Low Critical Current Density Research Articles

Sweep-Rate Dependent Reversal of Magnetic Moment in Superconducting Films

We report on a dynamical effect in YBa 2 Cu 3 O x (YBCO) films and YBCO/CMR (Colossal Magnetoresistive) multilayers, in which variations in the magnetic field sweep rate can change, or reverse the sign of, the magnetic moment. The effect occurs in thin films having a low critical current density, either intrinsically or as a result of operation near the critical temperature. We attribute this effect to the large demagnetizing field present in the films, leading to a relaxation in the non-equilibrium vortex distribution, in which current near the edge of the film changes direction. We have investigated this effect in a variety of systems by magnetometery, and provide some magneto-optical evidence of its mechanism.

Enhancement of critical current densities of powder-in-tube processed MgB2 tapes by using MgH2 as a precursor powder

We investigated the effect of starting powder on the microstructure and superconducting properties of MgB2 tapes prepared by a powder-in-tube technique. Two kinds of powder mixtures, Mg + B and MgH2 + B, were used as precursors. When using the mixture of Mg + B, carbon-steel (CS) sheaths reacted with B, and FexB (x = 1 and 2) was formed. This brought about a decrease in the density of the MgB2 core and, hence, a low critical current density (Jc). On the other hand, we found that the mixture of MgH2 + B was effective in suppressing the formation of FexB and obtaining a high density of the core layer. The Jc values of the tapes prepared using the mixture of MgH2 + B were about twice as large as those prepared using the mixture of Mg + B and reached 1600 A cm−2 at 4.2 K and 10 T. This is higher than that of the CS-sheathed MgB2 tapes prepared with MgB2 powder.

Current transport in interface-engineered high- Tc Josephson junctions

The mechanisms of current transport in interface-engineered junctions (IEJs) were investigated based on more than 400 junctions fabricated under various process conditions. We confirmed that the I c R n values of IEJs scaled with the square root of the Josephson critical current density ( J c) universally, as long as the junctions had a low critical current density of less than 1×10 4 A/cm 2 at 4.2 K. These low- J c junctions exhibited d I/d V profiles peculiar to inelastic tunneling via two or less localized states with some remnant of a smeared superconducting gap. In contrast, we could not observe a universal scaling relation between I c R n and J c among higher- J c junctions. The maximum I c R n values of IEJs having J c ranging from 10 4 to 10 6 A/cm 2 were limited to 2–3 mV at 4.2 K. The d I/d V profiles of these high- J c junctions differed considerably from those of low- J c junctions, and were characterized by a highly smeared-out superconducting gap with a reproducible sub-gap structure. The differential conductance at high bias voltages increased slightly with decreasing temperature. These results indicate that a simple tunneling picture is inadequate to describe IEJs with high- J c values.

Microstructure and electrical properties of Bi 1.8Pb 0.4Sr 2Ca 2.1Cu 3O x tapes with CeO 2 ceramic barriers

The AC losses in multifilamentary tapes can be reduced mainly by insulating barriers (ceramics or alloys) and by twisting the filaments. We have prepared multifilamentary Bi 1.8Pb 0.4Sr 2Ca 2.1Cu 3O x (BSCCO)/Ag tapes with the “powder-in-tube” process where the single filaments were surrounded with a ceramic layer. The microstructure and the electrical properties of tapes with CeO 2 barriers and without barriers will be compared. All tapes with ceramic barriers have broader transition to superconductivity and lower critical current densities than pure Ag tapes. We have found a melt-forming reaction between the CeO 2 barriers and the BSCCO filaments during heat treatment, resulting in a drastic j c degradation. The problem of interaction is more pronounced in the case of tapes with more filaments. Due to the thinner Ag interlayers in these tapes, a direct contact of BSCCO and the insulating ceramic by cracks or spikes cannot be avoided during processing.

Observation of zero-crossing steps in surface junctions of Bi 2Sr 2CaCu 2O 8+δ single crystals

We have studied the ac Josephson effect using an intrinsic Josephson junction fabricated on the surface of Bi 2Sr 2CaCu 2O 8+δ single crystals. Irradiated with external microwave of f = 76 and 94 GHz, the I–V characteristics show clear constant-voltage steps crossing the zero-current line. These steps satisfy the ac Josephson relation and the step heights show the Bessel-function behavior with increasing microwave power, corresponding to step numbers up to n = 4. The intrinsic surface junction with a very low critical current density meets the criterion for observing zero-crossing steps.

