Nb Bridges Research Articles

Nanobubble-Induced Aggregation of Ultrafine Particles: A Molecular Dynamics Study.

Nanobubble-induced aggregation (NBIA) of fine and ultrafine particles in liquid is a promising method for enhancing floatation rates in mineral processing, cleaning contaminants from water, and reviving marine ecosystems. Although the current experimental techniques can measure the nanobubble capillary force between two surfaces with controlled approach speed, they are not capable of imaging NBIA dynamics of fine/ultrafine particles by real-time observation with nanoscale spatial resolution. In this work, we use molecular dynamics (MD) simulations to study dynamics of NBIA of Ag particles in a Lennard-Jones fluid system. The molecular-level modeling allows us to study microscopic details of NBIA dynamics that are inaccessible by current experimental means. Using MD simulations, we investigated the effects of NB size, surface wettability, surface roughness, and contact line pinning on NBIA dynamics. Our modeling results show that both concave NB bridges between two hydrophobic surfaces and convex NB bridges between two hydrophilic surfaces can result in an attractive nanobubble capillary force (NBCF) that causes the aggregation of Ag particles in liquids. The equilibrium separation between two fully aggregated particles can be well predicted by the improved capillary force model. We also observe that the change of contact angle after the contact line pinning occurs at the sharp edge of a particle, which slows the aggregation process. Our thermodynamics analysis shows that there is a critical contact angle below which the merged surface NBs will detach from the surface instead of causing aggregation. The prediction of the critical contact angle is corroborated by our MD simulation results.

Phonon-engineered Nb film as a Mott-insulating tunnel-junction network

Herein, phonon engineering is applied to a metal. Suppressed phonon branches are expected to enhance electron–phonon interactions in metals. Independent resistance and magnetization measurements have confirmed the Mott transition, which has never been observed in pure, undoped metals. By repeatedly cooling and warming a periodically pierced Nb film, narrow Nb bridges left between adjacent thru-holes are converted to Mott-insulating bridges. The continuum in which these Mott-insulating bridges connect adjacent metallic Nb islands forms a tunnel-junction network, which can be used to implement a massively integrated single-electron device.

Boronic acid derivatized lanthanide-polyoxometalates with novel B-OH-Ln and B-O-Nb bridges.

Three new boronic acid derivatized lanthanide-polyoxometalates (BA-Ln-POMs) were synthesized, in which boronic acids served as bi- or tri-dentate ligands and exhibited interesting coordination ability towards POMs and LnIII. The pH was found to be crucial in controlling the coordination modes of boronic acid and the formation of B-O-Ln and B-O-Nb bridges. Compared with the boronic acid-free Ln-POM, a longer lifetime was observed for compound 2Eu due to the coordination of boronic acids to the EuIII center.

Progress in Development of the Superconducting Bolometer With Microwave Bias and Readout

Our new detector and readout concept brings together TES (Transition Edge Sensor) and MKID (Microwave Kinetic Inductance Detector) technologies and exploits the idea of a microwave-induced superconducting transition in a small thin-film microbridge. The superconducting transition of the bridge manifests itself as variation in the Q -factor of niobium resonators at 5-8 GHz, somewhat similar to MKID operation. We present data showing the potential for developing this concept into multipixel detector arrays. Single-pixel sensitivity was measured at 4.5 K for an input band of 600-700 GHz using a prototype 10-nm-thick Nb bridge of size 1 μm × 500 nm. Radiation from human skin was detected with a resolution better than 1 K/rtHz, which is encouraging for terahertz imaging applications. To further improve device sensitivity, we are also developing Hf-based devices that operate near 0.35 K. Details about the physics and stability of these devices are discussed.

Critical current of Nb, NbN, and TaN thin-film bridges with and without geometrical nonuniformities in a magnetic field

The dependence of the critical current ${I}_{\mathrm{C}}$ on magnetic field has been systematically studied in thin-film superconducting Nb, NbN, and TaN bridges with widths smaller than the Pearl length. We have shown that application of a magnetic field of a proper orientation to the bridges with bends results in a suppression of the current crowding effect. This leads to an increase up to about 30% of the critical current in such bridges with respect to ${I}_{\mathrm{C}}$ measured at zero magnetic fields. We have demonstrated a good qualitative and quantitative agreement between experimental results and theoretical predictions obtained in frame of the London and Ginzburg-Landau models for a dominating edge pinning mechanism of vortices. Also we observed oscillations of ${I}_{\mathrm{C}}$ at magnetic fields, which increase with a decrease of the width of superconducting stripe. These oscillations are qualitatively similar to the Fraunhofer-like dependence of a critical current on the magnetic field in wide Josephson junctions. In the frame of the Ginzburg-Landau model, a penetration and pining of vortex chain is considered as a mechanism responsible for this nonmonotonic behavior of the critical current.

