Weakly Linked Research Articles

Conundrum of strongly coupled supercurrent flow in both under- and overdoped Bi-2212 round wires

Understanding what makes Bi$_2$Sr$_2$Ca$_1$Cu$_2$O$_x$ (Bi-2212) the only high critical current density ($J_c$), high temperature superconductor (HTS) capable of being made as a round wire (RW) is important intellectually because high $J_c$ RW Bi-2212 breaks the paradigm that forces biaxially textured REBCO and uniaxially textured (Bi,Pb)$_2$Sr$_2$Ca$_2$Cu$_3$O$_x$ (Bi-2223) into tape geometries that reproduce the strong anisotropy of the native crystal structure and force expensive fabrication routes to ensure the best possible texture with minimum density of high angle grain boundaries. The biaxial growth texture of Bi-2212 developed during a partial melt heat treatment should favor high $J_c$, even though its $\sim$15$^{\circ}$ full width at half maximum (FWHM) grain-to-grain misorientation is well beyond the commonly accepted strong-coupling range. Its ability to be strongly overdoped should be valuable too, since underdoped cuprate grain boundaries are widely believed to be weakly linked. Accordingly, we here study property changes after oxygen underdoping the optimized, overdoped wire. While $J_c$ and vortex pinning diminish significantly in underdoped wires, we were not able to develop the prominent weak-link signature (a hysteretic $J_c$(H) characteristic) evident in even the very best Bi-2223 tapes with a $\sim$ 15$^{\circ}$ FWHM uniaxial texture. We attribute the high $J_c$ and lack of weak link signature in our Bi-2212 round wires to the high-aspect ratio, large-grain, basal-plane-faced grain morphology produced by partial-melt processing of Bi-2212 which enables $c$-axis Brick-Wall current flow when $ab$-plane transport is blocked. We conclude that the presently optimized biaxial texture of Bi-2212 intrinsically constitutes a strongly coupled current path, regardless of its oxygen doping state.

Josephson detection of time-reversal symmetry broken superconductivity in SnTe nanowires

A Josephson junction (JJ) couples the supercurrent flowing between two weakly linked superconductors to the phase difference between them via a current-phase relation (CPR). While a sinusoidal CPR is expected for conventional junctions with insulating weak links, devices made from some exotic materials may give rise to unconventional CPRs and unusual Josephson effects. In this work, we present such a case: we investigate the proximity-induced superconductivity in SnTe nanowires by incorporating them as weak links in JJs and observe a deviation from the standard CPR. We report on indications of an unexpected breaking of time-reversal symmetry in these devices, detailing the unconventional characteristics that reveal this behavior. These include an asymmetric critical current in the DC Josephson effect, a prominent second harmonic in the AC Josephson effect, and a magnetic diffraction pattern with a minimum in critical current at zero magnetic field. The analysis examines how multiband effects and the experimentally visualized ferroelectric domain walls give rise to this behavior, giving insight into the Josephson effect in materials that possess ferroelectricity and/or multiband superconductivity.

Shifts in fine root traits within and among species along a fine-scale hydrological gradient

Lessons from above-ground trait ecology and resource economics theory may not be directly translatable to below-ground traits due to differences in function, trade-offs and environmental constraints. Here we examine root functional traits within and across species along a fine-scale hydrological gradient. We ask two related questions: (1) What is the relative magnitude of trait variation across the gradient for within- versus among-species variation? (2) Do correlations among below-ground plant traits conform with predictions from resource-economic spectrum theory? We sampled four below-ground fine-root traits (specific root length, branching intensity, root tissue density and root dry matter content) and four above-ground traits (specific leaf area, leaf size, plant height and leaf dry matter content) in vascular plants along a fine-scale hydrological gradient within a wet heathland community in south-eastern Australia. Below-ground and above-ground traits were sampled both within and among species. Root traits shifted both within and among species across the hydrological gradient. Within- and among-species patterns for root tissue density showed similar declines towards the wetter end of the gradient. Other root traits showed a variety of patterns with respect to within- and among-species variation. Filtering of species has a stronger effect compared with the average within-species shift: the slopes of the relationships between soil moisture and traits were steeper across species than slopes of within species. Between species, below-ground traits were only weakly linked to each other and to above-ground traits, but these weak links did in some cases correspond with predictions from economic theory. One of the challenges of research on root traits has been considerable intraspecific variation. Here we show that part of intraspecific root trait variation is structured by a fine-scale hydrological gradient, and that the variation aligns with among-species trends in some cases. Patterns in root tissue density are especially intriguing and may play an important role in species and individual response to moisture conditions. Given the importance of roots in the uptake of resources, and in carbon and nutrient turnover, it is vital that we establish patterns of root trait variation across environmental gradients.

