Magnetic field hysteresis of critical currents in Bi(2223)/Ag tapes and a way to overcome it

Experiments showing hysteresis of critical currents versus the external magnetic field I_c(B_e) were performed with two multifilamentary Bi(2223)/Ag tapes. The I_c(B_e) hysteresis is observable in the transversal as well as in the longitudinal orientation of the long axis of the tape with respect to the magnetic field. Based on the idea that the hysteresis is the effect of trapped flux in a network of well-connected current paths, a way to overcome this effect has been proposed and experimentally verified. The induced frozen-in screening currents are split into several parallel current patterns by cycling the external magnetic field around the adjusted value. Using the proposed method, the ‘neutral’ I_c(B_e) characteristics have been found. Approximate calculations of the penetration depth of the trapped flux show that the network of well-connected current paths could be formed by several disk-shaped grains (≈ Φ8 × 0.4 μ) stacked into more or less axially ordered (quasi cylindrical) colonies of average dimensions estimated to ≈ Φ8 × 4 μm.     

Flux trapping and its effect on AC susceptibility of granular superconductors

Experimental data on the real (x′) and imaginary (x″) parts of AC magnetic susceptibility as a function of DC bias field (H) shows the effect of trapped flux in granular highT _c superconductors. The aim was to substantiate our recent theoretical findings on the basis of a two-component critical state model suitable for granular highT _c superconductors. Stress has been given to understanding the origin of hysteresis inx′(H) andx″(H). It was seen in the experimental data that above a certain value of DC field range irreversibility appears inx′(H) andx″(H) creating hysteresis like loops. Comparison of data with calculated loops shows good agreement.x′(H) andx″(H) curves show considerable asymmetry in presence of trapped flux.     

Superconducting NbN Thin-Film Nanowire Detectors for Time-of-Flight Mass Spectrometry

Superconducting nanowire detectors (SND) have been applied for time-of-flight mass spectrometry (TOF-MS) for the first time. In this study, we used the SND, which consists of a very thin niobium nitride (NbN) film having a nanowire meander pattern with a thickness of 6.8 nm and a width of 200 nm on a MgO substrate. The experiments were carried out for Angiotensin I and bovine serum albumin (BSA). These biomolecules were ionized by laser radiation with matrix-assisted laser desorption ionization (MALDI). The ions were accelerated by a static high voltage of 17.5 kV, and incident on the NbN meander, which is dc-biased below a superconducting critical current (I _c). It was found that the output pulses have a rise time of about 640 ps, which is extremely faster than superconducting tunnel junction (STJ) detectors, and a fall time of about 50 ns. Moreover, we investigated the bias current dependence of output pulses, and confirmed that molecules can be detected even for bias currents of about 50% of I _c.      

Preparation and superconductivity of NbN powder by a vapour phase reaction

The formation of superconducting NbN powders by the vapour phase reaction of the NbCl₄-NH₃-H₂ system has been investigated. The properties of NbN powders changed with the reaction temperature, mixing temperature of NbCl₄ and NH₃, and gas composition ratio ([NH₃]/[NbCl₄]). The reaction system gave NbN powders consisting of particles with sizes less than 50 nm. The process of particle formation is discussed. The NbN powders produced had vacancies at both the Nb and N sublattices. The vacancies influenced the superconducting transition temperature (T _c) of NbN, the highestT _c observed being 14.1 K.     

High Magnetic Fields in Superconducting Permanent Magnets

We report on the temperature dependence of the maximum trapped field B _o(T) in the high-T _c superconductor YBa₂Cu₃O_7–x (YBCO). Trapped fields of bulk melt-textured YBCO samples are limited by their pinning behaviour and mechanical properties. The mechanical properties of bulk YBCO samples were improved by the addition of silver. Furthermore, the YBCO disks were encapsulated in steel tubes in order to reinforce the material against the large tensile stress acting during the magnetizing process. High trapped fields up to 13.3 T were measured at 33 K on the surface of a single YBCO disk containing Ag additions and reinforced with steel tubes. The flux pinning properties of this YBCO disk were improved by neutron irradiation and Zn doping resulting in a significant shift of the trapped field curve B _o(T) to higher temperatures. In YBCO mini-magnets, consisting of two YBCO disks, maximum trapped fields up to 16.0 T were achieved at 24 K by a combination of Zn-doping and Ag addition. The improved reinforcement, which was used in this case, was found to withstand the maximum trapped field of 16 T without cracking.     

