Annular Junctions Research Articles

Magnetic field penetration in a long Josephson junction imbedded in a wide stripline

The dependence of the first critical field of long linear and annular Josephson junctions on the width of the surrounding stripline, the so called idle region, is investigated experimentally. The stripline modifies the effective Josephson length λeff in the junction. The experimental data are compared with the theory by Caputo et al. [J. Appl. Phys. 85, 7291 (1999)] and good agreement is found. The dependence of the first critical field on the width of the surrounding stripline can be well explained using the same λeff for both annular and linear junctions.

Fiske resonances in annular Josephson junctions

The resonances in two-dimensional annular Josephson junctions have been investigated from both a theoretical and experimental point of view. We obtained the analytical expression for the amplitude modulation of the Fiske resonances in the case of an externally applied magnetic field and the determination of the voltage positions of the Fiske steps as a function of the junction geometrical parameters (inner and outer radius). The experiments have been carried out on Nb-based annular junctions with finite ring width. The dependence of the maximum amplitude of the first and second Fiske step as a function of the external magnetic field is investigated in detail for a wide range of ring widths. The agreement found between experiments and the theoretical model is excellent.

Nonlinear resonance between a soliton and Josephson plasma waves: experiment and theory

Resonant interaction of a soliton (Josephson fluxon) with its self-generated Josephson plasma waves is studied experimentally, numerically, and analytically. An externally applied magnetic field H forms a cos-like potential relief for the soliton in the annular junction. Soliton motion under the influence of the bias current leads to an emission of plasma waves, which gives rise to a resonance at a certain soliton velocity. This resonance on the current–voltage characteristics shows a clear backbending accompanied by a negative differential resistance. Our analysis quantitatively explains the observed effect.

Loudspeaker for high frequencies

A loudspeaker for high frequencies (1) including a vibrating diaphragm (4) connected to a moving electromagnetic coil (3) and involving a central annular part (9) and a peripheral annular part (10) connected together along an annular junction (11). The diaphragm (4) is made from a flexible and elastic material so as to exhibit a defined deformation during the motion of the electromagnetic coil (3), and the annular parts (9, 10) are domed and are connected directly via their free end (13, 14) to a fixed structure of the loudspeaker (1).

Two-dimensional flow through an annular junction

Two-dimensional flow through an annular junction

Magnetic properties of annular Josephson junctions for radiation detection: Experimental results

We report on experimental results about Josephson junctions for radiation detection with an annular geometry. The observed suppression of the critical Josephson current and the presence of one single resonance step in the current–voltage characteristic, as predicted by the theory, strongly support the idea that this geometry could be very convenient for radiation detection. The results were obtained when one single magnetic fluxon was trapped in the central hole, and they represent the first experimental confirmation of the theoretical predictions in the case of annular junctions with finite ring width.

Enhancement of Ocular Drug Penetration

Although new drugs have recently been developed within the field of ophthalmology, the eye's various defense mechanisms make it difficult to achieve an effective concentration of these drugs within the eye. Drugs administered systemically have poor access to the inside of the eye because of the blood-aqueous and blood-retinal barriers. And although topical instillation of drugs is very popular in ophthalmology, topically applied drugs are rapidly eliminated from the precorneal area. In addition, the cornea, considered a major pathway for ocular penetration of topically applied drugs, is an effective barrier to drug penetration, since the corneal epithelium has annular tight junctions (zonula occludens), which completely surround and effectively seal the superficial epithelial cells. Various drug-delivery systems have been developed to increase the topical bioavailability of ophthalmic drugs by enhancement of the ocular drug penetration. The first approach is to modify the physicochemical property of drugs by chemical and pharmaceutical means. An optimum promoiety can be covalently bound to a drug molecule to obtain a prodrug that can chemically or enzymatically be converted to the active parent drug, either within the cornea or after the corneal penetration. Along these same lines, the transient formation of a lipophilic ion pair by ionic bonding is also useful for improving ocular drug penetration. The second approach is to modify the integrity of the corneal epithelium transiently by coadministration of an amphiphilic substance or by chelating agents that act as drug-penetration enhancers. The third approach modifies the integrity of the corneal epithelium transiently by physical techniques including iontophoresis and phonophoresis. This paper reviews the absorption behavior and ocular membranes penetration of topically applied drugs, and the various approaches for enhancement of ocular drug penetration in the eye.