A mixed-addenda Nb/W polyoxometalate containing dimeric Dawson subunit: Synthesis, structure, and characterization

A mixed-addenda Nb/W polyoxometalates, K4Na4[H6P2W12Nb4O59(NbO2)2]2·48H2O (1), has been isolated under acidic condition. The single crystal structural analysis showed that the six vacant sites of the lacunary {P2W12} precursor are occupied by six Nb atoms, two at the polar sites and four at the equatorial sites to form a {P2W12Nb6} mixed-addenda subunit. Two {P2W12Nb6} subunits are joined together in an inversion mode by two Nb–O–Nb bridges from the Nb atoms at equatorial sites to form a dimeric unit, which can be considered as a Nb12 core wrapped by two lacunary {P2W12} polyoxoanions. It is notable that the terminal peroxo groups bonded niobium atoms in the Nb12 core are only partially eliminated during the formation of the dimeric unit.

Fabrication of SQUIDs with Nb/Ru/Sr2RuO4 junctions

We have fabricated superconducting quantum interference devices (SQUIDs) using Nb/Ru/Sr2RuO4 junctions. The superconducting loop of a Nb-Sr2RuO4 hybrid dc SQUID is composed of Nb, Sr2RuO4 and two Nb/Ru/Sr2RuO4 junctions, and made by building a Nb bridge between two individual Ru micro inclusions on the ab-plane surface of the Ru-Sr2RuO4 eutectic system. We measure the critical current between Nb and Sr2RuO4 parts of such dc-SQUIDs, which oscillates with every flux quantum through the SQUID loop.

Nb Lateral Josephson Junction Induced by Inverse Proximity Effect With NiFe

We utilize the inverse proximity effect in superconductor-ferromagnet bilayer to generate lateral Josephson junctions. The weak link is created by a ferromagnetic NiFe strip across a superconductor Nb bridge due to the inverse proximity effect. The critical currents of the junctions were measured at different temperatures and magnetic fields. The junctions exhibit a modulation of the critical current in perpendicular magnetic field similar to a Fraunhofer interference pattern which proves the dc Josephson effect. The weak link region induced by the NiFe in Nb persists down to 2 K.

Constructing nanosized polyanions with diverse structures by the self-assembly of W/Nb mixed-addendum polyoxometalate and lanthanide ion

The reaction activity of W/Nb mixed-addendum POMs towards lanthanide has been demonstrated by experimental and computational methods. Utilizing the nucleophilicity of Ot(Nb) (Ot: terminal oxygen), six Nb–O–Eu bridge-based POMs with different structures have been isolated by the reactions of {GeW9Nb3O40} and lanthanide ions Eu3+ under different conditions, including a dimmer Cs3K4[(Ge2W18Nb6O78)Eu(H2O)4]·23H2O (1), three tetramers Cs8K9[(Ge4W36Nb12O156)Eu(H2O)3]·25H2O (2), Cs12K2[Cs(GeW9Nb3O40)4(SO4)Eu5(H2O)36]·61H2O (3), Cs12.5K1.5[Cs2(GeW9Nb3O40)4Eu4(H2O)22]·28H2O (4), a one-dimensional compound Cs11.5[(GeW9Nb3O40)4Eu5.5(H2O)26]·24H2O (5), and a two-dimensional compound CsK2.25[GeW9Nb3O40Eu1.25(H2O)12]·8H2O (6). In the polyanions of 1–6 (1a–6a), all the EuIII atoms are selectively bonded to Ot(Nb) of {GeW9Nb3O40} fragments, indicating the high nucleophilicity of Ot(Nb). The Density Functional Theory (DFT) calculation on anion [GeW9Nb3O40]7− further approves this. Comparing with the (Nb)O–Eu bonds, the formation of Nb–O–Nb bridges needs stronger acidity. Thus, weaker acidity (pH>1.5) is favorable to the former. The Cs+ cation plays an important templating role in the formation and stability of polyanions 1a–5a. The photoluminescence behaviors of 1 and 2 have been investigated and compound 1 exhibits selective luminescence response to malonic acid.

Magnetic Interactions of the 15‐Electron Nb6F12 Clusters in Nb6F15

AbstractThe crystal structure of Nb6F15 consists of Nb6F12 clusters that are corner‐shared through linear Nb–F–Nb bridges to form two simple cubic lattices, which interpenetrate each other with F···F van der Waals contacts only. With 15 valence electrons in Nb–Nb bonding states each cluster carries one unpaired spin. 19F NMR and EPR spectroscopic measurements suggest antiferromagnetic ordering in Nb6F15 below 6 K. A spin‐dimer analysis based on tight binding calculations shows that the correct magnetic structure of Nb6F15 associated with this ordering should have the spin arrangement in which two interpenetrating ferromagnetic simple cubic lattices are antiferromagnetically coupled.

