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 $\to$ Infectious $\to$ 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

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