Static properties of stacked Josephson junctions: Comparison of experiments with the inductively coupled sine-Gordon model

We report on the static properties of twofold stacks of long Josephson junctions fabricated with the $\mathrm{Nb}/\mathrm{Al}\ensuremath{-}{\mathrm{AlO}}_{x}/\mathrm{Nb}$ technology. The current-voltage characteristics of these structures typically show three different values of the critical current for a constant external magnetic field. This property can be explained by the inductive coupling model. Numerical calculations based on a system of coupled sine-Gordon equations show that the effective magnetic field in one of the junctions depends on whether the other junction is in the zero voltage state or not. This leads to different critical currents for each junction. This phenomenon was observed for the annular and straight overlap junctions with various parameters.

ERCP imaging.

Localization and spread of pancreatic diseases has been a reality since the availability of endoscopic pancreatography in the late 1960s, particularly after development of a fiberscope for this purpose. Endoscopic pancreatography allowed the clinician to discern the position and site of cancers, cysts, and localized inflammations. Ductal anomalies, such as annular pancreas, nonfusion, and anomalous junction of pancreatobiliary connection, were clearly recognized also. The pancreatography opacified the real lumen instead of the virtual images of ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI), so that precise studies comparing histopathologic details could be performed. The pancreatic ductal information visualized by using the fiberscope changed our understanding of chronic inflammation of the pancreas. Chronic diffuse pancreatitis, upstream pancreatitis, ductitis, and duct-narrowing pancreatitis were specified from their pathogenetic differences. Although the noninvasive methods such as US and CT will be the first choice for the diagnosis of pancreatic diseases, pancreatography will still be important for the morphologic studies of the pancreas in various disease conditions.

Variable Conformation of GAP Junctions Linking Bone Cells: A Transmission Electron Microscopic Study of Linear, Stacked Linear, Curvilinear, Oval, and Annular Junctions

There is a marked variability in the conformation of bone cell gap junctions in newborn murine cortical bone as defined by transmission electron microscopy (TEM). Studies were done in newborn BALB/c mouse and Sprague-Dawley rat femurs and tibias. Femoral and tibial cortices were dissected into 1 mm3 fragments and prepared in standardized fashion using modified Karnovsky fixation, 7.5% EDTA decalcification, 1% osmium tetroxide-sym collidine buffer with 1% lanthanum nitrate postfixation, Epon resin, 60 nm sections, lead citrate/uranyl acetate staining, and examination at 60 kV. Previous TEM descriptions of bone junctions have, with rare exceptions, noted only isolated linear or mildly curvilinear structures. In this study we noted gap junctional shapes on thin-section TEM preparations of osteoblasts and osteocytes to be extremely variable and complex encompassing linear, curvilinear, stacked linear, oval, and annular conformations. Multiple observations revealed linear gap junctions linking surface osteoblast cell bodies; linear, curvilinear, stacked linear, and oval junctions linking osteoblast processes in osteoid; linear and curvilinear junctions where cell processes joined with osteocyte cell bodies and each of the five conformations linking osteocyte processes within canaliculi. The annular junctions were found within osteoblast and osteocyte cytoplasm and in osteocyte cell processes within canaliculi. The annular junctions are intracellular, degenerating structures which appear as ultrastructural markers of gap junction involution. The more complex shapes reported here must be considered in (1) interpreting quantitative studies using freeze-fracture replicas, thin sections, and confocal microscopy immunolabeled junction connexin-43 components and (2) assessing gap junction biogenesis and turnover. 3-D reconstruction of bone junctions will enhance our understanding of these complex conformations.

Annular Josephson tunnel junctions in an external magnetic field: The dynamics

We have investigated the dynamical states in annular Josephson tunnel junctions in the presence of an externally applied uniform magnetic field parallel to the barrier plane. For electrically small and overdamped junctions we show that the Kulik perturbative theory for the Fiske singularities can easily and successfully be extended to the annular geometry even in the case of trapped fluxons. For electrically long annular junctions we present the results of the numerical simulations. Further, we report on the magnetic-field dependence of the amplitude of the so-called Fiske steps in specially designed high dissipation Nb/Al-${\mathrm{AlO}}_{\mathrm{x}}$-Al/Nb annular junctions having different normalized circumferences.

Annular Josephson junctions as superconductive nuclear particle detectors

A new geometrical configuration for superconductive tunnel junctions as high energy resolution particle detectors is presented. The proposed configuration is based on annular junctions, in which fluxons are trapped in the central hole during the superconducting transition. Such a configuration overcomes some important drawbacks of the traditional geometries. The suppression of the Josephson critical current by a magnetic field that does not need to be sustained during the whole detection time and the absence of Fiske steps, which compromise the stability of the bias point, are automatically reached with the new configuration.

