332 research outputs found
Non-Equilibrium Casimir Force between Vibrating Plates
We study the fluctuation-induced, time-dependent force between two plates confining a correlated fluid which is driven out of equilibrium mechanically by harmonic vibrations of one of the plates. For a purely relaxational dynamics of the fluid we calculate the fluctuation-induced force generated by the vibrating plate on the plate at rest. The time-dependence of this force is characterized by a positive lag time with respect to the driving. We obtain two distinctive contributions to the force, one generated by diffusion of stress in the fluid and another related to resonant dissipation in the cavity. The relation to the dynamic Casimir effect of the electromagnetic field and possible experiments to measure the time-dependent Casimir force are discussed. © 2013 Hanke
Critical Adsorption on Defects in Ising Magnets and Binary Alloys
Long-range correlations in a magnet close to its critical point or in a binary alloy close to a continuous order-disorder transition can substantially enhance the effect of local perturbations. It is demonstrated using a position-space renormalization procedure that quasi-one-dimensional defects which break the symmetry of the order parameter have pronounced effects: They cause long-range critical adsorption profiles and give rise to new universal critical exponents, which are identified and calculated using field-theoretical methods
Single DNA conformations and biological function
From a nanoscience perspective, cellular processes and their reduced in vitro
imitations provide extraordinary examples for highly robust few or single
molecule reaction pathways. A prime example are biochemical reactions involving
DNA molecules, and the coupling of these reactions to the physical
conformations of DNA. In this review, we summarise recent results on the
following phenomena: We investigate the biophysical properties of DNA-looping
and the equilibrium configurations of DNA-knots, whose relevance to biological
processes are increasingly appreciated. We discuss how random DNA-looping may
be related to the efficiency of the target search process of proteins for their
specific binding site on the DNA molecule. And we dwell on the spontaneous
formation of intermittent DNA nanobubbles and their importance for biological
processes, such as transcription initiation. The physical properties of DNA may
indeed turn out to be particularly suitable for the use of DNA in nanosensing
applications.Comment: 53 pages, 45 figures. Slightly revised version of a review article,
that is going to appear in the J. Comput. Theoret. Nanoscience; some typos
correcte
Density of states of a damped quantum oscillator
We calculate the density of states of a damped quantum-mechanical harmonic oscillator which is described by a Caldeira-Leggett type model with Ohmic dissipation and a Drude-like cutoff. From the exact expression for the associated partition function, we derive the asymptotic behavior of the density of states using Tauberian theorems. An effective algorithm to evaluate the density of states is presented and examples are given. It is pointed out that the calculated density of states is an experimentally accessible quantity
A Healthy and Ecologically Balanced Environment: An Argument for a Third Generation Right
Chymotrypsin-like serine proteases are found in high abundance in mast cell granules. By site-directed mutatgenesis, we have previously shown that basic amino acids in positions 143 and 192 (Arg and Lys respectively) of the human mast cell chymase are responsible for an acidic amino acid residue preference in the P2' position of substrates. In order to study the influence of these two residues in determining the specificity of chymase inhibitors, we have synthesized five different potent inhibitors of the human chymase. The inhibitory effects of these compounds were tested against the wild-type enzyme, against two single mutants Arg143Gln and Lys192Met and against a double mutant, Arg143Gln+Lys192Met. We observed a markedly reduced activity of all five inhibitors with the double mutant, indicating that these two basic residues are involved in conferring the specificity of these inhibitors. The single mutants showed an intermediate phenotype, with the strongest effect on the inhibitor by the mutation in Lys192. The Lys192 and the double mutations also affected the rate of cleavage of angiotensin I but did not seem to affect the specificity in the cleavage of the Tyr(4)-Ile(5) bond. A more detailed knowledge about which amino acids that confer the specificity of an enzyme can prove to be of major importance for development of highly specific inhibitors for the human chymase and other medically important enzymes
Core-Collapse Supernovae: Reflections and Directions
Core-collapse supernovae are among the most fascinating phenomena in
astrophysics and provide a formidable challenge for theoretical investigation.
