Dependence Of Critical Current Research Articles

Manipulating Josephson junctions in thin-films by nearby vortices

It is shown that a vortex trapped in one of the banks of a planar edge-type Josephson junction in a narrow thin-film superconducting strip can change drastically the dependence of the junction critical current on the applied field, Ic(H). When the vortex is placed at certain discrete positions in the strip middle, the pattern Ic(H) has zero at H=0 instead of the traditional maximum of ‘0-type’ junctions. The number of these positions is equal to the number of vortices trapped at the same location. When the junction–vortex separation exceeds ∼W, the strip width, Ic(H) is no longer sensitive to the vortex presence. The same is true for any separation if the vortex approaches the strip edges.

Electromigration kinetics and critical current of Pb-free interconnects

Electromigration kinetics of Pb-free solder bump interconnects have been studied using a single bump parameter sweep technique. By removing bump to bump variations in structure, texture, and composition, the single bump sweep technique has provided both activation energy and power exponents that reflect atomic migration and interface reactions with fewer samples, shorter stress time, and better statistics than standard failure testing procedures. Contact metallurgies based on Cu and Ni have been studied. Critical current, which corresponds to the Blech limit, was found to exist in the Ni metallurgy, but not in the Cu metallurgy. A temperature dependence of critical current was also observed.

Josephson Effect in Graphene: Comparison of Real and Pseudo Vector Potential Barriers

The Josephson currents through real vector potential (RVP) and pseudo vector potential (PVP) barriers in graphene are investigated. In graphene, the pseudo vector potential may be caused by a local strain. The comparison of supercurrents induced by the two type-barriers is focused. As a result, we find that not only will the RVP induce a transition Josephson current from the 0 → π state but also causes the difference in the phases of the order parameters of the two superconducting graphene layers to shift from φ → φ. The critical current is PVP-independent around the neutrality point while it strongly depends on the RVP. The vector potential dependence of critical current is found to be perfectly linear for both PVP and RVP barriers.

The effect of bias current asymmetry on the flux-flow steps in the grain boundary YBaCuO long Josephson junctions

The current-voltage characteristics and critical current versus magnetic field dependence of long 24°[001]-tilt YBa2Cu3O7−δ bicrystal grain-boundary junctions are studied both experimentally and theoretically. For the opposite magnetic field directions, the flux-flow steps with significantly different height and slope are observed. It is demonstrated that the most probable reason of this discrepancy is recently predicted asymmetry of spatial bias current distribution due to crystallographic anisotropy of bicrystal substrates [Kupriyanov et al., JETP Lett. 95, 289 (2012)].

Long-range triplet Josephson current modulated by the interface magnetization texture

We investigate the Josephson coupling between two s-wave superconductors separated by the ferromagnetic trilayers with noncollinear magnetization. We find that the dependence of long-range triplet critical current on the strength of exchange field and thickness of the interface layers shows an oscillatory behavior, when the interface magnetizations are orthogonal to the central magnetization. This feature of the critical current is induced by the spatial oscillations of the triplet state in the interface layer. In addition, when the misalignment angle between interface magnetization and central ferromagnet increases from 0 to , the critical current exhibits a nonmonotonic behavior under the condition that the original state of the junction with parallel magnetizations is the π-state.

Maximum Entropy Distributions Describing Critical Currents in Superconductors

Maximum entropy inference can be used to find equations for the critical currents (Jc) in a type II superconductor as a function of temperature, applied magnetic field, and angle of the applied field, θ or φ . This approach provides an understanding of how the macroscopic critical currents arise from averaging over different sources of vortex pinning. The dependence of critical currents on temperature and magnetic field can be derived with logarithmic constraints and accord with expressions which have been widely used with empirical justification since the first development of technical superconductors. In this paper we provide a physical interpretation of the constraints leading to the distributions for Jc(T) and Jc(B), and discuss the implications for experimental data analysis. We expand the maximum entropy analysis of angular Jc data to encompass samples which have correlated defects at arbitrary angles to the crystal axes giving both symmetric and asymmetric peaks and samples which show vortex channeling behavior. The distributions for angular data are derived using combinations of first, second or fourth order constraints on cot θ or cot φ . We discuss why these distributions apply whether or not correlated defects are aligned with the crystal axes and thereby provide a unified description of critical currents in superconductors. For J//B we discuss what the maximum entropy equations imply about the vortex geometry.

