Critical Current Reduction Research Articles

Controlled reduction of critical current densities in YBCO thin films

The reduction of critical currents in HTS films has proven to be of primary importance for the fabrication of devices sensitive to small applied magnetic fields ( >

RF coupling to single crystal BSCCO under c-axis bias

We have explored rf coupling to BSCCO single crystals with c-axis bias currents over a wide range of frequencies. A structure was developed that afforded the necessary impedance transformations at several frequencies from dc to 35 GHz. At low frequencies, connections were made to a coaxial cable. At X-band, the sample was placed in front of a short section of waveguide, forming a resonator. Above 12 GHz, the sample mount formed a coplanar resonator. At 35 GHz it was used as the end plate of a cylindrical cavity with degenerate mode spoilers and a loaded Q-factor of 3800 at 38.3 GHz. Fabricated BSCCO samples showed evidence of hysteretic Josephson junction stack behavior at 77 K. Application of low frequency rf signals caused steps to appear in the sample I-V. At frequencies above 6.4 GHz only a reduction of critical current was observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The influence of compressive and tensile axial strain on the critical properties of Nb/sub 3/Sn conductors

Various Nb/sub 3/Sn conductors are investigated in an axial strain experiment. The superconducting samples are soldered to a substrate that is bent to generate a compressive or tensile axial strain. Especially in the compressive strain range the critical current reduction is smaller than predicted by the well-known scaling law. The influence of the transverse strain components is investigated in a tape conductor by changing the thermal strain. It is found that the axial strain experiments can be described by an upper-critical field that depends on the deviatoric component of the strain tensor only. Finally a comparison of the critical temperature and the critical current as a function of axial strain is made. >

Calculation of the critical current reduction in a brittle wound multifilamentary wire due to external forces

A simple model is presented that can describe the electro-mechanical state of a multifilamentary wire. An elastic cylinder model is used to derive the strain state analytically. Axial and transverse forces came a position dependent critical current density in the wire. The integral critical current of such a wire is calculated. The effect of two different parameters, the hydrostatic and the deviatoric strain, on the critical current is compared. The critical current reduction of a model wire due to various external loads in a Nb/sub 3/Sn wire is analysed. Finally the position of the superconducting filaments is considered. >

Enhancement of critical current density and reduction of proximity coupling in fine filamentary NbTi with Si and Mn alloyed Cu matrices

Critical current density J/sub c/ and magnetization data are given on fine filamentary NbTi materials with a series of different copper alloy matrices containing Si, Mn, or both. By applying the correct thermomechanical treatment, materials of this type can give high J/sub c/'s with minimal electrical coupling. The use of Si has a particular advantage in that it greatly reduces intermetallic compound formation and may, therefore, obviate the need for a barrier around the filaments under certain conditions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Critical current degradation in Nb/sub 3/Sn cables under transverse pressure

The critical current degradation of a few sample Rutherford-type Nb/sub 3/Sn cables is investigated as a function of transverse pressure. A comparison is made between Nb/sub 3/Sn strands produced by the powder-in-tube, bronze, and modified jelly roll processes. The (keystoned) Rutherford cables are charged at 11 T under transverse pressures up to 250 MPa. Large differences in critical current reduction are observed, ranging from 6 to about 60% at 200 MPa, depending on the type of Nb/sub 3/Sn. It appears that about 40% of the total reduction is irreversible. Moreover, the irreversible part shows relaxation, and a partial recovery is possible by thermal cycling.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The critical current in a NbTi tape measured in different directions of magnetic field and the current reduction due to the self field

With reference to the application of NbTi tape in a superconducting thermal switch, the critical current of a 20- mu m-thick NbTi tape was measured in several directions of the magnetic field. The critical current was found to behave strongly anisotropically, due to the deformation of the NbTi. The tape is extrasensitive to the component of the magnetic field perpendicular to the surface. Without an external field this component of the self-field reduces the critical current far below its intrinsic value. A one-dimensional model can describe the reduction of critical current due to the self-field in a thin tape. >

Critical current density across 45° misoriented grain boundaries in YBCO thin films

The question of critical current reduction across 45° misoriented grain boundaries was addressed in this study. The reduction in J c for a polycrystalline c-axis perpendicular film with complete and partial in plane a-axis texturing was calculated using a limiting-pathe model. The numerical results were then compared to experimental results obtained with YBCO on YSZ substrates, which had a particular texturing that is most suited for this study. The results indicated that the J c reduction was not so large as indicated in the bicrystal studies of Dimos et al.

An Analysis of Heat Transfer in Josephson Junction Devices

Josephson junctions are electronic devices made from superconducting materials that cycle between resistive and nonresistive states. Heat generated in the resistive state causes a temperature rise, which may adversely affect electrical behavior, by reducing the critical Josephson current. In this work, temperature distributions and resulting reductions in critical current are calculated for Josephson junctions made from low and high-temperature superconductors. It is found that an unacceptable reduction in critical current may occur for junctions made from high-temperature materials. This problem can almost certainly be overcome, but perhaps at the expense of one advantage of Josephson junctions, namely compactness.

Effect of twin-boundary pinning on flux-lattice melting in YBa 2Cu 3O 7−δ

The observed reduction of critical currents with Fe doping in YBa 2 Cu 3 O 7− δ can be explained by the reduction of the flux lattice melting temperature with doping. A quantitative comparison to theory suggests that the flux lattice interacts predominantly with twin boundaries, and that the flux lattice correlation length is approximately ten times the average twin spacing.

Temperature and magnetic field dependence of transport critical current density in YBa 2Cu 3O 7 − δ ceramics

A systematic experimental study of transport critical current density of sintered high T c ceramics was performed. A new contactless method was developed to measure the transport critical current in magnetic fields up to 6 T at temperatures ranging from 5 K to T c. Reduction of critical currents due to trapped magnetic flux in the sample and the critical exponent characterizing the transition of the granular system to the coherent (zero voltage) state were determined from the measurements.

Controllable reduction of critical currents in YBa2Cu3O7−δ films

The critical currents in high quality thin films of the high Tc superconductor, YBa2Cu3O7−δ, can be controllably reduced by orders of magnitude using ion irradiation. This reduction in critical current occurs without substantial decrease in Tc or increase in room-temperature resistivity. Using this technique, we have fabricated weak links that exhibit an ac Josephson effect at 77 K.