They mark the spectacular end of the lives of massive stars and, in an
explosive eruption, release as much energy as the sun produces during its whole
life. A better understanding of the astrophysical role of supernovae as birth
sites of neutron stars, black holes, and heavy chemical elements, and more
reliable predictions of the observable signals from stellar death events are
tightly linked to the solution of the long-standing puzzle how collapsing stars
achieve to explode. In this article our current knowledge of the processes that
contribute to the success of the explosion mechanism are concisely reviewed.
After a short overview of the sequence of stages of stellar core-collapse
events, the general properties of the progenitor-dependent neutrino emission
will be briefly described. Applying sophisticated neutrino transport in
axisymmetric (2D) simulations with general relativity as well as in simulations
with an approximate treatment of relativistic effects, we could find successful
neutrino-driven explosions for a growing set of progenitor stars. First results
of three-dimensional (3D) models have been obtained, and magnetohydrodynamic
simulations demonstrate that strong initial magnetic fields in the pre-collapse
core can foster the onset of neutrino-powered supernova explosions even in
nonrotating stars. These results are discussed in the context of the present
controversy about the value of 2D simulations for exploring the supernova
mechanism in realistic 3D environments, and they are interpreted against the
background of the current disagreement on the question whether the standing
accretion shock instability (SASI) or neutrino-driven convection is the crucial
agency that supports the onset of the explosion.Comment: 36 pages, 20 figures (43 eps files); submitted to Progress of
Theoretical and Experimental Physics (PTEP
Polymers interacting with spherical and rodlike particles
The interaction of a long flexible polymer chain with mesoscopic particles of spherical or elongated cylindrical shape is investigated by field-theoretic methods using the polymer-magnet analogy. In the case that these particles are immersed in a dilute polymer solution and exhibit purely repulsive surfaces we study density profiles for monomers and chain ends near such a particle, the change of configurational entropy by immersing a particle into the solution, and the depletion interaction between a particle and a distant planar wall. Both ideal chains and chains with an excluded-volume interaction are considered. We also analyze particle surfaces with a short-ranged attraction and the adsorption-desorption transition for an ideal polymer chain. Properties such as the number of surface contacts are evaluated both in the adsorbed limit, in which the thickness of the adsorbed layer is much smaller than the unperturbed polymer size so that ground-state dominance applies, and at the adsorption threshold
Kinematics of outer halo globular clusters: M 75 and NGC 6426
Globular clusters (GCs) and their dynamic interactions with the Galactic
components provide an important insight into the structure and formation of the
early Milky Way. Here, we present a kinematic study of two outer halo GCs based
on a combination of VLT/FORS2, VLT/FLAMES, and Magellan/MIKE low- and
high-resolution spectroscopy of 32 and 27 member stars, respectively. Although
both clusters are located at Galactocentric distances of 15 kpc, they have
otherwise very different properties. M 75 is a luminous and metal-rich system
at [Fe/H] = dex, a value that we confirm from the calcium triplet
region. This GC shows mild evidence for rotation with an amplitude of A5 km s. One of the most metal-poor GCs in the Milky Way (at
[FeII/H] = dex), NGC 6426 exhibits marginal evidence of internal
rotation at the 2 km s level. Both objects have velocity dispersions
that are consistent with their luminosity. Although limited by small-number
statistics, the resulting limits on their ratios suggest
that M 75 is a slow rotator driven by internal dynamics rather than being
effected by the weak Galactic tides at its large distances. Here, M 75 () is fully consistent with the properties of other, younger
halo clusters. At , NGC 6426 appears to have a
remarkably ordered internal motion for its low metallicity, but the large
uncertainty does not allow for an unambiguous categorization as a fast rotator.
An accretion origin of M 75 cannot be excluded, based on the eccentric orbit,
which we derived from the recent data release 2 of Gaia, and considering its
younger age.Comment: 9 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
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