9,827 research outputs found
Evaluation of configurational entropy of a model liquid from computer simulations
Computer simulations have been employed in recent years to evaluate the
configurational entropy changes in model glass-forming liquids. We consider two
methods, both of which involve the calculation of the `intra-basin' entropy as
a means for obtaining the configurational entropy. The first method involves
the evaluation of the intra-basin entropy from the vibrational frequencies of
inherent structures, by making a harmonic approximation of the local potential
energy topography. The second method employs simulations that confine the
liquid within a localized region of configuration space by the imposition of
constraints; apart from the choice of the constraints, no further assumptions
are made. We compare the configurational entropies estimated for a model liquid
(binary mixture of particles interacting {\it via} the Lennard-Jones potential)
for a range of temperatures, at fixed density.Comment: 10 pages, 5 figures, Proceedings of "Unifying Concepts in Glass
Physics" Trieste 1999 (to appear in J. Phys. Cond. Mat.
Local effective dynamics of quantum systems: A generalized approach to work and heat
By computing the local energy expectation values with respect to some local
measurement basis we show that for any quantum system there are two
fundamentally different contributions: changes in energy that do not alter the
local von Neumann entropy and changes that do. We identify the former as work
and the latter as heat. Since our derivation makes no assumptions on the system
Hamiltonian or its state, the result is valid even for states arbitrarily far
from equilibrium. Examples are discussed ranging from the classical limit to
purely quantum mechanical scenarios, i.e. where the Hamiltonian and the density
operator do not commute.Comment: 5 pages, 1 figure, published versio
A New Version of Reimers' law of Mass Loss Based on a Physical Approach
We present a new semi-empirical relation for the mass loss of cool stellar
winds, which so far has frequently been described by "Reimers' law".
Originally, this relation was based solely on dimensional scaling arguments
without any physical interpretation. In our approach, the wind is assumed to
result from the spill-over of the extended chromosphere, possibly associated
with the action of waves, especially Alfven waves, which are used as guidance
in the derivation of the new formula. We obtain a relation akin to the original
Reimers law, but which includes two new factors. They reflect how the
chromospheric height depends on gravity and how the mechanical energy flux
depends, mainly, on effective temperature. The new relation is tested and
sensitively calibrated by modelling the blue end of the Horizontal Branch of
globular clusters. The most significant difference from mass loss rates
predicted by the Reimers relation is an increase by up to a factor of 3 for
luminous late-type (super-)giants, in good agreement with observations.Comment: 12 pages, 4 figures, accepted by ApJ Letter
Enhanced dispersion interaction between quasi-one dimensional conducting collinear structures
Recent investigations have highlighted the failure of a sum of terms
to represent the dispersion interaction in parallel metallic, anisotropic,
linear or planar nanostructures [J. F. Dobson, A. White, and A. Rubio, Phys.
Rev. Lett. 96, 073201 (2006) and references therein]. By applying a simple
coupled plasmon approach and using electron hydrodynamics, we numerically
evaluate the dispersion (non-contact van der Waals) interaction between two
conducting wires in a collinear pointing configuration. This case is compared
to that of two insulating wires in an identical geometry, where the dispersion
interaction is modelled both within a pairwise summation framework, and by
adding a pinning potential to our theory leading to a standard oscillator-type
model of insulating dielectric behavior. Our results provide a further example
of enhanced dispersion interaction between two conducting nanosystems compared
to the case of two insulating ones. Unlike our previous work, this calculation
explores a region of relatively close coupling where, although the electronic
clouds do not overlap, we are still far from the asymptotic region where a
single power law describes the dispersion energy. We find that strong
differences in dispersion attraction between metallic and semiconducting /
insulating cases persist into this non-asymptotic region. While our theory will
need to be supplemented with additional short-ranged terms when the electronic
clouds overlap, it does not suffer from the short-distance divergence exhibited
by purely asymptotic theories, and gives a natural saturation of the dispersion
energy as the wires come into contact.Comment: 10 pages, 5 figures. Added new extended numerical calculations, new
figures, extra references and heavily revised tex
New method for the time calibration of an interferometric radio antenna array
Digital radio antenna arrays, like LOPES (LOFAR PrototypE Station), detect
high-energy cosmic rays via the radio emission from atmospheric extensive air
showers. LOPES is an array of dipole antennas placed within and triggered by
the KASCADE-Grande experiment on site of the Karlsruhe Institute of Technology,
Germany. The antennas are digitally combined to build a radio interferometer by
forming a beam into the air shower arrival direction which allows measurements
even at low signal-to-noise ratios in individual antennas. This technique
requires a precise time calibration. A combination of several calibration steps
is used to achieve the necessary timing accuracy of about 1 ns. The group
delays of the setup are measured, the frequency dependence of these delays
(dispersion) is corrected in the subsequent data analysis, and variations of
the delays with time are monitored. We use a transmitting reference antenna, a
beacon, which continuously emits sine waves at known frequencies. Variations of
the relative delays between the antennas can be detected and corrected for at