Mechanical and electro-mechanical analysis in differently stabilized GdBCO coated conductor tapes with stainless steel substrate

The understanding of the strain dependence of critical current, <TEX>$I_c$</TEX>, in the reversible region is important for the evaluation of the performance of coated conductor (CC) tapes in practical applications. In this study, the stress/strain tolerance of <TEX>$I_c$</TEX> in GdBCO CC tapes with stainless steel substrate stabilized by additional Cu and brass laminate was analyzed quantitatively through <TEX>$I_c$</TEX>-strain measurement at 77 K under self-field. The variation in irreversible strain limits of CC tapes by the addition of stabilizing layers was analyzed through the consideration of the pre-strain induced on the GdBCO coating film. The results were then compared with the ones previously reported for GdBCO CC tapes with Hastelloy substrate. As a result, GdBCO CC tapes with stainless steel substrate showed much higher strain tolerance of <TEX>$I_c$</TEX> as compared with those adopting Hastelloy substrate.

Mo/Au Bilayer Superconducting Transition Edge Sensor Tuning With Surface Modification Structures

We have conducted an experimental study tuning the critical temperature, transition width, normal resistance, and critical current of Mo/Au bilayer transition edge sensors (TES) using Nb stripes on the TES surface. The Nb stripes modify the TES parameters through the lateral proximity effect. We summarize the dependence of the shift in transition temperature and the broadening of the transition width on the separation length between the Nb stripes and leads. We calculate the Ginzburg-Landau coherence length of the TES in two methods. One takes advantage of the dependence of the TES normal resistance on the number of the TES and Nb contacts. Another utilizes the temperature dependence of the TES critical current. The two methods are in good agreement with a coherence length ξ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ≈ 0.4 μm.

Critical Current Density and Temperature Dependence of Nb-Al Oxide-Nb Junction Resistance and Implications for Room Temperature Characterization

Room temperature junction resistance measurements are commonly used for screening Josephson-based circuits because testing is much easier than at cryogenic temperatures and can even be carried out at the wafer level. The value of ambient testing depends on the existence of a strong correspondence between the measured resistance at room temperature and the resistance and critical current obtained at the ultimate operating temperature. We have systematically studied the temperature dependence of junction resistance in order to quantify the emergence, with increasing critical current density, of parasitic contributions from non-uniform currents flowing in the Nb films, which tend to limit the value of room temperature screening. We will describe our measurements and our approach to correcting for these parasitic effects.

Quantum phase slips in Josephson junction rings

We study quantum phase-slip (QPS) processes in a superconducting ring containing $N$ Josephson junctions and threaded by an external static magnetic flux ${\ensuremath{\Phi}}_{B}$. In such a system, a QPS consists of a quantum tunneling event connecting two distinct classical states of the phases with different persistent currents [Matveev et al., Phys. Rev. Lett. 89, 096802 (2002)]. When the Josephson coupling energy ${E}_{J}$ of the junctions is larger than the charging energy ${E}_{C}={e}^{2}/2C$, where $C$ is the junction capacitance, the quantum amplitude for the QPS process is exponentially small in the ratio ${E}_{J}/{E}_{C}$. At given magnetic flux, each QPS can be described as the tunneling of the phase difference of a single junction of almost $2\ensuremath{\pi}$, accompanied by a small harmonic displacement of the phase difference of the other $N\ensuremath{-}1$ junctions. As a consequence, the total QPS amplitude ${\ensuremath{\nu}}_{\mathrm{ring}}$ is a global property of the ring. Here, we study the dependence of ${\ensuremath{\nu}}_{\mathrm{ring}}$ on the ring size $N$, taking into account the effect of a finite capacitance ${C}_{0}$ to ground, which leads to the appearance of low-frequency dispersive modes. Josephson and charging effects compete and lead to a nonmonotonic dependence of the ring's critical current on $N$. For $N\ensuremath{\rightarrow}\ensuremath{\infty}$, the system converges either towards a superconducting or an insulating state, depending on the ratio between the charging energy ${E}_{0}={e}^{2}/2{C}_{0}$ and the Josephson coupling energy ${E}_{J}$.

