3,102 research outputs found
Two-dimensional Copolymers and Multifractality: Comparing Perturbative Expansions, MC Simulations, and Exact Results
We analyze the scaling laws for a set of two different species of long
flexible polymer chains joined together at one of their extremities (copolymer
stars) in space dimension D=2. We use a formerly constructed field-theoretic
description and compare our perturbative results for the scaling exponents with
recent conjectures for exact conformal scaling dimensions derived by a
conformal invariance technique in the context of D=2 quantum gravity. A simple
MC simulation brings about reasonable agreement with both approaches. We
analyse the remarkable multifractal properties of the spectrum of scaling
exponents.Comment: 5 page
Shape characteristics of the aggregates formed by amphiphilic stars in water: dissipative particle dynamics study
We study the effect of the molecular architecture of amphiphilic star
polymers on the shape of aggregates they form in water. Both solute and solvent
are considered at a coarse-grained level by means of dissipative particle
dynamics simulations. Four different molecular architectures are considered:
the miktoarm star, two different diblock stars and a group of linear diblock
copolymers, all of the same composition and molecular weight. Aggregation is
started from a closely packed bunch of molecules immersed into
water. In most cases, a single aggregate is observed as a result of
equilibration, and its shape characteristics are studied depending on the
aggregation number . Four types of aggregate shape are observed:
spherical, rod-like and disc-like micelle and a spherical vesicle. We estimate
"phase boundaries" between these shapes depending on the molecular
architecture. Sharp transitions between aspherical micelle and a vesicle are
found in most cases. The pretransition region shows large amplitude
oscillations of the shape characteristics with the oscillation frequency
strongly dependent on the molecular architecture.Comment: 10 pages, 7 figure
A Simple Particle Action from a Twistor Parametrization of AdS_5
The SO(4,2) isometries of AdS_5 are realized non-linearly on its
horospherical coordinates (x^m,\rho). On the other hand, Penrose twistors have
long been known to linearly realize these symmetries on 4-dimensional Minkowski
space, the boundary of AdS_5, parametrized by x^m. Here we extend the twistor
construction and define a pair of twistors, allowing us to include a radial
coordinate in the construction. The linear action of SO(4,2) on the twistors
induces the correct isometries of AdS_5. We apply this new construction to the
study of the dynamics of a massive particle in AdS_5. We show that in terms of
the twistor variables the action takes a simple form of a 1-dimensional gauge
theory. Our result might open up the possibility to find a simple worldvolume
action also for the string propagating on AdS_5.Comment: 11 pages, LaTe
Polycrystalline silicon study: Low-cost silicon refining technology prospects and semiconductor-grade polycrystalline silicon availability through 1988
Photovoltaic arrays that convert solar energy into electrical energy can become a cost effective bulk energy generation alternative, provided that an adequate supply of low cost materials is available. One of the key requirements for economic photovoltaic cells is reasonably priced silicon. At present, the photovoltaic industry is dependent upon polycrystalline silicon refined by the Siemens process primarily for integrated circuits, power devices, and discrete semiconductor devices. This dependency is expected to continue until the DOE sponsored low cost silicon refining technology developments have matured to the point where they are in commercial use. The photovoltaic industry can then develop its own source of supply. Silicon material availability and market pricing projections through 1988 are updated based on data collected early in 1984. The silicon refining industry plans to meet the increasing demands of the semiconductor device and photovoltaic product industries are overviewed. In addition, the DOE sponsored technology research for producing low cost polycrystalline silicon, probabilistic cost analysis for the two most promising production processes for achieving the DOE cost goals, and the impacts of the DOE photovoltaics program silicon refining research upon the commercial polycrystalline silicon refining industry are addressed
Entropy-induced separation of star polymers in porous media
We present a quantitative picture of the separation of star polymers in a
solution where part of the volume is influenced by a porous medium. To this
end, we study the impact of long-range-correlated quenched disorder on the
entropy and scaling properties of -arm star polymers in a good solvent. We
assume that the disorder is correlated on the polymer length scale with a
power-law decay of the pair correlation function . Applying
the field-theoretical renormalization group approach we show in a double
expansion in and that there is a range of
correlation strengths for which the disorder changes the scaling
behavior of star polymers. In a second approach we calculate for fixed space
dimension and different values of the correlation parameter the
corresponding scaling exponents that govern entropic effects. We
find that , the deviation of from its mean field value
is amplified by the disorder once we increase beyond a threshold. The
consequences for a solution of diluted chain and star polymers of equal
molecular weight inside a porous medium are: star polymers exert a higher
osmotic pressure than chain polymers and in general higher branched star
polymers are expelled more strongly from the correlated porous medium.
