Quantum Interference in Superconducting Wire Networks and Josephson Junction Arrays: Analytical Approach based on Multiple-Loop Aharonov-Bohm Feynman Path-Integrals

We investigate analytically and numerically the mean-field superconducting-normal phase boundaries of two-dimensional superconducting wire networks and Josephson junction arrays immersed in a transverse magnetic field. The geometries we consider include square, honeycomb, triangular, and kagome' lattices. Our approach is based on an analytical study of multiple-loop Aharonov-Bohm effects: the quantum interference between different electron closed paths where each one of them encloses a net magnetic flux. Specifically, we compute exactly the sums of magnetic phase factors, i.e., the lattice path integrals, on all closed lattice paths of different lengths. A very large number, e.g., up to $10^{81}$ for the square lattice, exact lattice path integrals are obtained. Analytic results of these lattice path integrals then enable us to obtain the resistive transition temperature as a continuous function of the field. In particular, we can analyze measurable effects on the superconducting transition temperature, $T_c(B)$, as a function of the magnetic filed $B$, originating from electron trajectories over loops of various lengths. In addition to systematically deriving previously observed features, and understanding the physical origin of the dips in $T_c(B)$ as a result of multiple-loop quantum interference effects, we also find novel results. In particular, we explicitly derive the self-similarity in the phase diagram of square networks. Our approach allows us to analyze the complex structure present in the phase boundaries from the viewpoint of quantum interference effects due to the electron motion on the underlying lattices.Comment: 18 PRB-type pages, plus 8 large figure

AC Josephson effect in Nb3Sn thin-film bridges

The AC Josephson effect has been observed in Nb3Sn variable thickness bridges, at liquid hydrogen temperatures. The microwave radiation power dependence of the critical current and the current steps height have been studied. The results are in agreement with a theory based on vortex motion in the superconducting thin-film bridges. We consider these results as indicating good prospects for the use of high temperature superconducting alloys in cryogenic electronics.On présente les résultats d'observations de l'effet Josephson non stationnaire dans des microponts à épaisseur variable de Nb3Sn à la température de l'hydrogène liquide. Le courant critique et l'amplitude des premiers paliers induits sont étudiés en fonction de la puissance hyperfréquence. Les résultats obtenus correspondent à la théorie des tourbillons en mouvement dans les ponts supraconducteurs, ils mettent en évidence les avantages de l'utilisation d'alliages à haute température supraconductrice dans le domaine de la cryoélectronique