Predicted Performance of Superconductor-Insulator-Superconductor Mixers with Inhomogeneous Distributed Junction Arrays

The mixing properties of a new type of distributed superconductor-insulator-superconductor (SIS) junction array, which consists of different dimensions of junctions and spacings between every two junctions, have been theoretically investigated using the quantum theory of mixing. A set of dimensions of junctions and spacings between every two junctions of the distributed junction array have been determined so as to minimize the reflection coefficient of an S-parameter, S11, at the input port in the array using microwave computer-aided design, assuming that each SIS junction is represented by a parallel combination of linear normal-state resistance and geometrical capacitance. When the array is composed of well optimized set of dimensions of junctions and spacings between every two junctions, it is demonstrated that excellent mixing properties are attained over a broad-band in spite of the low critical current density of an SIS junction. It is also found that the degradation of mixing properties, which is always observed at certain frequencies in a conventional array with the equal dimensions of junctions and spacings, can be considerably suppressed in the array with a well optimized set of dimensions and spacings of junctions.

Harmonic current-phase relation in Nb–Al-based superconductor/insulator/normal conductor/insulator/superconductor-type Josephson junctions between 4.2 K and the critical temperature

For a study of the supercurrent–phase relationship, we have fabricated Nb/Al/AlOx/Al/AlOx/Al/Nb-based Josephson junctions of low critical current density. At 4.2 K, junctions of a nominal area of 10×10 μm2 showed a nonhysteretic dc current–voltage characteristic with a critical current of about 10 μA. Similar junctions were part of washer-type rf superconductor quantum interference devices. These samples were inductively coupled to high-quality tank circuits for measurements at 4.2 K or at higher temperatures up to values close to the critical temperature of niobium. In this range, the current–phase relation, as deduced from impedance measurements of the phase-biased junction, did not deviate significantly from the sinusoidal shape.

An experimental study of heating due to the AC loss in a BSCCO cylinder

Due to the very low thermal conductivity of high temperature superconductors (HTSC), AC losses can result in a non-uniform temperature distribution inside a sample. We report the results of an experimental investigation of the temperature distribution in the wall of the BSCCO hollow cylinder in AC magnetic field. The BSCCO cylinder has a relatively low critical current density of 550 A/cm 2 (at 77 K) measured under DC conditions at 1 μV/cm criterion. The temperature of the interior of the cylinder relative to the outside coolant raised 2.5 K at low external magnetic field (∼0.04 T). The change in temperature is increased linearly with frequency and as the third power of the applied magnetic field. The experimental results are compared with the theoretical predictions obtained in the framework of the critical state model complemented by the heat equation.

Studies of the improvement in microstructure of Y2O3 bufferlayers and its effect on YBa2Cu3O7-x film growth

Two kinds of Y2O3 buffer layers have been directlydeposited on cube-textured Ni substrates by electron beam evaporation underdifferent gas pressures. One is deposited at 10-5 Torr air (asingle-layer buffer). In order to reveal the improvement of the microstructure,the other is formed at first under 5×10-4 Torr and then10-6 Torr of N2 (a double-layer buffer). As a result, thedouble-buffer layer obtained is more than 97% {100} orientation and is arelatively denser structure. YBa2Cu3O7-x overlayersare deposited by pulsed-laser deposition on these buffer layers. Large YBa2Cu3O7-x pop-off areas are seen for the single-buffersamples. These areas are eliminated on the double-layer buffer. However, thecritical current does not increase appreciably for the YBa2Cu3O7-x film on the double-layer buffer, even though themicrostructure of the buffer is improved. Other defects probably yieldedpercolative current flow and a low critical current density in the YBa2Cu3O7-x layers.

Galvanic corrosion behavior of titanium implants coupled to dental alloys.

The corrosion of five materials for implant suprastructures (cast-titanium, machined-titanium, gold alloy, silver-palladium alloy and chromium-nickel alloy), was investigated in vitro, the materials being galvanically coupled to a titanium implant. Various electrochemical parameters E(CORR), i(CORR) Evans diagrams, polarization resistance and Tafel slopes) were analyzed. The microstructure of the different dental materials was observed before and after corrosion processes by optical and electron microscopy. Besides, the metallic ions released in the saliva environment were quantified during the corrosion process by means of inductively coupled plasma-mass spectrometry technique (ICP-MS). The cast and machined titanium had the most passive current density at a given potential and chromium-nickel alloy had the most active critical current density values. The high gold content alloys have excellent resistance corrosion, although this decreases when the gold content is lower in the alloy. The palladium alloy had a low critical current density due to the presence of gallium in this composition but a selective dissolution of copper-rich phases was observed through energy dispersive X-ray analysis.

