on
a square lattice are investigated by means of the quantum Monte Carlo method
with the continuous-time loop algorithm. It is found that the critical
concentration of magnetic sites is independent of the spin size S, and equal to
the two-dimensional percolation threshold. However, the existence of quantum
fluctuations makes the critical exponents deviate from those of the classical
percolation transition. Furthermore, we found that the transition is not
universal, i.e., the critical exponents significantly depend on S.Comment: RevTeX, 4 pages including 5 EPS figure
Classical Correlation-Length Exponent in Non-Universal Quantum Phase Transition of Diluted Heisenberg Antiferromagnet
Critical behavior of the quantum phase transition of a site-diluted
Heisenberg antiferromagnet on a square lattice is investigated by means of the
quantum Monte Carlo simulation with the continuous-imaginary-time loop
algorithm. Although the staggered spin correlation function decays in a power
law with the exponent definitely depending on the spin size , the
correlation-length exponent is classical, i.e., . This implies that
the length scale characterizing the non-universal quantum phase transition is
nothing but the mean size of connected spin clusters.Comment: 4 pages, 3 figure
Early Iron Age agriculture: archaeobotanical evidence from an underground granary at Overbygård in northern Jutland, Denmark
Disorder Induced Phase Transition in a Random Quantum Antiferromagnet
A two-dimensional Heisenberg model with random antiferromagnetic
nearest-neighbor exchange is studied using quantum Monte Carlo techniques. As
the strength of the randomness is increased, the system undergoes a transition
from an antiferromagnetically ordered ground state to a gapless disordered
state. The finite-size scaling of the staggered structure factor and
susceptibility is consistent with a dynamic exponent .Comment: Revtex 3.0, 10 pages + 5 postscript figures available upon request,
UCSBTH-94-1
Testosterone Replacement in Hypogonadal Men With Type 2 Diabetes and/or Metabolic Syndrome (the TIMES2 Study)
Archaeobotanical investigations in the Aisne valley, northern France, from the neolithic up to the early Middle Ages
Testosterone treatment is not associated with increased risk of adverse cardiovascular events: results from the Registry of Hypogonadism in Men (RHYME).
SummaryAims The aim of this study was to assess cardiovascular (CV) safety of testosterone replacement therapy (TRT) in a large, diverse cohort of European men with hypogonadism (HG). Methods The Registry of Hypogonadism in Men (RHYME) was designed as a multi-national, longitudinal disease registry of men diagnosed with hypogonadism (HG) at 25 clinical sites in six European countries. Data collection included a complete medical history, physical examination, blood sampling and patient questionnaires at multiple study visits over 2–3 years. Independent adjudication was performed on all mortalities and CV outcomes. Results Of 999 patients enrolled with clinically diagnosed HG, 750 (75%) initiated some form of TRT. Registry participants, including both treated and untreated patients, contributed 23 900 person-months (99.6% of the targeted) follow-up time. A total of 55 reported CV events occurred in 41 patients. Overall, five patients died of CV-related causes (3 on TRT, 2 untreated) and none of the deaths were adjudicated as treatment-related. The overall CV incidence rate was 1522 per 100 000 person-years. CV event rates for men receiving TRT were not statistically different from untreated men (P=.70). Regardless of treatment assignment, CV event rates were higher in older men and in those with increased CV risk factors or a prior history of CV events. Conclusions Age and prior CV history, not TRT use, were predictors of new-onset CV events in this multi-national, prospective hypogonadism registry
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