Magnetic and Thermal Properties of HTS Bulk Magnet in the Pulsed-Field Magnetizing Process

In order to enhance the field-trapping ability of high T _c superconducting melt-textured bulk materials which act as quasi-permanent magnets when they capture the external magnetic fields, it is important to enhance the mechanical toughness of the materials to stand the stress induced by the magnetic repulsive force and thermal expansion. We adopted a dense Dy123-based bulk material with reduced void concentration in the experiment. Since the heat generations due to the flux motions in the samples results in the degradation of J _c, the time evolutions of the trapped magnetic fields and the temperature rises during and after the pulsed-field magnetizing processes were precisely measured at the same time to evaluate the penetrating flux motions and the heat generations in the sample. A single and couple of the magnetic pulsed fields with various intensities were successively applied to the sample at 30 K. The single magnetic field application exhibited a peak effect in the trapped-field behavior and tended to decline due to the heat generation. In the iterative pulsed-field application, the behaviors of the trapped fields and the temperature changes were found to be inverse between the first and the second pulsed-field applications. This implies that the flux penetration behavior into the sample magnet at the second field application is strongly restricted by the presence of former trapped fields which were formed by the first field applications.     

Effect of Proton Irradiation on the Critical Current in Tl2Ca2BaCu2O8 Bicrystal Josephson Junctions

Josephson junctions made by depositing Tl₂Ba₂CaCu₂O₈ (T1-2212) high-temperature supeconducting films on a SrTiO₃ bicrystal were irradiated at room temperature with 2-MeV protons. Changes in the critical currentI _c and the temperatureT* at whichI _c approaches zero are reported as functions of the proton fluence. Following an initial drop at low fluences. similar to a pretreatment stage.I _c becomes significantly more resistant to radiation damage. At low fluences, the sensitivity ofI _c to radiation damage is apparently dominated by the presence of grains or bridges at the junction. At high fluences the dominant effect appears to be radiation-induced decreases in the transition temperatureT _c of the banks.     

Impact of Gd Doping on Morphology and Superconductivity of NbN Sputtered Thin Films

We report the effect of Gd inclusion in the NbN superconductor thin films. The films are deposited on single crystalline Silicon (100) by DC reactive sputtering technique, i.e., deposition of Nb and Gd in presence of reactive N₂ gas. The fabricated relatively thick films (400 nm) are crystallized in cubic structure. These films are characterized for their morphology, elemental analysis, and roughness by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDAX), and Atomic Force Microscopy (AFM) respectively. The optimized film (maximum T _c ) is achieved with gas ratio of Ar:N₂ (80:20) for both pristine and Gd-doped films. The optimized NbN film possesses T _c (R=0) in zero and 140 kOe fields are at 14.8 K and 8.8 K, respectively. The Gd-doped NbN film showed T _c (R=0) in zero and 130 kOe fields at 11.2 K and 6.8 K, respectively. The upper critical field H _c2(0) of the studied superconducting films is calculated from the magneto-transport [R(T)H] measurements using GL equations. It is found that Gd doping deteriorated the superconducting performance of NbN.     

Superconducting Single Photon Nanowire Detectors Development for IR and THz Applications

We present our progress in the development of superconducting single-photon detectors (SSPDs) based on meander-shaped nanowires made from few-nm-thick superconducting films. The SSPDs are operated at a temperature of 2–4.2 K (well below T _c ) being biased with a current very close to the nanowire critical current at the operation temperature. To date, the material of choice for SSPDs is niobium nitride (NbN). Developed NbN SSPDs are capable of single photon counting in the range from VIS to mid-IR (up to 6 μm) with a record low dark counts rate and record-high counting rate. The use of a material with a low transition temperature should shift the detectors sensitivity towards longer wavelengths. We present state-of-the art NbN SSPDs as well as the results of our recent approach to expand the developed SSPD technology by the use of superconducting materials with lower T _c , such as molybdenum rhenium (MoRe). MoRe SSPDs first were made and tested; a single photon response was obtained.      

Anisotropic I–V characteristics of spontaneously emerged periodic stripes of superconducting NbN thin films on Si trench sidewall by RF magnetron sputtering

To realize a highly integrated thin film superconductive coil, sputter-deposition of NbN on a trench sidewall of Si substrate was examined. A trench of 0.2 mm in depth was fabricated by Si Deep-RIE. A Nb target was sputtered in Ar/N₂ mixed gas flow. Thanks to the periodic corrugated structure formed by repeated Deep-RIE process, it was found that NbN superconducting lateral stripes connected with each other with thinner NbN thin film were spontaneously emerged. The superconducting properties along and across the stripes were completely different. Along the stripes, the I–V curves showed supercurrent. Additionally, the undulations were observed in I–V curves below Tc indicating the existence of the inhomogeneity of such as thickness, having different J_c depending on the position in the stripes. The normal conductivity along the stripes is almost five times higher than that across the stripes. Across the stripes, clear supercurrent was observed in the films formed in N₂ flow rate lower than 12%, indicating the stripes were connected with a thin superconductive layer in between. In the films formed in N₂ flow rate over 14%, no supercurrent was observed but the conductance increased with temperature indicating tunneling conduction. The obtained novel structure can be looked upon as a spontaneously emerged multifilamentary superconducting wire possibly applicable to produce high magnetic field.     