Microstructural analysis of the compatibility of solution deposited buffer layers with the TFA process for YBCO

ABSTRACTIn this paper, the feasibility of applying solution deposition processes for the fabrication of coated conductors has been explored. The crystal and chemical compatibility of the buffer layers processed using metalorganic decomposition with the Y123 deposition using the trifluoroacetate process has been studied. Two buffer layer materials have been used, namely, barium zirconate and strontium titanate. The measured superconducting properties of these conductors were correlated with the microstructure observed on these samples using SEM and cross-sectional TEM. In case of barium zirconate buffer layers, though there exists a very good structural and chemical compatibility between the buffer layer and the Y123, the presence of surface defects in the buffer layer causes compositional heterogeneity and randomly oriented grains in the Y123 film. This leads to poor superconducting properties. In case of strontium titanate buffer layers, due to the excellent crystal and chemical compatibility, and the absence of surface defects, high critical current densities (of the order of 106A/cm2 at 77K and self field) were obtained. However, TEM cross section studies reveals the presence of a significant portions of a-oriented Y123 crystallites which could lead to lower critical current densities. Further studies of the TFA process is required to eliminate the occurrence of a-oriented Y123 in the microstructure. This could lead to further improvements in the properties.

The upper critical field of the Chevrel phase superconductor lead-molybdenum-sulphide doped with gadolinium

Chevrel phase superconductors with upper critical field values of up to 50 T, are candidate materials for the next generation of high field magnets. We have investigated the effect of doping lead-molybdenum-sulphide with the magnetic ion gadolinium to improve the critical parameters and the flux pinning in this material. Samples were prepared at 1500/spl deg/C to ensure the gadolinium was uniformly distributed throughout the material and then sintered using a hot isostatic press operating at a pressure of 2000 atmospheres. Calorimetric measurements have been completed In magnetic fields up to 15 T as a function of temperature. The temperature dependence of the upper critical field of these isotropic Chevrel phase materials are presented. Upper critical field values of more than 60 T have been determined for both doped and undoped samples. The calorimetric data reported here show that material with low levels of gadolinium doping offers the possibility of strong flux pinning and high critical current densities in high magnetic fields. In highly doped materials, the low critical current density and irreversibility fields found are probably due to a reduction in the fundamental properties and not poor grain boundaries.

Distribution of flux pinning strength in a superconducting Bi-2223 silver-sheathed tape

The distribution of the flux pinning strength was estimated from a measured current–voltage curve for a monofilamentary Bi-2223 silver-sheathed tape in a magnetic field parallel to the c-axis. It is found that the distribution can be fitted well to the Weibull function of a very wide distribution. This wide distribution seems to be caused by the low critical current density at weak links. The obtained Weibull function can be approximated fairly well by a simpler distribution function with which the scaling of current–voltage curves is analyzed using the flux creep–flow model. The obtained scaling parameters agree well with the experimental results.

Self-field effects on critical current density and current-voltage characteristics in superconducting YBaCuO thick films

The self-field and percolative influences on transport measurements of polycrystalline bridges engraved on YBaCuO thick film have been investigated. A maximum in the dependence of the critical current density on cross-sectional area of the bridge (A = 0.003 mm2-0.3 mm2) has been found experimentally, in samples with low critical current densities (Jc<50 A cm-2). The result of the measurements are in agreement with Mulet and coworkers, who have predicted that, under certain conditions, the self-field effects on transport measurements are negligible and the Jc dependence on the sample dimensions is determined by the percolative character of the transport current. Self-field influences have also been observed in current-voltage characteristics, which have been analysed using the Ambegaokar-Halperin phase-slip theory. By allowing the noise parameter () to change with temperature, magnetic field and transport current, adequate agreement between theoretical and experimental current-voltage characteristics has been obtained. The dependence of the noise parameter with the transport current is demonstrated to be related with the self-field.

The effects of Cs doping, heat treatments on the phase formation and superconducting properties of (Bi,Pb)–Sr–Ca–Cu–O ceramics

The influence of Cs doping and heat treatments on the phase formation, microstructure development and normal, superconducting state properties of (Bi,Pb)–Sr–Ca–Cu–O superconductors was systematically investigated. Samples with nominal composition Bi 1.7Pb 0.3Sr 2Ca 2Cu 3Cs x O y , x=0.0–1.0, were prepared and characterized by X-ray diffraction, scanning electron microscopy, electron probe microanalysis, resistivity, low field magnetic susceptibility and critical current density measurements. Addition of Cs to Bi 1.7Pb 0.3Sr 2Ca 2Cu 3O y was found to be effective in forming the high- T c (2223) phase. For x=0.4–0.8, we observed an ∼93% high- T c (2223) phase. The superconducting properties were enhanced with Cs doping.