Penetration depth of MgB2 measured using Josephson junctions and SQUIDs

The penetration depth of MgB2 was measured using two methods of different mechanisms. The first method used MgB2 Josephson junctions and the magnetic field dependence of the junction critical current. The second method deduced the penetration depth from the inductance of a MgB2 microstrip used to modulate the voltage of a MgB2 DC SQUID. The two methods showed a consistent value of the low-temperature penetration depth for MgB2 to be about 40 nm. Both the small penetration depth value and its temperature dependence are in agreement with a microscopic theory for MgB2 in the clean limit.

Effect of Subgrain Structure on the Critical Current Density in HTS Films

We have suggested a model of film–substrate interface and calculated its energy. The calculations have shown that rotational relaxation occurs due to the finite size of clusters and to the non-equilibrium effect of the film growth. We have predicted the real subgrain size and the expected rotation of domains. In some kinds of high temperature superconducting films, such as the mostly used YBCO, domains can be subdivided into twin domains. The twin structure has smaller scale since it is being formed at tetra-ortho transition during cooling and/or oxygen annealing when other defects are already formed in tetra-phase during deposition. Usually, twin boundaries have no strong influence on the critical current because they introduce almost no perturbation in the superconducting order parameter. We have shown that when a strain is applied, a difference between critical temperatures of a- and b-oriented twin domains appears and results in an additional pinning and unusual reversible strain dependence of critical current. The suggested model explains experimental strain dependences of the critical current.

Tensile strain dependence of critical current of RHQ-Nb3Al wires

Nb3Al superconducting wires produced by rapid heating and quenching (RHQ) have been developed for high-field accelerator magnets. It is known that critical currents of A15 superconductors (e.g., Nb3Al and Nb3Sn) have a dependence on stress/strain. It is thus important to clarify the stress/strain behavior of the RHQ-Nb3Al wires for the development of high-field accelerator magnets. We recently started experimentally investigating the strain dependence of the critical current of the RHQ-Nb3Al wires with a Ta interfilament by measuring their critical currents under longitudinal tensile strains. To evaluate the effect of residual strain in Nb3Al filaments induced by thermal contraction of the materials in the wire, neutron diffraction measurements were performed at room temperature.

Geometry-related magnetic interference patterns in longSNSJosephson junctions

We have measured the critical current dependence on the magnetic flux of two long $SNS$ junctions differing by the normal wire geometry. The samples are made by a Au wire connected to W contacts, via focused ion beam assisted deposition. We could tune the magnetic pattern from the monotonic Gaussian-like decay of a quasi-one-dimensional (1D) normal wire to the Fraunhofer-like pattern of a square normal wire. We explain the monotonic limit with a semiclassical 1D model, and we fit both field dependencies with numerical simulations of the two-dimensional Usadel equations. Furthermore, we observe both integer and fractional Shapiro steps. The magnetic flux dependence of the integer steps reproduces as expected that of the critical current ${I}_{c}$, while fractional steps decay slower with the flux than ${I}_{c}$.

Critical current of a granular d-wave superconductor

The twist angle γ dependence of the Josephson critical current of d-wave superconductors in one junction and a granular system is considered. Our results show that the twist angle γ dependence of the d-wave Josephson critical current is the same for one junction and a granular system. The magnetic field dependences of the critical-current of a granular d-wave superconductor has also been determined by considering the rectangular and circular junction model of an array of small superconducting particles which interacting by Josephson coupling through insulating barriers. We will show that in the case of circular model, the critical current of the Josephson current is larger than that of rectangular one.