Soliton bunching in annular Josephson junctions

By studying soliton (fluxon) motion in long annular Josephson junctions it is possible to avoid the influence of the boundaries and soliton-soliton collisions present in linear junctions. A new experimental design consisting of a niobium coil placed on top of an annular junction has been used to insert individual fluxons or antifluxons into the junction in a controllable way. The dynamical behavior of different numbers of trapped fluxons was investigated. In addition, we were able to change the junction parameters by changing temperature. In some of the zero-field steps, the experiments reveal a small jump to higher values of the voltage at the top of the steps. This phenomenon can be caused by a crossover from a nonbunched state to a bunched state of the fluxon motion. By performing direct numerical simulations using the perturbed sine-Gordon equation with parameter values determined from the experiments we have been able to confirm the above explanation.

Lorentz contraction of flux quanta observed in experiments with annular Josephson tunnel junctions.

Vortices (magnetic flux quanta) in Josephson tunnel junctions can move at velocities near the propagation velocity of light in the junction, and they undergo the Lorentz contraction. In annular junctions, pairs of vortices and antivortices are created; these move in opposite directions and collide with each other. Using low-temperature scanning electron microscopy, we can visualize the collision region. We observe the contraction of the collision region with increasing vortex velocity. With the assumption that the length of the collision region is proportional to the length of the vortices, we can directly image the Lorentz contraction of magnetic flux quanta.

Structure and turnover of junctional complexes between principal cells of the rat epididymis

The epididymal junctional complex between adjacent principal cells is composed of apically located gap, adherens and tight junctions. Tight junctions between adjacent epithelial cells lead to the formation of the blood-epididymal barrier. The objectives of this study were to examine the structure of the epididymal junctional complex in the different regions of the epididymis and to review the regulation of epithelial cadherin in the rat epididymis. Changes in the structure of the junctional complex, at the level of the electron microscope, were evident when comparing the initial segment to other regions of the epididymis. In the initial segment, the tight junction spanned a considerable length of the apical plasma membrane but had few desmosomes. In the other regions of the epididymis, the span of merging plasma membranes was considerably reduced, but in these regions, numerous desmosomes were present in the apical region. Several examples of what appeared to be a loss of portions of the plasma membrane of adjacent principal cells were evident along the entire epididymis. Such images as the invagination of a portion of the lateral plasma membrane of one principal cell into another, constriction of the invaginated area and eventual detachment leading to the formation of annular junctions suggest that there is a turnover of plasma membranes. The formation of cellular junctions involves the interactions of cell adhesion proteins followed by the addition of junctional proteins which assemble into tight and gap junctions. Epithelial cadherin (E-Cad), a calcium-dependent cell adhesion protein, was localized to the principal cells of the epididymis. Immunocytochemistry at the level of the electron microscope showed that E-Cad was present between the lateral plasma membranes of adjacent principal cells, both in the region of the junctional complex and in the deeper lying areas. E-Cad was also present in annular junctions located in close proximity to the junctional complex, indicating that these structures were related to the plasma membrane. E-Cad mRNA levels are regulated during postnatal epididymal development. In the caput-corpus epididymidis, E-Cad mRNA concentrations increase to peak at 42 days of age. This is well correlated with the conversion of testosterone to dihydrotestosterone in the epididymis. In the cauda epididymidis, however, E-Cad mRNA concentrations do not increase as a function of age, indicating that this protein is regulated in a segment-specific manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Freeze‐fracture study of cell junctions in the epididymis and vas deferens of a seasonal breeder: The mink (Mustela vison)