Preparation of Nb/Al–AlO x/Nb Josephson junctions with critical current densities down to 1 A/cm 2

A new method for preparing Nb/Al–AlO x /Nb Josephson junctions with very low critical current densities is reported: an ultraviolet light assisted aluminum oxidation process in pure oxygen atmosphere. A better reproducibility of the critical current density of the junctions is reached in comparison to junctions prepared by the double oxidation technique. Series arrays of Josephson tunnel junctions with up to 960 junctions are produced and tested for Josephson voltage standard applications. The thickness of the aluminum oxide barrier is determined by Rutherford backscattering spectrometry and is compared with the results of specific capacitance measurements.

Superconducting YNi 2B 2C thin films prepared by pulsed laser deposition

Thin films of the intermetallic superconducting compound YNi 2B 2C have been prepared by pulsed laser deposition. The YNi 2B 2C phase was formed during the deposition process at a substrate temperature of 860°C (in situ) or after deposition by post-annealing. Polycrystalline, c-axis oriented films were obtained on MgO and Al 2O 3 substrates. Post-annealed and in situ grown YNi 2B 2C films showed inductively measured transition temperatures up to 15 K with typical transition widths of ≈ 0.5 K. The electrical and magnetic properties of the films like π( T), B c2( T), J c( T) and M( H) at 5 K were measured. The B c2( T) dependence showed a linear behaviour and the low temperature critical current density J c(5 K) was 8 × 10 5 A/cm 2.

The effective dissipation in Nb/AlOx/Nb Josephson tunnel junctions by return current measurements

Measurements of temperature dependence of the return current in high quality Nb/AlOx/Nb Josephson junctions are presented. From the experimental data, we obtain the effective resistance, i.e., the effective dissipation, for the retrapping process, according to the generalized resistively shunted junction model proposed by Chen, Fisher, and Leggett. We present a careful analysis, based on a comparison between the measured temperature dependencies of both the return and the quasiparticle tunneling current. We find that the junction subgap conductance, which includes the quasiparticle and the quasiparticle-pair interference terms, is responsible for the return process. The measurements have been performed on various samples, in a wide range of critical current densities from 50 to 2250 A/cm2, covering different damping regimes and spanning over the high and low temperature limits. Junctions with low critical current density show ideal dissipation which makes the return current scale with temperature according to the BCS exponential behavior without flattening out effects. This result may be relevant for the possible use of Nb/AlOx/Nb junctions in macroscopic quantum coherence experiments, which strongly require a very low dissipation.

Distributed quasiparticle mixing in a non-linear transmission line at sub-millimeter wavelengths

A new class of distributed mixer based on the non-linear quasiparticle tunnel current in a superconductor-insulator-superconductor transmission line has been proposed and tested. The Nb/Al/AlOx/Nb junctions used in our mixers are typically 200 nm wide and are between 1 and 2 guided wavelengths long. Compared with the traditional sub-millimeter lumped element quasiparticle mixer, the distributed mixer requires lower critical current densities, lower magnetic field as well as simpler tuning circuitry. Experiments have been performed in various sub-millimeter frequency bands. Receiver noise temperatures of 66 K at 460 GHz, 210 K at 630 GHz and 600 K at 780 GHz have been measured. These noise temperatures are comparable to or slightly better than the noise temperatures of the corresponding lumped element mixers.

Corrosion behaviour of monocrystalline titanium nitride

The corrosion behaviour of monocrystalline and highly textured TiN films deposited onto single crystalline MgO has been investigated. It was found that films with high defect concentrations and elements of polycrystallinity had a reduced corrosion resistance. These films had low corrosion potentials and high critical and passive current densities. For high quality films the [110]- and [111]-orientations generally displayed better corrosion properties than films with [100]-orientation. Corrosion had different effects on films of different orientations: On [100]-oriented films loose rectangularly shaped flakes were observed. [110]-oriented films cracked because of compressive stresses, which in many cases caused the films to peel off during corrosion. On [111]-oriented films, finally, triangular pits were defected after corrosion. Films with high corrosion potentials had low values of critical and passive current densities. Non-destructive electrochemical analysis in combination with defect analysis can thus be used to predict the behaviour of these films on anodic polarisation.