15,037 research outputs found
Melt viscosities of lattice polymers using a Kramers potential treatment
Kramers relaxation times and relaxation times and
for the end-to-end distances and for center of mass diffusion are
calculated for dense systems of athermal lattice chains. is defined
from the response of the radius of gyration to a Kramers potential which
approximately describes the effect of a stationary shear flow. It is shown that
within an intermediate range of chain lengths N the relaxation times
and exhibit the same scaling with N, suggesting that N-dependent
melt-viscosities for non-entangled chains can be obtained from the Kramers
equilibrium concept.Comment: submitted to: Journal of Chemical Physic
Superhalogen and Superacid
A superhalogen and a corresponding Br{\o}nsted
superacid were designed and investigated on DFT and DLPNO-CCSD(T) levels of
theory. Calculated compounds have outstanding electron affinity and
deprotonation energy, respectively. We consider superacid
to be able to protonate molecular nitrogen. The
stability of these structures is discussed, while some of the previous
predictions concerning Br{\o}nsted superacids of record strength are doubted.Comment: 11 pages (main paper), 32 pages (supporting information), 10 figures,
10 tables, 62 reference
Electromagnetic Vacuum of Complex Media: Dipole Emission vs. Light Propagation, Vacuum Energy, and Local Field Factors
We offer a unified approach to several phenomena related to the
electromagnetic vacuum of a complex medium made of point electric dipoles. To
this aim, we apply the linear response theory to the computation of the
polarization field propagator and study the spectrum of vacuum fluctuations.
The physical distinction among the local density of states which enter the
spectra of light propagation, total dipole emission, coherent emission, total
vacuum energy and Schwinger-bulk energy is made clear. Analytical expressions
for the spectrum of dipole emission and for the vacuum energy are derived.
Their respective relations with the spectrum of external light and with the
Schwinger-bulk energy are found. The light spectrum and the Schwinger-bulk
energy are determined by the Dyson propagator. The emission spectrum and the
total vacuum energy are determined by the polarization propagator. An exact
relationship of proportionality between both propagators is found in terms of
local field factors. A study of the nature of stimulated emission from a single
dipole is carried out. Regarding coherent emission, it contains two components.
A direct one which is transferred radiatively and directly from the emitter
into the medium and whose spectrum is that of external light. And an indirect
one which is radiated by induced dipoles. The induction is mediated by one (and
only one) local field factor. Regarding the vacuum energy, we find that in
addition to the Schwinger-bulk energy the vacuum energy of an effective medium
contains local field contributions proportional to the resonant frequency and
to the spectral line-width.Comment: Typos fixed, journal ref. adde
Instanton Theory of Burgers Shocks and Intermittency
A lagrangian approach to Burgers turbulence is carried out along the lines of
the field theoretical Martin-Siggia-Rose formalism of stochastic hydrodynamics.
We derive, from an analysis based on the hypothesis of unbroken galilean
invariance, the asymptotic form of the probability distribution function of
negative velocity-differences. The origin of Burgers intermittency is found to
rely on the dynamical coupling between shocks, identified to instantons, and
non-coherent background fluctuations, which, then, cannot be discarded in a
consistent statistical description of the flow.Comment: 7 pages; LaTe
Scaled Particle Theory for Hard Sphere Pairs. I. Mathematical Structure
We develop an extension of the original Reiss-Frisch-Lebowitz scaled particle
theory that can serve as a predictive method for the hard sphere pair
correlation function g(r). The reversible cavity creation work is analyzed both
for a single spherical cavity of arbitrary size, as well as for a pair of
identical such spherical cavities with variable center-to-center separation.
These quantities lead directly to prediction of g(r). Smooth connection
conditions have been identified between the small-cavity situation where the
work can be exactly and completely expressed in terms of g(r), and the
large-cavity regime where macroscopic properties become relevant. Closure
conditions emerge which produce a nonlinear integral equation that must be
satisfied by the pair correlation function. This integral equation has a
structure which straightforwardly generates a solution that is a power series
in density. The results of this series replicate the exact second and third
virial coefficients for the hard sphere system via the contact value of the
pair correlation function. The predicted fourth virial coefficient is
approximately 0.6 percent lower than the known exact value. Detailed numerical
analysis of the nonlinear integral equation has been deferred to the sequel
(following paper
The Local Bubble and Interstellar Material Near the Sun
The properties of interstellar matter (ISM) at the Sun are regulated by our
location with respect to the Local Bubble (LB) void in the ISM. The LB is
bounded by associations of massive stars and fossil supernovae that have
disrupted natal ISM and driven intermediate velocity ISM into the LB interior
void. The Sun is located in such a driven ISM parcel. The Local Fluff has a
bulk velocity of 19 km/s in the LSR, and an upwind direction towards the center
of the gas and dust ring formed by the Loop I supernova remnant interaction
with the LB. When the ram pressure of the LIC is included in the total LIC
pressure, and if magnetic thermal and cosmic ray pressures are similar, the LIC
appears to be in pressure equilibrium with the local hot bubble plasma.Comment: Proceedings of Symposium on the Composition of Matter, honoring
Johannes Geiss on the occasion of his 80th birthday. Space Science Reviews
(in press
Analogy between turbulence and quantum gravity: beyond Kolmogorov's 1941 theory
Simple arguments based on the general properties of quantum fluctuations have
been recently shown to imply that quantum fluctuations of spacetime obey the
same scaling laws of the velocity fluctuations in a homogeneous incompressible
turbulent flow, as described by Kolmogorov 1941 (K41) scaling theory. Less
noted, however, is the fact that this analogy rules out the possibility of a
fractal quantum spacetime, in contradiction with growing evidence in quantum
gravity research. In this Note, we show that the notion of a fractal quantum
spacetime can be restored by extending the analogy between turbulence and
quantum gravity beyond the realm of K41 theory. In particular, it is shown that
compatibility of a fractal quantum-space time with the recent Horava-Lifshitz
scenario for quantum gravity, implies singular quantum wavefunctions. Finally,
we propose an operational procedure, based on Extended Self-Similarity
techniques, to inspect the (multi)-scaling properties of quantum gravitational
fluctuations.Comment: Sliglty modified version of the article about to appear in IJMP
Stiff polymer in monomer ensemble
We make use of the previously developed formalism for a monomer ensemble and
include angular dependence of the segments of the polymer chains thus
described. In particular we show how to deal with stiffness when the polymer
chain is confined to certain regions. We investigate the stiffness from the
perspectives of a differential equation, integral equations, or recursive
relations for both continuum and lattice models. Exact analytical solutions are
presented for two cases, whereas numerical results are shown for a third case.Comment: 10 pages, including 6 figure
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