Current transport, magnetic and elemental properties of densified Ag-sheathed Ba1−xKxFe2As2 tapes

Understanding the mechanisms that limit the current transport in granular iron-based superconductors is necessary to unlock their potential for magnet applications. This paper reports the transport critical current surface IC(T,H) of powder-in-tube Ag/Ba1−xKxFe2As2 tapes densified by flat rolling and uniaxial hot pressing. We investigate the amplitude of the isothermal-IC(H)-curve magnetic hysteresis, which is a marker for the electrical connectivity between superconducting grains. This allows us to demonstrate that, in spite of an improved connectivity, the percolative path through weakly linked grains continues to play a role in the IC properties of hot-pressed conductors. The limitations to the current transport due to weak links depend on the orientation of the magnetic field and are more severe when H is on the tape plane. This finding provides a justification to the puzzling evidence that IC(H) can be smaller for magnetic fields applied on the tape plane and not perpendicular to it. Finally, we report an unexpected increase of IC upon augmenting the magnetic field from 0 to ∼2 T. SQUID measurements reveal a superparamagnetic-like magnetic background that takes place in the same range of magnetic fields. A microscopic study of the conductor-core elemental composition provides evidences that these features can be both related to the presence of Fe-rich non-stoichiometric regions in the superconducting core. On a more general note, this study points out that further advances in the in-field current transport properties of powder-in-tube Ag/Ba1−xKxFe2As2 conductors are possible and rely to a large extent on solutions to improve the inter-grain connectivity and the quality of the superconducting phase.

One-Dimensional Transport of Bosons between Weakly Linked Reservoirs

We study a flow of ultracold bosonic atoms through a one-dimensional channel that connects two macroscopic three-dimensional reservoirs of Bose-condensed atoms via weak links implemented as potential barriers between each of the reservoirs and the channel. We consider reservoirs at equal chemical potentials so that a superflow of the quasicondensate through the channel is driven purely by a phase difference 2Φ imprinted between the reservoirs. We find that the superflow never has the standard Josephson form ∼sin2Φ. Instead, the superflow discontinuously flips direction at 2Φ=±π and has metastable branches. We show that these features are robust and not smeared by fluctuations or phase slips. We describe a possible experimental setup for observing these phenomena.

Upper critical field, critical current density and thermally activated flux flow in CaFFe0.9Co0.1As superconductor

In this paper, we report the synthesis, structure, transition temperature, upper critical field, critical current density and thermally activated flux flow in the CaFFe0.9Co0.1As superconductor. Superconductivity arises at 23 K by Co substitution at the site of Fe atoms and the upper critical field is estimated as 102 T using the Werthamer–Helfand–Hohenberg formula. The flux-flow activation energy (U0/kB) varies from 3230 K and 4190 K in a field of 9 T and 1 T, respectively. At 2 K, the Jc is found to be approximately 4 × 103 A cm−2 and 0.3 × 103 A cm−2 in zero and 6 T field, respectively. Transmission electron microscopy analysis shows an amorphous region surrounding most of the grains which is likely to be present in the form of amorphous and weak link grain boundaries in this compound. It seems that most of the current is hindered by mis-aligned grains, amorphous grain boundaries and impurities, which are invariably found between the grains. The presence of the weakly linked granules and their weakly pinned intergranular Josephson vortices are responsible for both low Jc and the Arrhenius temperature dependence of resistivity.

Resistive transition in granular disordered highTcsuperconductors: A numerical study

The resistive transition of granular high-T$_c$ superconductors, characterized by either weak (YBCO-like) or strong (MgB$_2$-like) links, occurs through a series of avalanche-type current density rearrangements. These rearrangements correspond to the creation of resistive layers, crossing the whole specimen approximately orthogonal to the current density direction, due to the simultaneous transition of a large number of weak-links or grains. The present work shows that exact solution of the Kirchhoff equations for strongly and weakly linked networks of nonlinear resistors, with Josephson junction characteristics, yield the subsequent formation of resistive layers within the superconductive matrix as temperature increases. Furthermore, the voltage noise observed at the transition is related to the resistive layer formation process. The noise intensity is estimated from the superposition of voltage drop elementary events related to the subsequent resistive layers. At the end of the transition, the layers mix-up, the step amplitude decreases and the resistance curve smoothes. This results in the suppression of noise, as experimentally found. Remarkably, a scaling law for the noise intensity with the network size is argued. It allows to extend the results to networks with arbitrary size and, thus, to real specimens.

