3,528 research outputs found
Automatic sample rotator for metallographic polishing
Simple, inexpensive device can be attached to most metallographic sample polishing tables. It provides a suitable surface finish for microscopic examination or photography of surface details of the samples
Thermodynamic constraints on fluctuation phenomena
The relationships between reversible Carnot cycles, the absence of perpetual
motion machines and the existence of a non-decreasing, globally unique entropy
function forms the starting point of many textbook presentations of the
foundations of thermodynamics. However, the thermal fluctuation phenomena
associated with statistical mechanics has been argued to restrict the domain of
validity of this basis of the second law of thermodynamics. Here we demonstrate
that fluctuation phenomena can be incorporated into the traditional
presentation, extending, rather than restricting, the domain of validity of the
phenomenologically motivated second law. Consistency conditions lead to
constraints upon the possible spectrum of thermal fluctuations. In a special
case this uniquely selects the Gibbs canonical distribution and more generally
incorporates the Tsallis distributions. No particular model of microscopic
dynamics need be assumed.Comment: 12 pages, 24 figure
Precise overgrowth composition during biomineral culture and inorganic precipitation
We introduce a method to analyze element ratios and isotope ratios in mineral overgrowths. This general technique can quantify environmental controls on proxy behavior for a range of cultured biominerals and can also measure compositional effects during seeded mineral growth. Using a media enriched in multiple stable isotopes, the method requires neither the mass nor the composition of the initial seed or skeleton to be known and involves only bulk isotope measurements. By harnessing the stability and sensitivity of bulk analysis the new approach promises high precision measurements for a range of elements and isotopes. This list includes trace species and select non-traditional stable isotopes, systems where sensitivity and external reproducibility currently limit alternative approaches like secondary ion mass spectrometry (SIMS) and laser ablation mass spectrometry. Since the method separates isotopically labeled growth from unlabeled material, well-choreographed spikes can resolve the compositional effects of different events through time. Among other applications, this feature could be used to separate the impact of day and night on biomineral composition in organisms with photosymbionts
Dynamical description of vesicle growth and shape change
We systematize and extend the description of vesicle growth and shape change
using linear nonequilibrium thermodynamics. By restricting the study to shape
changes from spheres to axisymmetric ellipsoids, we are able to give a
consistent formulation which includes the lateral tension of the vesicle
membrane. This allows us to generalize and correct a previous calculation. Our
present calculations suggest that, for small growing vesicles, a prolate
ellipsoidal shape should be favored over oblate ellipsoids, whereas for large
growing vesicles oblates should be favored over prolates. The validity of this
prediction is examined in the light of the various assumptions made in its
derivation.Comment: 6 page
Heat transfer between nanoparticles: Thermal conductance for near-field interactions
We analyze the heat transfer between two nanoparticles separated by a
distance lying in the near-field domain in which energy interchange is due to
Coulomb interactions. The thermal conductance is computed by assuming that the
particles have charge distributions characterized by fluctuating multipole
moments in equilibrium with heat baths at two different temperatures. This
quantity follows from the fluctuation-dissipation theorem (FDT) for the
fluctuations of the multipolar moments. We compare the behavior of the
conductance as a function of the distance between the particles with the result
obtained by means of molecular dynamics simulations. The formalism proposed
enables us to provide a comprehensive explanation of the marked growth of the
conductance when decreasing the distance between the nanoparticles
alpha^2 corrections to parapositronium decay: a detailed description
We present details of our recent calculation of alpha^2 corrections to the
parapositronium decay into two photons. These corrections are rather small and
our final result for the parapositronium lifetime agrees well with the most
recent measurement. Implications for orthopositronium decays are briefly
discussed.Comment: 18 pages, late
A Skyrme-type proposal for baryonic matter
The Skyrme model is a low-energy effective field theory for QCD, where the
baryons emerge as soliton solutions. It is, however, not so easy within the
standard Skyrme model to reproduce the almost exact linear growth of the
nuclear masses with the baryon number (topological charge), due to the lack of
Bogomolny solutions in this model, which has also hindered analytical progress.
Here we identify a submodel within the Skyrme-type low energy effective action
which does have a Bogomolny bound and exact Bogomolny solutions, and therefore,
at least at the classical level, reproduces the nuclear masses by construction.
Due to its high symmetry, this model qualitatively reproduces the main features
of the liquid droplet model of nuclei. Finally, we discuss under which
circumstances the proposed sextic term, which is of an essentially geometric
and topological nature, can be expected to give a reasonable description of
properties of nuclei.Comment: 11 pages, 2 figures, latex. v3: Extended and revised version, some
clarifications added. Some references and 2 figures added. v4: matches
published versio
Soft Photon Spectrum in Orthopositronium and Vector Quarkonium Decays
QED gauge invariance, when combined with analyticity, leads to constraints on
the low energy end of the emitted photon spectra. This is known as Low's
theorem. It is shown that the Ore-Powell result, as well as further
developments for the orthopositronium differential decay rate, are in
contradiction with Low's theorem, i.e. that their predicted soft photon spectra
are incorrect.
A solution to this problem is presented. The implications for the
orthopositronium lifetime puzzle, the charmonium rho-pi puzzle, the prompt
photon spectrum in inclusive quarkonium decays and the extraction of alpha_S
from quarkonium annihilation rates are briefly commented.Comment: LaTeX, 10 page
Semi-classical equation of state and specific heats for neutron-star inner crust with proton shell corrections
An approach to the equation of state for the inner crust of neutron stars
based on Skyrme-type forces is presented. Working within the Wigner-Seitz
picture, the energy is calculated by the TETF (temperature-dependent extended
Thomas-Fermi) method, with proton shell corrections added self-consistently by
the Strutinsky-integral method. Using a Skyrme force that has been fitted to
both neutron matter and to essentially all the nuclear mass data, we find
strong proton shell effects: proton numbers = 50, 40 and 20 are the only
values possible in the inner crust, assuming that nuclear equilibrium is
maintained in the cooling neutron star right down to the ambient temperature.
Convergence problems with the TETF expansion for the entropy, and our way of
handling them, are discussed. Full TETF expressions for the specific heat of
inhomogeneous nuclear matter are presented. Our treatment of the electron gas,
including its specific heat, is essentially exact, and is described in detail.Comment: 41 pages, 6 figure
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