7,242 research outputs found
Spin transport in a one-dimensional anisotropic Heisenberg model
We analytically and numerically study spin transport in a one-dimensional
Heisenberg model in linear-response regime at infinite temperature. It is shown
that as the anisotropy parameter Delta is varied spin transport changes from
ballistic for Delta<1 to anomalous at the isotropic point Delta=1, to diffusive
for finite Delta>1, ending up as a perfect isolator in the Ising limit of
infinite Delta. Using perturbation theory for large Delta a quantitative
prediction is made for the dependence of diffusion constant on Delta.Comment: 5 pages, 4 figures; v2.: few comments added and typos corrected;
published versio
Static aerodynamic characteristics of three blunted sixty-degree half-angle cones at Mach numbers from 0.60 to 1.30
Subsonic and transonic static aerodynamic characteristics data on 60 deg half-angle cone
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Climate and plant community diversity in space and time.
Climate strongly shapes plant diversity over large spatial scales, with relatively warm and wet (benign, productive) regions supporting greater numbers of species. Unresolved aspects of this relationship include what causes it, whether it permeates to community diversity at smaller spatial scales, whether it is accompanied by patterns in functional and phylogenetic diversity as some hypotheses predict, and whether it is paralleled by climate-driven changes in diversity over time. Here, studies of Californian plants are reviewed and new analyses are conducted to synthesize climate-diversity relationships in space and time. Across spatial scales and organizational levels, plant diversity is maximized in more productive (wetter) climates, and these consistent spatial relationships are mirrored in losses of taxonomic, functional, and phylogenetic diversity over time during a recent climatic drying trend. These results support the tolerance and climatic niche conservatism hypotheses for climate-diversity relationships, and suggest there is some predictability to future changes in diversity in water-limited climates
Slow nucleic acid unzipping kinetics from sequence-defined barriers
Recent experiments on unzipping of RNA helix-loop structures by force have
shown that about 40-base molecules can undergo kinetic transitions between two
well-defined `open' and `closed' states, on a timescale = 1 sec [Liphardt et
al., Science 297, 733-737 (2001)]. Using a simple dynamical model, we show that
these phenomena result from the slow kinetics of crossing large free energy
barriers which separate the open and closed conformations. The dependence of
barriers on sequence along the helix, and on the size of the loop(s) is
analyzed. Some DNAs and RNAs sequences that could show dynamics on different
time scales, or three(or more)-state unzipping, are proposed.Comment: 8 pages Revtex, including 4 figure
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