277 research outputs found
Magnetic flux locking in two weakly coupled superconducting rings
We have analyzed the quantum interference effects in the macroscopic
''superconducting molecule''. The composite system consists of two massive
superconducting rings, each interrupted by a Josephson junction, which are at
the same time weakly coupled with one another. The special case of coupling via
the Josephson four-terminal junction is considered. The structure of the
macroscopic quantum states in an applied magnetic field is calculated. It is
shown, that depending on the values of the magnetic fluxes through each ring,
the system displays two groups of states, the ''orthostates'' with both induced
currents going in the same direction, and the ''parastates'' with the opposite
currents and with the total induced flux locked to zero value. The transition
to the flux locked state with changing of the total applied flux is sudden and
is preserved in a certain interval which is determined by the difference of the
fluxes applied through each ring. It makes the system sensitive to small
gradients of the external magnetic field.Comment: 8 pages in Latex, 3 figures (eps
Ballistic Four-Terminal Josephson Junction: Bistable States and Magnetic Flux Transfer
The macroscopic quantum interference effects in ballistic Josephson
microstructures are investigated. The studied system consists of four bulk
superconductors (terminals) which are weakly coupled through the mesoscopic
rectangular normal metal (two dimensional electron gas). We show that nonlocal
electrodynamics of ballistic systems leads to specific current-phase relations
for the mesoscopic multiterminal Josephson junction. The nonlocal coupling of
supercurrents produces the "dragging" effect for phases of the superconducting
order parameter in different terminals. The composite Josephson junction, based
on this effect, exhibits the two -level system behaviour controlled by the
external magnetic flux. The effect of magnetic flux transfer in a system of
nonlocally coupled superconducting rings is studied.Comment: 12pages LaTex, 6 figures; e-mail: [email protected]
Stochastic SIR epidemics in a population with households and schools
We study the spread of stochastic SIR (Susceptible Infectious
Recovered) epidemics in two types of structured populations, both consisting of
schools and households. In each of the types, every individual is part of one
school and one household. In the independent partition model, the partitions of
the population into schools and households are independent of each other. This
model corresponds to the well-studied household-workplace model. In the
hierarchical model which we introduce here, members of the same household are
also members of the same school.
We introduce computable branching process approximations for both types of
populations and use these to compare the probabilities of a large outbreak. The
branching process approximation in the hierarchical model is novel and of
independent interest. We prove by a coupling argument that if all households
and schools have the same size, an epidemic spreads easier (in the sense that
the number of individuals infected is stochastically larger) in the independent
partition model. We also show by example that this result does not necessarily
hold if households and/or schools do not all have the same size
DC SQUID based on the mesoscopic multiterminal Josephson junction
A theory is offered for a novel device, mesoscopic four-terminal SQUID. The
studied system consists of a mesoscopic four-terminal junction, one pair of
terminals of which is incorporated in a superconducting ring and the other one
is connected with a transport circuit. The nonlocal weak coupling between the
terminals leads to effects of phase dragging and magnetic flux transfer. The
behaviour of a four-terminal SQUID, controlled by the external parameters, the
applied magnetic flux and the transport current is investigated. The critical
current and the current voltage characteristics as functions of magnetic flux
are calculated. In the nonlocal mesoscopic case they depend not only on the
magnitude of the applied flux but also on its sign, allowing measurement of the
direction of the external magnetic field.Comment: 11 pages, 4 figures, presented at the EUCAS 2001 conferenc
Rational design of purely peptidic amphiphiles for drug delivery applications
A broad range of new properties is emerging from supramolecular aggregates. Self-assembled structures of purely peptidic amphiphiles exploit these properties to produce biocompatible, biodegradable, smart materials for drug administration. This thesis explores the design, synthesis, purification, characterization of purely peptidic amphiphiles, and evaluates potential applications.
The first chapter provides a general introduction to the field of self-assembly, and of drug delivery as compared to nature’s delivery mechanisms. The benefit of amino acid-based molecules in producing smart materials for drug delivery applications is highlighted via biocompatibility and biodegradability. Synthetic strategies and purification methods are discussed. Finally, gramicidin A (gA) – a naturally occurring, short, hydrophobic, membrane-integrated peptide for producing the amphiphilic peptides presented here – is introduced.
Chapter 2 presents an initial approach to produce self-assembled structures from purely peptidic amphiphiles. The undecamer used features a repetitive L-tryptophan and D-leucine [LW-DL] motif, representing the hydrophobic block, and an N-terminally attached hydrophilic (lysine or acetylated lysine) section. Besides solid-phase peptide synthesis and purification, the process that self-assembles micelles and spherical peptide particles, or “peptide beads”, was characterized as a function of temperature and solvent composition by electron paramagnetic resonance, dynamic and static light scattering, fluorimetry and electron microscopy. Equilibrium between single peptide molecules, micelles and peptide beads is then presented.
Chapter 3 examines the structure of self-assembled peptide beads of diameters from 200 – 1500 nm. The beads were analyzed by electron- and atomic force microscopy, static and dynamic light-, and small angle X-ray scattering. The peptide beads result from hierarchical organization of micellar-like subunits and confirm the concept of multicompartment micelles. An improved understanding of the beads’ capacity to embed hydrophobic and hydrophilic payloads emerges and provides perspectives for drug delivery applications.