The Effect of Gd Concentration on the Physical and Magnetic Properties of Bi1.7Pb0.3-xGdxSr2Ca3Cu4O12+y Superconductors

Bi–Pb–Gd–Sr–Ca–Cu–O bulk samples with nominal composition Bi_1.7Pb _0.3-xGd_xSr₂Ca₃Cu₄O_12+y (x=0.01, 0.05, 0.075, 0.10) were prepared by the melt-quenching method. The effects of different Gd doping on the structure have been investigated by electrical resistance, scanning electron micrographs, XRD, magnetization and magnetic hysteresis loop measurements. The magnetization measurements have been carried out as a function of magnetic field for fields up to 5 kOe at temperatures well below the zero resistance temperatures of the annealed samples. It has been found that the high-T_c superconducting phase, (2 2 2 3), is formed in the sample A with concentration x = 0.01, annealed at 840°C for 120 h. However, with increasing Gd³⁺ doping for Pb²⁺ the (2 2 2 3) phase gradually transforms into the (2 2 1 2) phase. The magnitudes of magnetization and initial susceptibility, | M | and | dM/dH|, and the hysteresis loop areas decrease with increasing Gd concentration x and/or temperature T. The fast decreases in | M|, | dM/dH |, and the hysteresis loop areas related to the superconducting volume, with increasing x and/or T seem to imply an existence of flux pinning centres in our samples. In order to support this implication the critical current densities J_c, of the samples, have been estimated at two fixed temperatures, 9 and 30 K. Our data have indicated that J_c decreases with increasing temperature and/or Gd concentration, as expected.     

A Statistical Model of <Emphasis Type="Italic">I</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> Changes for Bending Bi-2223 Tapes

Bi-2223 tape was one of the high-temperature superconductors with commercial applications. One of the applications was bending and winding Bi-2223 tape into solenoids to produce high magnetic fields. To study bending properties, three multifilamentary of Bi-2223 tapes sheathed with silver alloys were manufactured. Bending experiments for the tapes were performed, and critical currents I _c of tapes with definite bending radius were measured. And, current transferring mechanisms in filaments were analyzed, as well. Experimental results showed that silver alloy sheathed tapes had better bending properties than pure silver-sheathed one. On the contrary, to describe bending radius dependence of I _c , a statistical model was suggested. The model expected that bending radius dependence of I _c was following an exponential law that was quantitatively expressed by mathematic expressions. Bending dependence of I _c could be calculated from the expression and calculated results agreed with experimental data very well. Therefore, the suggested model has successfully explained the experimental results.     

Evidence for unpinned magnetic flux vortices above theIctransition in the 2223-phase high-Tcsuperconductor

A similarity in the dc voltage-current (V-I) curves for both direct and alternating transport currents is used to propose that unpinned flux vortices are generated above theI _ctransition for dc transport currents, when Abrikosov flux vortices begin to penetrate the superconductor. Two methods can be used to give a dc voltage drop for an ac transport current: (1) if there is a slight dc offset voltage in the ac current which favors vortex loop collapse as in a traditional dcI _ctest, or (2) if an asymmetric transverse magnetic field is present which favors vortex loop collapse for current in one direction over the reverse direction.     

Observation of Re-entrant Resistance in NbN/NbO/Co Trilayer

Transport measurements on a niobium nitride (NbN) film covered with a ferromagnetic cobalt layer are reported here. The sample shows a superconducting transition (T _SC) at 6.5?K. In the superconducting state, a dip in resistance is observed at a temperature referred to as T _MIN. Below T _MIN, resistance reappears, with a magnitude of about 1% of the normal-state resistance. The observed resistance is found to decrease on increasing the applied current?(I). A?possible reason for re-entrant resistance might be the vortex dynamics in NbN superconducting layer due to the stray fields created by the Co layer. Further confirmation of the observed behavior is obtained from the plot of resistance versus probe current measured at various temperatures in close vicinity of?T _SC.     