Extensive Characterization of the 1 mm PIT &lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;${\rm Nb}_{3}{\rm Sn}$&lt;/tex&gt;&lt;/formula&gt; Strand for the 13-T FRESCA2 Magnet

In the framework of the EuCARD program, CERN is participating in the development of a 13 T 100-mm-aperture dipole magnet to upgrade the superconducting cable test facility FRESCA at CERN. The conductor candidates for building this magnet are two 1-mm Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands: the Powder In Tube (PIT) produced by Bruker-EAS and the 132/169 RRP by Oxford Superconducting Technology (OST). Recently the PIT strand has been extensively characterized by CERN in collaboration with the University of Geneva (UniGe). The critical current dependence on the magnetic field and on the axial strain has been measured at different temperatures. Furthermore, the strand magnetization has been measured at different temperature using a vibrating sample magnetometer. Finally the magneto-thermal stability of this strand was studied by measuring the quench current between 0 T and 12 T at 1.9 K and 4.3 K. The experimental results are compared with an optimized scaling law for the critical current of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands. In this paper the results obtained for the PIT strand are summarized and discussed.

Transport critical current dependence on magnetic fields in ceramic high temperature superconducting samples

The expression for the Josephson current flowing through the rectangular junctions was averaged over the values determining its properties in the case when magnetic field is less then first critical magnetic field of superconducting grains. The averaging results permit to explain the earlier performed experimental investigations of the scaling properties of critical state in random 3-d Josephson net created by transport current. At low magnetic field the average Josephson current is a quadratic function of the magnetic field and at high field the average current is an inverse quadratic function.

Strong vortex pinning in FeSe0.5Te0.5 epitaxial thin film

We report on the magnetic field and angular dependence of the critical current density of epitaxial FeTe0.5Se0.5 thin films. The films exhibit high critical current values and weak dependence on the applied magnetic field. The Jc is larger for field parallel to the c-axis, which is the opposite behavior of what expected from the critical field anisotropy. The analysis of the activation energy for vortex motion indicates that the single pinning regime holds up to 9 T, suggesting that correlated pinning centers are more effective than the vortex-vortex interaction even at the largest applied fields. Scanning tunneling microscope analysis indicates threading dislocations as possible pinning centers.

Critical current behaviour of YBCO thin films described by vortex pinning on low-angle domain boundaries and vortex creep

A pinning potential for vortices was introduced assuming plastic pinning of vortex lattice on chains of out-of-plane individual edge dislocations that form low-angle domain boundaries in high quality YBa2Cu3O7 thin films. Using this pinning potential with the classical Kim–Anderson vortex creep approach a model for critical current dependence on field has been successfully developed. The model shows a plausible description of Jc(Ba) over the entire field range. Electrical field criterion is incorporated providing the ability to compare measurements made with different criteria (including different measurement techniques). Applicability of this model has been verified by experimental data obtained by direct transport and magnetisation measurements on high quality films grown by pulsed laser deposition. Pinning potential obtained from the fitting procedure is consistent with theoretical predictions. The model showed that the effective pinning landscape changes under influence of external conditions.

Passive Superconducting Flux Conservers for Rotating-Magnetic-Field-Driven Field-Reversed Configurations

The Princeton Field-Reversed Configuration (PFRC) experiment employs an odd-parity rotating-magnetic-field (RMFo) current drive and plasma heating system to form and sustain high-β plasmas. For radial confinement, an array of coaxial, internal, passive flux conserver (FC) rings applies magnetic pressure to the plasma while still allowing radio-frequency RMFo from external coils to reach the plasma. The 3-ms pulse duration of the present experiment is limited by the skin time τfc of its room-temperature copper FC rings. To explore plasma phenomena with longer characteristic times, the pulse duration of the next-generation PFRC-2 device will exceed 100 ms, necessitating FC rings with τfc > 300 ms. In this paper we review the physics of internal, discrete, passive FCs and describe the evolution of the PFRC's FC array. We then detail new experiments that have produced higher-performance FC rings that contain embedded high-temperature superconducting (HTS) tapes. Several HTS tape winding configurations have been studied, and a wide range of extended skin times, from 0.4 s to >103 s, has been achieved. The new FC rings must carry up to 2.5 kA of current to balance the expected PFRC-2 plasma pressure, so the dependence of the HTS-FC critical current on the winding configuration and operating temperature has also been studied. From these experiments, the key HTS-FC design considerations have been identified and HTS-FC rings with the desired performance characteristics have been produced.