26,824 research outputs found
Negative Specific Heat in a Quasi-2D Generalized Vorticity Model
Negative specific heat is a dramatic phenomenon where processes decrease in
temperature when adding energy. It has been observed in gravo-thermal collapse
of globular clusters. We now report finding this phenomenon in bundles of
nearly parallel, periodic, single-sign generalized vortex filaments in the
electron magnetohydrodynamic (EMH) model for the unbounded plane under strong
magnetic confinement. We derive the specific heat using a steepest descent
method and a mean field property. Our derivations show that as temperature
increases, the overall size of the system increases exponentially and the
energy drops. The implication of negative specific heat is a runaway reaction,
resulting in a collapsing inner core surrounded by an expanding halo of
filaments.Comment: 12 pages, 3 figures; updated with revision
Infrared properties of SiC particles
We present basic laboratory infrared data on a large number of SiC
particulate samples, which should be of great value for the interpretation of
the 11.3 micron feature observed in the spectra of carbon-rich stars. The
laboratory spectra show a wide variety of the SiC phonon features in the 10-13
micron wavelength range, both in peak wavelength and band shape. The main
parameters determining the band profile are morphological factors as grain size
and shape and, in many cases, impurities in the material. We discovered the
interesting fact that free charge carriers, generated e.g. by nitrogen doping,
are a very common characteristics of many SiC particle samples. These free
charge carriers produce very strong plasmon absorption in the near and middle
infrared, which may also heavily influence the 10-13 micron feature profile via
plasmon-phonon coupling.
We also found that there is no systematic dependence of the band profile on
the crystal type (alpha- vs. beta-SiC). This is proven both experimentally and
by theoretical calculations based on a study of the SiC phonon frequencies.
Further, we give optical constants of amorphous SiC. We discuss the
implications of the new laboratory results for the interpretation of the
spectra of carbon stars.Comment: 17 pages, 12 figures. To appear in A&
Estimation in a growth study with irregular measurement times
Between 1982 and 1988 a growth study was carried out at the Division of Pediatric Oncology of the University Hospital of Groningen. A special feature of the project was that sample sizes are small and that ages at entry may be very different. In addition the intended design was not fully complied with. This paper highlights some aspects of the statistical analysis which is based on (1) reference scores, (2) statistical procedures allowing for an irregular pattern of measurement times caused by missing data and shifted measurement times
Realistic Tight Binding Model for the Electronic Structure of II-VI Semiconductors
We analyze the electronic structure of group II-VI semiconductors obtained
within LMTO approach in order to arrive at a realistic and minimal tight
binding model, parameterized to provide an accurate description of both valence
and conduction bands. It is shown that a nearest-neighbor model is
fairly sufficient to describe to a large extent the electronic structure of
these systems over a wide energy range, obviating the use of any fictitious
orbital. The obtained hopping parameters obey the universal scaling law
proposed by Harrison, ensuring transferability to other systems. Furthermore,
we show that certain subtle features in the bonding of these compounds require
the inclusion of anion-anion interactions in addition to the nearest-neighbor
cation-anion interactions.Comment: 9 pages, 9 figure
Topology of RNA-RNA interaction structures
The topological filtration of interacting RNA complexes is studied and the
role is analyzed of certain diagrams called irreducible shadows, which form
suitable building blocks for more general structures. We prove that for two
interacting RNAs, called interaction structures, there exist for fixed genus
only finitely many irreducible shadows. This implies that for fixed genus there
are only finitely many classes of interaction structures. In particular the
simplest case of genus zero already provides the formalism for certain types of
structures that occur in nature and are not covered by other filtrations. This
case of genus zero interaction structures is already of practical interest, is
studied here in detail and found to be expressed by a multiple context-free
grammar extending the usual one for RNA secondary structures. We show that in
time and space complexity, this grammar for genus zero
interaction structures provides not only minimum free energy solutions but also
the complete partition function and base pairing probabilities.Comment: 40 pages 15 figure
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