Planar S-(S/F)-S Josephson junctions induced by the inverse proximity effect

We present a utilization of the inverse proximity effect in superconductor-ferromagnet (S-F) bilayer to generate lateral Josephson junctions. The weak link is created by a Pd0.95Fe0.05 strip across a Nb bridge. Close to TC and in perpendicular magnetic field the junctions exhibit a modulation of the critical current IC(B) similar to a Fraunhofer interference pattern which proves the dc Josephson effect. The structure contains three weak links (two different areas) in series which result in the observation of two periods scalable with the areas penetrated by magnetic flux. Measurements of Shapiro steps prove the presence of the ac Josephson effect.

Strong influence of a magnetic layer on the critical current of Nb bridge in finite magnetic fields due to surface barrier effect

We measured the critical current of the bilayer Nb/Co in the applied magnetic field. When the magnetic field was tilted to the axis which was perpendicular to the plane of the bilayer we observed a large difference in critical currents flowing in opposite directions. We found that the largest critical current of the bilayer exceeded the critical current of the superconductor without Co layer in a wide range of the tilted magnetic fields. The theory which takes into account the surface barrier effect for vortex entry and magnetic field of the magnetic layer gave a quantitative explanation of our experimental results.

Layered ferromagnet/superconductor heterostructures: Nonequilibrium quasiparticle dynamics and photodetector applications

Time-resolved optical pump-probe and photoimpedance studies of proximized ferromagnet/superconductor nanobilayers are presented. The weak ferromagnetic nature of an ultrathin ${\mathrm{Ni}}_{0.48}{\mathrm{Cu}}_{0.52}$ film makes it possible to observe the dynamics of the nonequilibrium superconductivity in $\mathrm{Ni}\mathrm{Cu}∕\mathrm{Nb}$ hybrids through time-resolved measurements of a near-surface optical reflectivity change, which is generated by femtosecond optical pump pulses and discussed within a nonequilibrium two-temperature electron-heating model. We observed that the NiCu overlay significantly reduced the slow bolometric contribution present in the photoresponse of a pure Nb film, resulting in a strong enhancement of the nonequilibrium kinetic-inductive component of the transient photoimpedance, measured as an $\ensuremath{\sim}700\text{\ensuremath{-}}\mathrm{ps}$-wide voltage waveform generated across an optically excited current-biased $\mathrm{Ni}\mathrm{Cu}∕\mathrm{Nb}$ bilayer microbridge. The sensitive picosecond photoresponse makes our $\mathrm{Ni}\mathrm{Cu}∕\mathrm{Nb}$ heterostructures suitable for ``engineered'' ultrafast superconducting photodetectors since the photoimpedance signals observed in plain Nb bridges were at least $10\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ long and were due to the light-induced simple-heating effect.

Quantitative analysis of the guidance of vortices in superconducting films with magnetic dots

The guidance of vortices in superconducting Nb films with periodic arrangements of magnetic Ni dots is studied via simultaneous four-probe measurements in standard and Hall configuration. Ni dots are fabricated via e-beam lithography and lift-off technology. Arrays with periodicity down to 400 nm and dot size of 200 nm are obtained. Subsequently, the arrays are covered with a thin superconducting Nb layer. Simultaneous Hall and standard resistive measurements of the flux dynamic in Nb bridges with and without magnetic dots demonstrate and quantify the guidance of flux via the magnetic dots. Guidance of vortices is only possible in a limited current regime; guidance improves with decreasing temperature. Within the identified current regime most of the vortices (>40% for zero field) and in the optimum case all of the vortices are guided by the magnetic dots.

Icosaniobate: A New Member of the Isoniobate Family.

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Icosaniobate: A New Member of the Isoniobate Family

Icosaniobate [Nb20O54]8- was synthesized by reacting [H4Nb6O19]4- with NO in tetrahydrofuran or MeNO2. A single-crystal X-ray diffraction study of its n-tetrabutylammonium salt [a = 17.7284(18) A, b = 33.542(3) A, c = 34.316(2) A, Z = 4, and space group P22(1)2(1)] revealed a dimeric structure where two decaniobate ions are condensed sharing two terminal O atoms. Unlike that in [(NbW5O18)2O]4-, the Nb-O-Nb bridges in icosaniobate are bent. The nonlinear bridging reduces the maximum possible symmetry of the dimeric anion to mm2, which it closely approximates.

