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]

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

On massive photons inside a superconductor as follows from London and Ginzburg-Landau theory

Quantum Matter and Optic

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

Quantum Monte Carlo Algorithm Based on Two-Body Density Functional Theory for Fermionic Many-Body Systems: Application to 3He

We construct a quantum Monte Carlo algorithm for interacting fermions using the two-body density as the fundamental quantity. The central idea is mapping the interacting fermionic system onto an auxiliary system of interacting bosons. The correction term is approximated using correlated wave functions for the interacting system, resulting in an effective potential that represents the nodal surface. We calculate the properties of 3He and find good agreement with experiment and with other theoretical work. In particular, our results for the total energy agree well with other calculations where the same approximations were implemented but the standard quantum Monte Carlo algorithm was usedComment: 4 pages, 3 figures, 1 tabl

Quantum Monte Carlo simulation of overpressurized liquid 4He

A diffusion Monte Carlo simulation of superfluid $^4$He at zero temperature and pressures up to 275 bar is presented. Increasing the pressure beyond freezing ($\sim$ 25 bar), the liquid enters the overpressurized phase in a metastable state. In this regime, we report results of the equation of state and the pressure dependence of the static structure factor, the condensate fraction, and the excited-state energy corresponding to the roton. Along this large pressure range, both the condensate fraction and the roton energy decrease but do not become zero. The roton energies obtained are compared with recent experimental data in the overpressurized regime.Comment: 5 pages, accepted for publication in Phys. Rev. Let

Conductance and supercurrent discontinuities in atomic size point contacts

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