7,992 research outputs found
Exploiting lattice potentials for sorting chiral particles
Several ways are demonstrated of how periodic potentials can be exploited for
sorting molecules or other small objects which only differ by their chirality.
With the help of a static bias force, the two chiral partners can be made to
move along orthogonal directions. Time-periodic external forces even lead to
motion into exactly opposite directions.Comment: 4 pages, 4 figure
Aging renewal theory and application to random walks
The versatility of renewal theory is owed to its abstract formulation.
Renewals can be interpreted as steps of a random walk, switching events in
two-state models, domain crossings of a random motion, etc. We here discuss a
renewal process in which successive events are separated by scale-free waiting
time periods. Among other ubiquitous long time properties, this process
exhibits aging: events counted initially in a time interval [0,t] statistically
strongly differ from those observed at later times [t_a,t_a+t]. In complex,
disordered media, processes with scale-free waiting times play a particularly
prominent role. We set up a unified analytical foundation for such anomalous
dynamics by discussing in detail the distribution of the aging renewal process.
We analyze its half-discrete, half-continuous nature and study its aging time
evolution. These results are readily used to discuss a scale-free anomalous
diffusion process, the continuous time random walk. By this we not only shed
light on the profound origins of its characteristic features, such as weak
ergodicity breaking. Along the way, we also add an extended discussion on aging
effects. In particular, we find that the aging behavior of time and ensemble
averages is conceptually very distinct, but their time scaling is identical at
high ages. Finally, we show how more complex motion models are readily
constructed on the basis of aging renewal dynamics.Comment: 21 pages, 7 figures, RevTe
SPH simulations of star/planet formation triggered by cloud-cloud collisions
We present results of hydrodynamic simulations of star formation triggered by
cloud-cloud collisions. During the early stages of star formation, low-mass
objects form by gravitational instabilities in protostellar discs. A number of
these low-mass objects are in the sub-stellar mass range, including a few
objects of planetary mass. The disc instabilities that lead to the formation of
low-mass objects in our simulations are the product of disc-disc interactions
and/or interactions between the discs and their surrounding gas.Comment: 8 pages, 7 figures; accepted for publication in the proceedings of
IAU Symposium 249: Exoplanets: Detection, Formation and Dynamics, Y.-S. Sun,
S. Ferraz-Mello & J.-L. Zhou (eds.), Cambridge University Pres
Quantitative sum rule analysis of low-temperature spectral functions
We analyze QCD and Weinberg-type sum rules in a low-temperature pion gas
using vector and axial-vector spectral functions following from the
model-independent chiral-mixing scheme. Toward this end we employ recently
constructed vacuum spectral functions with ground and first-excited states in
both channels and a universal perturbative continuum; they quantitatively
describe hadronic tau-decay data and satisfy vacuum sum rules. These features
facilitate the implementation of chiral mixing without further assumptions, and
lead to in-medium spectral functions which exhibit a mutual tendency of
compensating resonance and dip structures, suggestive for an approach toward
structureless distributions. In the sum rule analysis, we account for pion mass
corrections, which turn out to be significant. While the Weinberg sum rules
remain satisfied even at high temperatures, the numerical evaluation of the QCD
sum rules for vector and axial-vector channels reveals significant deviations
setting in for temperatures beyond ~140 MeV, suggestive of additional physics
beyond low-energy chiral pion dynamics.Comment: 8 pages, 3 figure
Melting of Branched RNA Molecules
Stability of the branching structure of an RNA molecule is an important
condition for its function. In this letter we show that the melting
thermodynamics of RNA molecules is very sensitive to their branching geometry
for the case of a molecule whose groundstate has the branching geometry of a
Cayley Tree and whose pairing interactions are described by the Go model.
Whereas RNA molecules with a linear geometry melt via a conventional continuous
phase transition with classical exponents, molecules with a Cayley Tree
geometry are found to have a free energy that seems smooth, at least within our
precision. Yet, we show analytically that this free energy in fact has a
mathematical singularity at the stability limit of the ordered structure. The
correlation length appears to diverge on the high-temperature side of this
singularity.Comment: 4 pages, 3 figure
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