Fabrication of Nb/NbOx/CoySi1−y junctions for superconductor-amorphous ferromagnet tunneling transport

Abstract Superconductor (Nb)–insulator (NbO x )–Co y Si 1− y alloy planar tunnel junctions have been fabricated with the purpose of investigating the spin polarization of the magnetic alloy. These simple transition metal–Si alloys present a crystalline to amorphous transition at y =0.75, that results in thin films with suitable magnetic properties such as low coercive fields and a well-defined uniaxial anisotropy. The insulator barrier is defined as a native oxide layer prepared by oxidizing the superconducting Nb bridge in a humid atmosphere. Transport measurements have revealed the appropriate characteristics of this barrier; in particular, they show an increase in the junction resistance as the temperature is decreased down to the critical temperature of the Nb layer.

Critical current of Nb and NbN thin‐film structures: The cross‐section dependence

We present results of a systematic study of the critical current in superconducting Nb and NbN thin film bridges. The bridges with a width from 50 nm to 10 µm were patterned from thin superconducting films by means of electron-beam lithography and ion milling. For both materials the nominal critical current density extrapolated to zero temperature varied with the bridge width and thickness. We attribute these variations to a fabrication-enhanced reduction of the effective, superconducting cross-section of the bridges with respect to their geometric cross-section and to an uneven distribution of the super-current over the superconducting core of the bridge. In very thin bridges, i.e. 5 nm and 8 nm for NbN and Nb, respectively, the nominal current density increased drastically when the bridge width became smaller than 500 nm. We associate the enhancement of the critical current density in narrow bridges with the crossover from depinning of magnetic vortices to either their nucleation or breaking of Cooper pairs. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Assembly of Hybrid Inorganic−Organic Materials from Octahedral Nb6 Clusters and Metal Complexes

The octahedral edge-bridged niobium cyano-chloride cluster [Nb6Cl12(CN)6]4- and the [Mn(salen)]+ metal complex have been used as building units to prepare solid-state materials with extended frameworks at room temperature through self-assembly processes. Three materials with different dimensionalities were prepared and characterized: (Me4N)4[Nb6Cl12(CN)6]·2MeOH (1) (0D), (Me4N)2[Mn(salen)]2[Nb6Cl12(CN)6] (2) (2D), and (Et4N)2[Mn(salen)(MeOH)]2[Nb6Cl12(CN)6]·2MeOH (3) (1D). 1 was used as cluster precursor for the preparation of 2 and 3. The framework dimensionality seems to be affected by the size of the template-counterion used. Single-crystal X-ray analysis revealed that 1 is based on discrete [Nb6Cl12(CN)6]4- separated by (Me4N)+ and MeOH molecules. 2 has a two-dimensional framework, in which each layer is formed by [Nb6Cl12(CN)6]4- clusters connected through four cyanide ligands to four different [Mn(salen)]+. Each manganese complex connects two clusters through Nb−CN−Mn−NC−Nb bridges, leading to the ...

Structure of zinc and niobium tellurite glasses by neutron and x-ray diffraction**Some of these results have already been presented in thesis work: Yousef E 2003 Astudy of some physical properties of tellurite glass (Al-Azhar University, AssiutEgypt).

Neutron and x-ray diffraction experiments of high resolving power with neutrons from aspallation source and high-energy photons from a synchrotron have been performed oncompositional series of binary Zn, Nb and on mixed Zn/Nb tellurite glasses. The Te–O,Zn–O and Nb–O coordination numbers are determined by Gaussian fitting of thefirst-neighbour peaks in the neutron and x-ray data simultaneously. The transition ofTeO4 toTeO3 units with increasing fraction of a second component is indicated by decreasing total Te–Ocoordination numbers. This transition appears different for glasses with ZnO orNb2O5 additions. Details of the Te–O peaks suggest there are two species of Te–O bonds withlengths of and nm. The change of their fractions shows excellent agreement with the existence ofTeO4 trigonalbipyramids and TeO3 trigonal pyramids. All oxygen atoms from ZnO andNb2O5 areused for rupture of Te–O–Te bridges, which is accompanied with a change of nearly all participatingTeO4 toTeO3 groups. The tendency for a change decreases for glasses of higher second component content which is accompanied by the occurrenceof TeO4 groups with non-bridging oxygens. The Nb tellurite glasses show transition tonetwork-forming behaviour with the formation of Nb–O–Nb bridges. The fractions ofTeO3 unitsof ternary Zn/Nb tellurite glasses agree with an additivity behaviour of the modifying effects of ZnOand Nb2O5 additions.