УСИЛЕНИЕ АДГЕЗИИ СТВОЛОВЫХ ПРОГЕНИТОРНЫХ КЛЕТОК К СИНТЕТИЧЕСКИМ МАТЕРИАЛАМ ВНЕКЛЕТОЧНЫМ МАТРИКСОМ

Background: Tissue-engineered vascular grafts of small diameter have being widely used in coronary artery bypass grafting. However, the rate of settling of these grafts with endothelial cells is insufficient. Aims:The aim of the study was to selucidate the adhesion of endothelial progenitor cells and mesenchymal stem cells to synthetic materials (polystyrol, polytetrafluoroethylene) preprocessed with extracellular matrix (gelatin, fibronectin, collagen) in vitro.Materials and methods: For the study,the endothelial progenitor cells (EPC) were isolated from the peripheral blood of patients with ischemic heart disease, and mesenchymal stem cells (MSC) — from bone marrow of Wistar rats. Commercial monoclonal antibodies for flow- cytometry were used to determine the phenotype of EPC. To isolate the early and late EPC from mononuclear cells of peripheral blood, cells were raised for 8 and 16 days on a gelatin or fibronectin based substrate. The commercial kits for enzyme linked immunoassay were applied to assess levels of cytokine production and nitric oxide by early and late EPC conditioned by gelatin or fibronectin on the 8th and 16th days of growth. To conduct the study the MSC were isolated from bone marrow of rats. To determine the attachment of adherent fraction of nucleated bone marrow cells, cell morphology, the adipogenic and osteogenic differentiation were evaluated. To assess migration of MSC in real time within 24 hours on the device Cell-IQ, we used «closure/wound healing» test. The commercial kits for enzyme linked immunoassay were applied to assess levels of cytokine production and nitric oxide by MSC; and components of the extracellular matrix (fibronectin, collagen I and type IV) ― to assess the increased adhesion of MSC to the polytetrafluoroethylene. Results: The results demonstrated that endothelial progenitor cells adhere to both gelatin and fibronectin and confirmed the influence of these extracellular matrix components on the cytokine levels produced by early and late endothelial cells. The combination of fibronectin with type I or IV collagen or the combination of thereof promotes the adhesion to polytetrafluoroethylene and colonization of the graft.Conclusions: Preprocessing of synthetic material (polystyrene, polytetrafluoroethylene) enhances adhesion and growth of EPC and MSC which can be implemented when creating tissue-engineered vascular grafts for small diameter coronary artery bypass grafting with specified conditions of settlement by the cells involved in neointima formation.Обоснование. Тканеинженерные сосудистые протезы малого диаметра нашли свое широкое применение при операциях шунтирования. Однако скорость заселения протезов эндотелиоцитами недостаточна.Цель исследования ― изучение адгезии эндотелиальных прогениторных клеток и мезенхимных стволовых клеток к синтетическим материалам (полистерол, политетрафторэтилен), предобработанным внеклеточным матриксом (желатин, фибронектин, коллаген) in vitro.Методы. Для проведения исследования эндотелиальные прогениторные клетки (ЭПК) выделяли из периферической крови больных ишемической болезнью сердца, мезенхимные стволовые клетки (МСК) ― из костного мозга крыс линии Wistar. Для определения фенотипа ЭПК использовали коммерческие моноклональные антитела для проточной цитофлоуметрии. Для получения ранних и поздних ЭПК из мононуклеаров периферической крови клетки растили в течение 8 и 16 сут на подложке из желатина или фибронектина. Для оценки уровней продукции цитокинов и оксида азота ранними и поздними ЭПК на 8-е и 16-е сут роста при кондиционировании с желатином или фибронектином использовали коммерческие наборы для твердофазного иммуноферментного анализа. Для проведения исследования МСК выделяли из костного мозга крыс. Для определения принадлежности прилипшей фракции ядросодержащих клеток костного мозга оценивали морфологию клеток, дифференцировку в адипогенном и остеогенном направлении. Для проведения оценки миграции МСК в режиме реального времени в течение 24 ч на приборе Cell-IQ использовали тест закрытия/заживления  раны. Для оценки уровней продукции цитокинов и оксида азота МСК использовали коммерческие наборы для твердофазного иммуноферментного анализа; для оценки усиления адгезии МСК к сосудистым протезам из политетрафторэтилена ― компоненты внеклеточного матрикса (фибронектин, коллаген I и IV типа).Результаты. Показано, что эндотелиальные прогениторные клетки,  адгезируют как к желатину, так и к фибронектину, продемонстрировано также влияние данных компонентов внеклеточного матрикса  на уровни продукции  цитокинов  ранними и поздними эндотелиальными клетками. Сочетание фибронектина с коллагеном I или IV типа или их комбинацией способствует адгезии к политетрафторэтилену и заселению графта.Заключение. Предобработка синтетического материала (полистирол, политетрафторэтилен) усиливает адгезию и рост ЭПК и МСК, что может быть использовано для создания тканеинженерных сосудистых протезов малого диаметра для аортокоронарного шунтирования с заданными условиями заселения их клетками, вовлеченными в формирование неоинтимы

Computational optimization of indirect-driven targets for ignition at the Iskra-6 laser facility

The Iskra-6 laser facility with energy up to 300 kJ is planned to be created in Russia. The results of 1D- and 2D-simulations of a single-shell cryogenic target and a double-shell non-cryogenic target to achieve thermonuclear ignition at the Iskra-6 laser facility are discussed

Thickness dependence of pinning mechanisms in granular Nb thin films

We have investigated the influence of thickness on the pinning properties of granular sputtered Nb films in a parallel magnetic field. The presence of a crossover in the pinning mechanisms at some critical thickness dc was established. Below dc the pinning is due to individual vortices. Above dc the commensurability between the vortex period and the film thickness occurs, causing a large increase in the critical current density due to surface pinning. It is shown that, in the vortex-free limit, when the critical current is determined by the depairing current, the effect of the commensurability is not present. © IOP Publishing Ltd