each recorded event by measuring the phases at the beacon frequencies.Comment: 9 pages, 9 figures, 1 table, pre-print of article published in
Nuclear Inst. and Methods in Physics Research, A, available at:
http://www.sciencedirect.com/science/article/B6TJM-4Y9CF4B-4/2/37bfcb899a0f387d9875a5a0729593a
Geometry: The leading parameter for the Poisson’s ratio of bending-dominated cellular solids
Control over the deformation behaviour that a cellular structure shows in response to imposed external forces is a requirement for the effective design of mechanical metamaterials, in particular those with negative Poisson’s ratio. This article sheds light on the old question of the relationship between geometric microstructure and mechanical response, by comparison of the deformation properties of bar-and-joint-frameworks with those of their realisation as a cellular solid made from linear-elastic material. For ordered planar tessellation models, we find a classification in terms of the number of degrees of freedom of the framework model: first, in cases where the geometry uniquely prescribes a single deformation mode of the framework model, the mechanical deformation and Poisson’s ratio of the linearly-elastic cellular solid closely follow those of the unique deformation mode; the result is a bending-dominated deformation with negligible dependence of the effective Poisson’s ratio on the underlying material’s Poisson’s ratio and small values of the effective Young’s modulus. Second, in the case of rigid structures or when geometric degeneracy prevents the bending-dominated deformation mode, the effective Poisson’s ratio is material-dependent and the Young’s modulus View the MathML sourceE˜cs large. All analysed structures of this type have positive values of the Poisson’s ratio and large values of View the MathML sourceE˜cs. Third, in the case, where the framework has multiple deformation modes, geometry alone does not suffice to determine the mechanical deformation. These results clarify the relationship between mechanical properties of a linear-elastic cellular solid and its corresponding bar-and-joint framework abstraction. They also raise the question if, in essence, auxetic behaviour is restricted to the geometry-guided class of bending-dominated structures corresponding to unique mechanisms, with inherently low values of the Young’s modulus
On the Response of an OST to a Point-like Heat Source
A new technique of superconducting cavity diagnostics has been introduced by
D. Hartrill at Cornell University, Ithaca, USA. Oscillating Superleak
Transducers (OST) detect the heat transferred from a cavity's quench point via
"Second Sound" through the superfluid He bath, needed to cool the
superconducting cavity. The observed response of an OST is a complex, but
reproducible pattern of oscillations. A small helium evaporation cryostat was
built which allows the investigation of the response of an OST in greater
detail. The distance between a point-like electrical heater and the OST can be
varied. The OST can be mounted either parallel or perpendicular to the plate,
housing the heat source. If the artificial quench-point releases an amount of
energy compatible to a real quench spot on a cavity's surface, the OST signal
starts with a negative pulse, which is usually strong enough to allow automatic
detection. Furthermore, the reflection of the Second Sound on the wall is
observed. A reflection coefficient R = 0.39 +- 0.05 of the glass wall is
measured. This excludes a strong influence of multiple reflections in the
complex OST response. Fourier analyses show three main frequencies, found in
all OST spectra. They can be interpreted as modes of an oscillating circular
membrane.Comment: 10 pages, 16 figure
Abstract basins of attraction
Abstract basins appear naturally in different areas of several complex
variables. In this survey we want to describe three different topics in which
they play an important role, leading to interesting open problems
Relativistic calculations of pionic and kaonic atoms hyperfine structure
We present the relativistic calculation of the hyperfine structure in pionic
and kaonic atoms. A perturbation method has been applied to the Klein-Gordon
equation to take into account the relativistic corrections. The perturbation
operator has been obtained \textit{via} a multipole expansion of the nuclear
electromagnetic potential. The hyperfine structure of pionic and kaonic atoms
provide an additional term in the quantum electrodynamics calculation of the
energy transition of these systems. Such a correction is required for a recent
measurement of the pion mass
High affinity immobilization of proteins using the CrAsH/TC tag
Protein microarrays represent important tools for biomedical analysis. We have recently described the use of the biarsenical-tetracysteine (TC) tag for the preparation of protein microarrays. The unique feature of this tag enables the site-specific immobilization of TC-containing proteins on biarsenical-modified surfaces, resulting in a fluorescence enhancement that allows the direct quantification of the immobilized proteins. Moreover, the reversibility of the binding upon incubation with large quantities of thiols permits the detachment of the proteins from the surface, thereby enabling recovery of the substrate to extend the life time of the slide. Herein, we describe our recent results that further extend the applicability of the CrAsH/TC tag to the fabrication of biochips. With this aim, the immobilization of proteins on surfaces has been investigated using two different spacers and two TC tags, the minimal TC sequence (CCPGCC) and an optimized motif (FLNCCPGCCMEP). While the minimal peptide motif enables a rapid recycling of the slide, the optimized TC sequence reveals an increased affinity due to its greater resistance to displacement by thiols. Moreover, the developed methodology was applied to the immobilization of proteins via on-chip ligation of recombinant protein thioesters
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