Chapter 4 presents a library of longer peptides, based on the full sequence of gA. The peptide design includes three parts: (a) a charged lysine part, (b) an acetylated lysine part and (c) a constant hydrophobic rod-like helix, based on gA. Stepwise replacement of lysine (K) with acetylated lysine (X) generated ten peptides: Ac-X8-gA and KmX8-m-gA (m ranging from 0 to 8). A change in the primary sequence caused a change in secondary structure. The transition reflected a change in the self-assembled structures from fibers to micelles. This demonstrates how even small point mutations influence the supramolecular outcome and serve as an important step to understanding and controlling self-assembly.
In Chapter 5, knowledge gained on gA-based peptides is applied to produce purely peptidic vesicles. The work here demonstrates that, to form such structures with short amphiphiles, additional stabilizing factors are necessary. Thus, we exploited different dimerization strategies to form stable peptide membranes and developed a general recipe to form purely peptidic vesicles. The vesicles demonstrated pH responsiveness and the capacity to embed hydrophilic and hydrophobic payloads in its structure.
Chapter 6 presents the potential of the beads in drug delivery applications. Hydrophobic and hydrophilic payload-filled beads are internalized by human cells. Further, a method to increase embedding efficiency for RNA/DNA payloads to 99% is presented. The internalization of the gene delivery vehicle into cells led to specific gene silencing. Delivery of co-embedded paclitaxel and doxorubicin was proven effective. The results also demonstrate that the new class of drug delivery material caused no measurable toxicity in the experiments. Thus, the material is suggested as a biocompatible drug delivery vehicle for gene therapy and multi-drug delivery.
In Chapter 7, self-assembly of the peptide is used to template-pack gold nanoparticles. The C-terminally cysteinated peptide Ac-X3-gT-C was used to coat gold nanoparticles and form gold core micelles. These micelles aggregate into composite peptide-gold nanoparticles in which the individual gold nanoparticles remain separated. Dense packing of the gold nanoparticles offers opportunities for new optical- and electronic properties and use in potential payload release from the beads by the typical gold nanoparticle radiation absorption effect.
The last chapter summarizes and discusses the achievements of this work. It gives an overview of on-going work and the prospect of worthwhile research. This includes, e.g. development of drug delivery systems, use of the presented peptidic self-assembly system as template material in nanosciences, and the use of the material to investigate cell uptake pathways of nano-sized objects
Operationalisering van het waterkwaliteitsmodel Schelde-estuarium: uitbreiding procesformuleringen met denitrificatie waterkolom
Evaluatie modellering organische microverontreinigingen Schelde-estuarium: verbeterde toepassing van SAWES-IMPAQT: verslag onderzoek
Modelling of trends in Twitter using retweet graph dynamics
In this paper we model user behaviour in Twitter to capture the emergence of
trending topics. For this purpose, we first extensively analyse tweet datasets
of several different events. In particular, for these datasets, we construct
and investigate the retweet graphs. We find that the retweet graph for a
trending topic has a relatively dense largest connected component (LCC). Next,
based on the insights obtained from the analyses of the datasets, we design a
mathematical model that describes the evolution of a retweet graph by three
main parameters. We then quantify, analytically and by simulation, the
influence of the model parameters on the basic characteristics of the retweet
graph, such as the density of edges and the size and density of the LCC.
Finally, we put the model in practice, estimate its parameters and compare the
resulting behavior of the model to our datasets.Comment: 16 pages, 5 figures, presented at WAW 201
Beleefd praten over water : hoe overheden met bewoners en belanghebbenden communiceren over veranderingen in het waterbeheer
Steeds vaker worden burgers actief betrokken bij het waterbeheer. Toch is participatie niet per definitie een garantie voor succes. Het slagen van burgerparticipatie hangt onder andere samen met het gestelde doel, het vinden van de juiste vorm en de mogelijkheid om het doel aan te laten sluiten bij bredere vormen van gebiedsontwikkeling. Alterra maakte drie praktijkhandelingen voor participatie, belevingsonderzoek en communicatie in het waterbeheer. Dit deel gaat over ‘beleefd praten over water’, oftewel het met respect communiceren over de belangen van alle betrokkenen. Het beschrijft inzichten in de communicatie van waterbeheerders met bewoners en belanghebbenden. Een goede communicatie kan bijdragen aan draagvlak voor veranderingen in het waterbeheer. Deze inzichten zijn gegroeid in een leergemeenschap waarin waterbeheerders en sociale wetenschappers ervaringen uitwisselden aan de hand van praktijkcases binnen het Leven met Water-project WaterTeken
Response of thin-film SQUIDs to applied fields and vortex fields: Linear SQUIDs
In this paper we analyze the properties of a dc SQUID when the London
penetration depth \lambda is larger than the superconducting film thickness d.
We present equations that govern the static behavior for arbitrary values of
\Lambda = \lambda^2/d relative to the linear dimensions of the SQUID. The
SQUID's critical current I_c depends upon the effective flux \Phi, the magnetic
flux through a contour surrounding the central hole plus a term proportional to
the line integral of the current density around this contour. While it is well
known that the SQUID inductance depends upon \Lambda, we show here that the
focusing of magnetic flux from applied fields and vortex-generated fields into
the central hole of the SQUID also depends upon \Lambda. We apply this
formalism to the simplest case of a linear SQUID of width 2w, consisting of a
coplanar pair of long superconducting strips of separation 2a, connected by two
small Josephson junctions to a superconducting current-input lead at one end
and by a superconducting lead at the other end. The central region of this
SQUID shares many properties with a superconducting coplanar stripline. We
calculate magnetic-field and current-density profiles, the inductance
(including both geometric and kinetic inductances), magnetic moments, and the
effective area as a function of \Lambda/w and a/w.Comment: 18 pages, 20 figures, revised for Phys. Rev. B, the main revisions
being to denote the effective flux by \Phi rather than
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