The present study used the freeze-fracture technique and vascular infusion of horseradish peroxidase (HRP) as a junction permeability tracer, visible in thin sections, to investigate potential seasonal variations in the mink epididymis and vas deferens cell junctions. The junctions were studied in kits during the neonatal period, during and after puberty, and during adulthood monthly throughout the annual reproductive cycle. Results showed the existence, at the time of birth, of a junctional complex joining ductal cells that reached the lumen of the epididymis and vas deferens. The lumenal impermeable segment of the junctional complex was characterized by the presence of an occluding zonule made up of continuous tight junctional ridges extending around the perimeter of the cell. The basal permeable segment of the junctional complex contained mainly discontinuous ridges with frequent forming gap and tight junctions next to adhering junctions. Receding annular junctions were present in the apical and lateral cytoplasm of principal and clear cells. The membrane domain apical to the occluding zonule bore 30-35 nm exo/endocytosis blebs and 40-60 crenations associated with deforming tight and gap junctions made up of randomly scattered particles. Patterns of junctional strands varied greatly from one principal and/or clear cell to another. However, cell junctions did not significantly vary from the neonatal period to adulthood nor from breeding to testicular regression. Anatomical subdivisions of the epididymis with fewer junctional strands per zonule and high diversity in their patterns exhibit no permeability differences to HRP when compared with subdivisions containing more numerous strands. The junctions were impermeable during the neonatal period and throughout the annual reproductive cycle. The following conclusions were reached: 1) a competent occluding zonule developed in the mink epididymis and vas deferens before it did in the testis; 2) the number of strands and the diversity of patterns did not correlate with permeability differences; 3) after the development of a lumen in the testicular excurrent duct, neither receding cellular changes caused by testicular regression nor seasonal passage of a bolus of sperm through the duct modified the occluding zonules; and 4) in the testicular excurrent duct, junction modulation (i.e., formation and deformation) paralleled that in the testis and followed the direction of the synthesis-transport-secretion process.

FLUXON GENERATION BY A CURRENT PULSE IN JOSEPHSON JUNCTIONS──THEORY AND SIMULATIONS

The problem of fluxon generation in Josephson junctions by a current pulse is addressed, with the help of the inverse scattering transform theory. The threshold conditions for soliton (breather or fluxon) generation are obtained analytically, with respect to relevant studies in the same direction. The reliability of the theory is examined by direct numerical experiments. Fairly good agreements exist between the theory and the numerical results, particularly in the case of semi-infinitely long junctions. Practically long (L～10λ) Josephson junctions are also explored, namely, overlap, in-line and circularly-symmetric annular junctions. Explained are those factors that complicate the generation processes suck as boundary effects, stability of soliton states against the driving current, soliton return effect and the applied magnetic field (in annular junctions only). The current pulse method is shown to be effective and practical for fluxon generation in Josephson junctions.

Bunching of fluxons in a long Josephson junction with surface losses.

It is known that the sine-Gordon model of a long Josephson junction with a surface-loss term predicts a spatially oscillating trailing tail of a strongly contracted fluxon. In the present work, it is demonstrated that this should give rise to bunched (bound) states of fluxons, although the onset of the bunching drastically differs from the standard situation recently investigated in terms of the perturbed nonlinear Schro\ifmmode\ddot\else\textasciidieresis\fi{}dinger model. Influence of the bunching on the I-V characteristic of an annular junction with a finite number of trapped fluxons is analyzed qualitatively. It is demonstrated that the bunching may set in with a small hysteresis, and it increases voltage at a given current. The latter effect has been observed in most recent experiments with the annular junction.

Soliton dynamics in Josephson tunnel junctions

The non-linear dynamics of Josephson junctions is well described by the perturbed sine-Gordon equation together with the appropriate boundary conditions. A possible solution of the pure sine-Gordon equation are solitons. In the junction the (quasi) solitons manifest themselves as Josephson vortices, each of them carrying one magnetic flux quantum. We used the technique of Low Temperature Scanning Electron Microscopy to study the soliton dynamics in superconducting tunnel junctions of various geometries. The electron beam power can be adjusted to work in different regimes. For high power, the beam acts as an active probe and we were able to introduce individual solitons in annular junctions one by one and studied multisoliton behavior without the influence of boundaries. For low power, the beam acts as a passive probe influencing the soliton dynamics in a well controlled way. Here, we investigated details of the dynamic soliton behavior.

Size effect in circularly symmetric, annular Josephson junctions and the role of an applied magnetic field.

The size effect in circularly symmetric, annular Josephson junctions is discussed in detail. For a junction of width W, we find that there exists a minimum inner radius ${\mathrm{\ensuremath{\rho}}}_{\mathrm{min}}$ for resonant soliton motion within the junction, with respect to the dissipative parameter and external magnetic field, and that the applied magnetic field plays an important role in reducing the size effect (${\mathrm{\ensuremath{\rho}}}_{\mathrm{min}}$ decreases remarkably in the presence of a negative external magnetic field). Because of the size effect, the range of the first current step vanishes if the inner radius of the annular junction is smaller than ${\mathrm{\ensuremath{\rho}}}_{\mathrm{min}}$. A qualitative interpretation is given for the size effect. On the basis of our results, future physical experiments are suggested.