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Food webs

Limits to the critical current inBi2Sr2Ca2Cu3Oxtape conductors: The parallel path model

An extensive overview of a model that describes current flow and dissipation in high-quality ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{Ca}}_{2}{\text{Cu}}_{3}{\text{O}}_{x}$ superconducting tapes is provided. The parallel path model is based on a superconducting current running in two distinct parallel paths. One of the current paths is formed by grains that are connected at angles below $4\ifmmode^\circ\else\textdegree\fi{}$. Dissipation in this strongly linked backbone occurs within the grains and is well described by classical flux-creep theory. The other current path, the weakly linked network, is formed by superconducting grains that are connected at intermediate angles $(4\ifmmode^\circ\else\textdegree\fi{}--8\ifmmode^\circ\else\textdegree\fi{})$ where dissipation occurs at the grain boundaries. However, grain boundary dissipation in this weakly linked current path does not occur through Josephson weak links, but just as in the strongly linked backbone, is well described by classical flux creep. The results of several experiments on ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{Ca}}_{2}{\text{Cu}}_{3}{\text{O}}_{x}$ tapes and single-grained powders that strongly support the parallel path model are presented. The critical current density of ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{Ca}}_{2}{\text{Cu}}_{3}{\text{O}}_{x}$ tapes can be scaled as a function of magnetic field angle over the temperature range from 15 K to 77 K. Expressions based on classical flux creep are introduced to describe the dependence of the critical current density of ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{Ca}}_{2}{\text{Cu}}_{3}{\text{O}}_{x}$ tapes on the magnetic field and temperature.

Current-induced suppression of superconductivity in a three-dimensional lattice of weakly linked indium grains in opal

The current-voltage characteristics of the metal-dielectric composite have been investigated in the range of the resistive state near the superconducting transition temperature Tc. The composite structure can be represented as a face-centered cubic lattice, which involves a large number of weakly linked indium nanograins and is stabilized in structural cavities of opal. The response to microwave radiation is used to characterize the resistive state of the composite. The comparative investigation into the current-voltage characteristics and the response of the composite to microwave radiation makes it possible to conclude that the weak links are superconducting in the region of critical current (Ic) of the composite as a whole. The transition of weak links to the resistive state occurs at currents immediately preceding the transition of the composite from the resistive state to the ohmic state. The model of resistivity of the indium-opal composite is proposed on the basis of morphological examinations. According to this model, the energy dissipation in the resistive state is brought about by the quasi-discrete (due to the quantization of the magnetic flux in circuits of a three-dimensional lattice comprised of multiply connected grains) redistribution of transport current over the cross-section of composite.

Prospective origins of prostate carcinoma. Prostatic intraepithelial neoplasia and atypical adenomatous hyperplasia.

The search for the precursor of prostatic adenocarcinoma has focused in recent years on two histopathologic findings: high grade prostatic intraepithelial neoplasia (PIN) and atypical adenomatous hyperplasia (AAH). This article describes the diagnostic criteria and clinical significance of PIN and AAH. PIN is the most likely precursor of prostate carcinoma. The continuum that culminates in high grade PIN and early invasive carcinoma is characterized by progressive basal cell layer disruption, loss of markers of secretory differentiation, nuclear and nucleolar abnormalities, increasing proliferative potential, increasing microvessel density, variation in DNA content, and allelic loss. Clinical studies suggest that PIN predates carcinoma by 10 years or more, with low grade PIN first appearing in men in their 30s. AAH is usually found in the transition zone, and shows a weak and inconclusive link to well differentiated adenocarcinoma of the transition zone. The significance of recognizing PIN is based on its strong association with prostatic carcinoma. PIN coexists with carcinoma in most cases, but retains an intact or fragmented basal cell layer, unlike carcinoma, which lacks a basal cell layer. High grade PIN in biopsies predicts the presence of carcinoma in subsequent biopsies, and PIN provides the highest risk ratio of all known predictive factors. This finding indicates the need for repeat biopsy and follow-up when PIN is identified in a biopsy, especially in patients with an elevated serum prostate specific antigen concentration. PIN also offers promise as an intermediate endpoint in studies of chemoprevention of prostatic carcinoma. Unlike PIN, AAH is weakly linked to carcinoma, and current data indicate that no follow-up is necessary for patients with this finding.