Surprisingly, polymer chains will prefer a stronger correlated medium to a less
or uncorrelated medium of the same density while the opposite is the case for
star polymers.Comment: 14 pages, 7 figure
Statistical analysis of 22 public transport networks in Poland
Public transport systems in 22 Polish cities have been analyzed. Sizes of
these networks range from N=152 to N=2881. Depending on the assumed definition
of network topology the degree distribution can follow a power law or can be
described by an exponential function. Distributions of paths in all considered
networks are given by asymmetric, unimodal functions. Clustering, assortativity
and betweenness are studied. All considered networks exhibit small world
behavior and are hierarchically organized. A transition between dissortative
small networks N=500 is observed.Comment: 11 pages, 17 figures, 2 tables, REVTEX4 forma
A role for the cleaved cytoplasmic domain of E-cadherin in the nucleus
Cell-cell contacts play a vital role in intracellular signaling, although the molecular mechanisms of these signaling pathways are not fully understood. E-cadherin, an important mediator of cell-cell adhesions, has been shown to be cleaved by γ-secretase. This cleavage releases a fragment of E-cadherin, E-cadherin C-terminal fragment 2 (E-cad/CTF2), into the cytosol. Here, we study the fate and function of this fragment. First, we show that coexpression of the cadherin-binding protein, p120 catenin (p120), enhances the nuclear translocation of E-cad/CTF2. By knocking down p120 with short interfering RNA, we also demonstrate that p120 is necessary for the nuclear localization of E-cad/CTF2. Furthermore, p120 enhances and is required for the specific binding of E-cad/CTF2 to DNA. Finally, we show that E-cad/CTF2 can regulate the p120-Kaiso-mediated signaling pathway in the nucleus. These data indicate a novel role for cleaved E-cadherin in the nucleus
Longitudinal spin-relaxation in nitrogen-vacancy centers in electron irradiated diamond
We present systematic measurements of longitudinal relaxation rates ()
of spin polarization in the ground state of the nitrogen-vacancy (NV) color
center in synthetic diamond as a function of NV concentration and magnetic
field . NV centers were created by irradiating a Type 1b single-crystal
diamond along the [100] axis with 200 keV electrons from a transmission
electron microscope with varying doses to achieve spots of different NV
center concentrations. Values of () were measured for each spot as a
function of .Comment: 4 pages, 8 figure
Star copolymers in porous environments: scaling and its manifestations
We consider star polymers, consisting of two different polymer species, in a
solvent subject to quenched correlated structural obstacles. We assume that the
disorder is correlated with a power-law decay of the pair correlation function
g(x)\sim x^{-a}. Applying the field-theoretical renormalization group approach
in d dimensions, we analyze different scenarios of scaling behavior working to
first order of a double \epsilon=4-d, \delta=4-a expansion. We discuss the
influence of the correlated disorder on the resulting scaling laws and possible
manifestations such as diffusion controlled reactions in the vicinity of
absorbing traps placed on polymers as well as the effective short-distance
interaction between star copolymers.Comment: 13 pages, 3 figure
Multifractality of Brownian motion near absorbing polymers
We characterize the multifractal behavior of Brownian motion in the vicinity
of an absorbing star polymer. We map the problem to an O(M)-symmetric
phi^4-field theory relating higher moments of the Laplacian field of Brownian
motion to corresponding composite operators. The resulting spectra of scaling
dimensions of these operators display the convexity properties which are
necessarily found for multifractal scaling but unusual for power of field
operators in field theory. Using a field-theoretic renormalization group
approach we obtain the multifractal spectrum for absorbtion at the core of a
polymer star as an asymptotic series. We evaluate these series using
resummation techniques.Comment: 18 pages, revtex, 6 ps-figure
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