6,886 research outputs found
Crossover behavior for long reptating polymers
We analyze the Rubinstein-Duke model for polymer reptation by means of
density matrix renormalization techniques. We find a crossover behavior for a
series of quantities as function of the polymer length. The crossover length
may become very large if the mobility of end groups is small compared to that
of the internal reptons. Our results offer an explanation to a controversy
between theory, experiments and simulations on the leading and subleading
scaling behavior of the polymer renewal time and diffusion constant.Comment: 4 Pages, RevTeX, and 4 PostScript figures include
Total energies from variational functionals of the Green function and the renormalized four-point vertex
We derive variational expressions for the grand potential or action in terms
of the many-body Green function which describes the propagation of
particles and the renormalized four-point vertex which describes the
scattering of two particles in many-body systems. The main ingredient of the
variational functionals is a term we denote as the -functional which plays
a role analogously to the usual -functional studied by Baym (G.Baym,
Phys.Rev. 127, 1391 (1962)) in connection with the conservation laws in
many-body systems. We show that any -derivable theory is also
-derivable and therefore respects the conservation laws. We further set
up a computational scheme to obtain accurate total energies from our
variational functionals without having to solve computationally expensive sets
of self-consistent equations. The input of the functional is an approximate
Green function and an approximate four-point vertex
obtained at a relatively low computational cost. The
variational property of the functional guarantees that the error in the total
energy is only of second order in deviations of the input Green function and
vertex from the self-consistent ones that make the functional stationary. The
functionals that we will consider for practical applications correspond to
infinite order summations of ladder and exchange diagrams and are therefore
particularly suited for applications to highly correlated systems. Their
practical evaluation is discussed in detail.Comment: 21 pages, 10 figures. Physical Review B (accepted
Development of a unified tensor calculus for the exceptional Lie algebras
The uniformity of the decomposition law, for a family F of Lie algebras which
includes the exceptional Lie algebras, of the tensor powers ad^n of their
adjoint representations ad is now well-known. This paper uses it to embark on
the development of a unified tensor calculus for the exceptional Lie algebras.
It deals explicitly with all the tensors that arise at the n=2 stage, obtaining
a large body of systematic information about their properties and identities
satisfied by them. Some results at the n=3 level are obtained, including a
simple derivation of the the dimension and Casimir eigenvalue data for all the
constituents of ad^3. This is vital input data for treating the set of all
tensors that enter the picture at the n=3 level, following a path already known
to be viable for a_1. The special way in which the Lie algebra d_4 conforms to
its place in the family F alongside the exceptional Lie algebras is described.Comment: 27 pages, LaTeX 2
Phase Coexistence of a Stockmayer Fluid in an Applied Field
We examine two aspects of Stockmayer fluids which consists of point dipoles
that additionally interact via an attractive Lennard-Jones potential. We
perform Monte Carlo simulations to examine the effect of an applied field on
the liquid-gas phase coexistence and show that a magnetic fluid phase does
exist in the absence of an applied field. As part of the search for the
magnetic fluid phase, we perform Gibbs ensemble simulations to determine phase
coexistence curves at large dipole moments, . The critical temperature is
found to depend linearly on for intermediate values of beyond the
initial nonlinear behavior near and less than the where no
liquid-gas phase coexistence has been found. For phase coexistence in an
applied field, the critical temperatures as a function of the applied field for
two different are mapped onto a single curve. The critical densities
hardly change as a function of applied field. We also verify that in an applied
field the liquid droplets within the two phase coexistence region become
elongated in the direction of the field.Comment: 23 pages, ReVTeX, 7 figure
Sorting living mesenchymal stem cells using a TWIST1 RNA-based probe depends on incubation time and uptake capacity
Bone marrow derived mesenchymal stromal cells (BMSCs) are multipotent progenitors of particular interest for cell-based tissue engineering therapies. However, one disadvantage that limit their clinical use is their heterogeneity. In the last decades a great effort was made to select BMSC subpopulations based on cell surface markers, however there is still no general consensus on which markers to use to obtain the best BMSCs for tissue regeneration. Looking for alternatives we decided to focus on a probe-based method to detect intracellular mRNA in living cells, the SmartFlare technology. This technology does not require fixation of the cells and allows us to sort living cells based on gene expression into functionally different populations. However, since the technology is available it is debated whether the probes specifically recognize their target mRNAs. We validated the TWIST1 probe and demonstrated that it specifically recognizes TWIST1 in BMSCs. However, differences in probe concentration, incubation time and cellular uptake can strongly influence signal specificity. In addition we found that TWIST1high expressing cells have an increased expansion rate compared to TWIST1low expressing cells derivedfrom the same initial population of BMSCs. The SmartFlare probes recognize their target gene, however for each probe and cell type validation of the protocol is necessary
Electrochemical methods for speciation of trace elements in marine waters. Dynamic aspects
The contribution of electrochemical methods
to the knowledge of dynamic speciation of toxic trace elements in marine waters is critically reviewed. Due to the importance
of dynamic considerations in the interpretation of the electrochemical signal, the principles and recent developments of kinetic features in the interconversion of metal complex species will be presented. As dynamic electrochemical
methods, only stripping techniques (anodic stripping voltammetry and stripping chronopotentiometry) will be used because they are the most important for the
determination of trace elements. Competitive ligand ex- change-adsorptive cathodic stripping voltammetry, which should be considered an equilibrium technique rather than a dynamic method, will be also discussed because the complexing parameters may be affected by some kinetic limitations if equilibrium before analysis is not attained and/or the flux of the adsorbed complex is in fluenced by the lability of the natural complexes in the water sample. For a correct data interpretation and system characterization the comparison of results obtained from different techniques seems essential in the articulation of a serious discussion of their meaning
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