Magnetovoltage Measurements and Hysteresis Effects in Polycrystalline Superconducting Y1Ba2Cu3O7−x /Ag in Weak Magnetic Fields

The magnetovoltage measurements (V–H curves) with different sweeping rates (dH/dt) of the external magnetic field in Ag-added polycrystalline YBa₂Cu₃O_7?x sample (YBCO/Ag) were investigated. The measurements of V–H curves were carried out as functions of the transport current (I) and temperature (T). Upon cycling H, all V–H curves measured for different values of I exhibit a clockwise hysteresis effects in forward region. The hysteresis effects in the V–H curve were interpreted in terms of two-level magnetic system, which considers the superposition of the external magnetic field and the local magnetic fields in the intergrain boundaries induced by magnetic dipole moment of neighbor superconducting grains. The analysis of magnetovoltage data showed that the flux trapping in the junction network has a negligible effect on the evolution of the V–H curves and the irreversibilities arise mainly from the flux trapping inside the grains. It is shown that the width of V–H curves shows a universal scaling behavior with respect to the applied current below the critical temperature T _c . The comparison of V–H curves of the YBCO/Ag sample with those of YBCO shows that adding Ag to the superconducting structure weakens the pinning properties of Josephson medium and provokes the instabilities in measured dissipation. The presence of Ag in the superconducting matrix causes marked decrease in hysteresis effects and makes the V–H curves dH/dt dependent. At high values of dH/dt, the instabilities and plateau regions in V–H curves increase significantly as compared to those of observed in YBCO. In addition, the interrelation between the evolution of V–H curves obtained for different values of I and the critical current I _c was demonstrated experimentally.     

Properties of n × n square arrays of proximity effect bridges

We have studied the I-V characteristics, the dc resistance at the origin R(T), and the critical current I _c (T) of arrays consisting of n × n (n = 10, 20, 30, and 40) In squares separated by Au/In regions. The superconducting squares have side d _s (In) from 7.5 to 15 m and d _N (Au/In) from 2.5 to 11 m. These devices show two transition temperatures in their R(T) curve; one at T _cG near T _c (In) and another one at T _c ^* above T _c (Au/In). The I-V curves for each temperature region resemble recent published results on NbN granular films. These arrays follow a formalism proposed by Wolf, Gubser, and Imry to describe the transition to a resistanceless state at T _c ^*. The measured values of I _c (T) are larger than expected from n noninteracting Bardeen-Johnson junctions in parallel, although the exponential behavior in temperature is followed. They are compatible with the idea of bands in the quasiparticle spectrum in the Au/In regions. Deviations in I _c (T) from an exponential behavior in T near T _c ^* have been analyzed by applying to SNS arrays ideas developed for phase transitions in SIS arrays. Evidence that at T _c ^* we may be observing a phase transition at T _cJ due to the Josephson coupling between In squares exists but it needs the development of theoretical work to be put on more solid grounds.Supported by the Swiss National Science Foundation.     

Study of galvanomagnetic effects in Bi-Sr-Ca-Cu-O films

Galvanomagnetic effect has been studied at 77 K on Bi-Sr-Ca-Cu-O films as a function of d.c. bias current. These films were prepared by spray pyrolysis and screen-printing techniques, withT _c (R=0) 80 K and 100 K respectively. Magnetic field dependence ofI _c of sprayed-film showed very small hysteresis whereas screen-printed film showed greater hysteresis. Application of small magnetic field on these films destroyed the zero-resistance state and a finite resistance was developed. The slope of the resistance vs magnetic field curveΔR/ΔH depended on the current flowing through the sample. To increaseΔR/ΔH, meander-shape pattern was prepared on the sprayed film. In generalΔR/ΔH increased by a factor of 10³ after sample patterning. The possibility of using these films as a sensitive magnetic field sensor is discussed.     

Temperature dependence and microwave enhancement of the critical current of superconducting microbridges

The temperature dependence of superconducting tin, thin-film, narrow bridges has been investigated. NearT _c the critical current varies with (1 – T/T_c ), with a distribution of values between 1 and 3/2. It is suggested that the values can be used to characterize the bridges. The microwave enhancement ofI _c andT _c was measured and these data were compared with a recent theory by Christiansenet al. on the microbridges. We found that their theory gives a correct prediction for the Dayem effect in the bulklike bridges (with =3/2) while their theoretical values for the junction-like bridges (with =1) are not in agreement with our data.Work supported in part by the National Science Foundation.     

Angular dependence of transport current near critical at fields to 4 T in metalorganic thin films

Values of the transport current have been obtained as a function of angle of the magnetic field to the plane of the thin film YBa₂Cu₃O_7–d (YBCO) prepared by metalorganic deposition. Current flowed in thea, b planes at various angles to the applied magnetic fields to 4.2 T. Films with thicknesses near 350 nm were studied. For these thin films andHc a secondary maximum is observed whenJ _c> 1 MA/cm₂. NearHa, b a large increase inI _c is observed, and the sharpness, field dependence, and field directional dependence is discussed. The response to currents aboveI _c is determined and the fit to a power lawVI _n shows a strong correlation betweenI _c andn.