3,482 research outputs found
Continuum Theory of Polymer Crystallization
We present a kinetic model of crystal growth of polymers of finite molecular
weight. Experiments help to classify polymer crystallization broadly into two
kinetic regimes. One is observed in melts or in high molar mass polymer
solutions and is dominated by nucleation control with , where is the growth rate and is the super-cooling. The
other is observed in low molar mass solutions (as well as for small molecules)
and is diffusion controlled with , for small . Our
model unifies these two regimes in a single formalism. The model accounts for
the accumulation of polymer chains near the growth front and invokes an
entropic barrier theory to recover both limits of nucleation and diffusion
control. The basic theory applies to both melts and solutions, and we
numerically calculate the growth details of a single crystal in a dilute
solution. The effects of molecular weight and concentration are also determined
considering conventional polymer dynamics. Our theory shows that entropic
considerations, in addition to the traditional energetic arguments, can capture
general trends of a vast range of phenomenology. Unifying ideas on
crystallization from small molecules and from flexible polymer chains emerge
from our theory.Comment: 37 double-spaced pages including 8 figures, submitted to the Journal
of Chemical Physic
Measuring entanglement in condensed matter systems
We show how entanglement may be quantified in spin and cold atom many-body
systems using standard experimental techniques only. The scheme requires no
assumptions on the state in the laboratory and a lower bound to the
entanglement can be read off directly from the scattering cross section of
Neutrons deflected from solid state samples or the time-of-flight distribution
of cold atoms in optical lattices, respectively. This removes a major obstacle
which so far has prevented the direct and quantitative experimental study of
genuine quantum correlations in many-body systems: The need for a full
characterization of the state to quantify the entanglement contained in it.
Instead, the scheme presented here relies solely on global measurements that
are routinely performed and is versatile enough to accommodate systems and
measurements different from the ones we exemplify in this work.Comment: 6 pages, 2 figure
Trapped ion chain as a neural network
We demonstrate the possibility of realizing a neural network in a chain of
trapped ions with induced long range interactions. Such models permit to store
information distributed over the whole system. The storage capacity of such
network, which depends on the phonon spectrum of the system, can be controlled
by changing the external trapping potential and/or by applying longitudinal
local magnetic fields. The system properties suggest the possibility of
implementing robust distributed realizations of quantum logic.Comment: 4 pages, 3 figure
On applying the set covering model to reseeding
The Functional BIST approach is a rather new BIST technique based on exploiting embedded system functionality to generate deterministic test patterns during BIST. The approach takes advantages of two well-known testing techniques, the arithmetic BIST approach and the reseeding method. The main contribution of the present paper consists in formulating the problem of an optimal reseeding computation as an instance of the set covering problem. The proposed approach guarantees high flexibility, is applicable to different functional modules, and, in general, provides a more efficient test set encoding then previous techniques. In addition, the approach shorts the computation time and allows to better exploiting the tradeoff between area overhead and global test length as well as to deal with larger circuits
Noncontact modulation calorimetry of metallic liquids in low Earth orbit
Noncontact modulation calorimetry using electromagnetic heating and radiative heat loss under ultrahigh-vacuum conditions has been applied to levitated solid, liquid, and metastable liquid samples. This experiment requires a reduced gravity environment over an extended period of time and allows the measurement of several thermophysical properties, such as the enthalpy of fusion and crystallization, specific heat, total hemispherical emissivity, and effective thermal conductivity with high precision as a function of temperature. From the results on eutectic glass forming Zr-based alloys thermodynamic functions are obtained which describe the glass-forming ability of these alloys
New Insights into Cosmic Ray induced Biosignature Chemistry in Earth-like Atmospheres
With the recent discoveries of terrestrial planets around active M-dwarfs,
destruction processes masking the possible presence of life are receiving
increased attention in the exoplanet community. We investigate potential
biosignatures of planets having Earth-like (N-O) atmospheres orbiting
in the habitable zone of the M-dwarf star AD Leo. These are bombarded by high
energetic particles which can create showers of secondary particles at the
surface. We apply our cloud-free 1D climate-chemistry model to study the
influence of key particle shower parameters and chemical efficiencies of NOx
and HOx production from cosmic rays. We determine the effect of stellar
radiation and cosmic rays upon atmospheric composition, temperature, and
spectral appearance. Despite strong stratospheric O destruction by cosmic
rays, smog O can significantly build up in the lower atmosphere of our
modeled planet around AD Leo related to low stellar UVB. NO abundances
decrease with increasing flaring energies but a sink reaction for NO with
excited oxygen becomes weaker, stabilizing its abundance. CH is removed
mainly by Cl in the upper atmosphere for strong flaring cases and not via
hydroxyl as is otherwise usually the case. Cosmic rays weaken the role of
CH in heating the middle atmosphere so that HO absorption becomes more
important. We additionally underline the importance of HNO as a possible
marker for strong stellar particle showers. In a nutshell, uncertainty in NOx
and HOx production from cosmic rays significantly influences biosignature
abundances and spectral appearance.Comment: Manuscript version after addressing all referee comments. Published
in Ap
Modeling transcriptional networks in Drosophila development at multiple scales
Quantitative models of developmental processes can provide insights at multiple scales. Ultimately, models may be particularly informative for key questions about network level behavior during development such as how does the system respond to environmental perturbation, or operate reliably in different genetic backgrounds? The transcriptional networks that pattern the Drosophila embryo have been the subject of numerous quantitative experimental studies coupled to modeling frameworks in recent years. In this review, we describe three studies that consider these networks at different levels of molecular detail and therefore result in different types of insights. We also discuss other developmental transcriptional networks operating in Drosophila, with the goal of highlighting what additional insights they may provide
- …