M. Tange

Superconducting properties of Bi-2212 whiskers

Abstract Bi 2 Sr 2 CaCu 2 O y whiskers were grown from pellets with the stoichiometric composition of single crystals by an annealing process. The whiskers show single step resistive transition and the temperature at zero resistance is about 80 K for the observed whiskers. The flux-flow resistance was measured in magnetic field perpendicular to the surface of the whisker and exhibited a profile similar to those observed in single crystals. The irreversibility lines of the whiskers are estimated and show an improved profile compared with the single crystal’s one. Thus the quality of the whiskers is as good as bulk single crystals.

Ca substitution effect for Sr upon superconductivity of Bi2.1CaySr1.9- yCuO6+δ

Physical properties of a compound expressed by Bi2.1CaySr1.9-yCuO6+δ are investigated by means of powder X-ray diffraction and magnetic measurement for as-sintered samples with varying Ca content y. Main crystal structure is found to be Bi-2201 phase as y is increased from 0.2 to 1.9. Bulk superconductivity is obtained from magnetic measurement. The critical temperature (Tc) is almost unchanged for the samples with various y in the range of 74–80 K, whereas the magnitude of diamagnetism is changed with roughly proportional to y in the range of 0<y≤0.95 and with abrupt decrease in 0.95<y≤1.8. The results suggest a cluster model about the role of Ca/Sr mixture on the superconductivity in Bi-2201 phase.

Annealing Effect of 80 K-Class Superconductivity of Ca-Doped B2S2CuO6+δ in Bi-2201 Phase

Annealing effect on magnetic properties of superconductivity is investigated for the crystals of Bi1.9CaySr1.9-yCuO6+δ (0≤y≤1.8) in Bi-2201 phase. It is found that carrier density is reduced with Ca doping in Ca-poor region of y≤0.8. In contrast, the excess oxygen δ is saturated with doping at about the optimal Tc in Ca-rich region of y>0.8. The results indicate possible oxygen deficiencies with Ca doping, which gives a new aspect to a model for the appearance of 80 K-class superconductivity.

Effect of Strip Width on Low-Field Peak Effect of Critical Current Density in Bi2Sr2CaCu2O8+δ Superconducting Strips

We investigate the dependence of the critical current density J c along the crystal a -axis on the magnetic field along the c -axis at high temperatures below the superconducting transition temperature in individual Bi 2 Sr 2 CaCu 2 O 8+δ (Bi-2212) whiskers with surface barriers by electrical transport measurements. J c shows a low-field peak effect; J c increases with the magnetic field in a low-field region and shows a peak at the threshold field H peak below 100 Oe. A comparison of the low-field peak effects in whiskers of different widths shows that H peak depends on the sample width (i.e., the lateral dimension); the lateral axis is perpendicular to the magnetic field and the direction of current flow. A decrease in the sample width w shifts H peak toward a higher field. This systematic variation in H peak with w implies that the low-field peak effect observed in the Bi-2212 whiskers is associated with the effect of the finite w on dilute vortices.

Optimization Of Pb And La Co‐Doped Bi2Sr2CuO6+δ Superconductors

We obtained zero‐resistance Tc of 43.8 K for the single crystals of Pb and La co‐doped Bi2Sr2CuO6+δ. The superconductivity with the onset of about 75 K was observed, and the annealing effect was investigated.

Enhancement Of Critical Current Density Under Low Magnetic Fields Around 70 K In Bi2Sr2CaCu2Oy Superconducting Whiskers

Electrical transport measurements for Bi2Sr2CaCu2Oy whiskers with a single‐step superconducting transition were carried out at high temperature ranging from 70 to 77 K below the superconducting transition temperature (= 81.5–82.5 K) to investigate critical current density (Jc) under magnetic fields along the crystal c axis. The Jc parallel to the crystal a axis, which was estimated from the V‐I curves, enhances under several ten oersteds, and then decreases with increasing the magnetic field. This peak effect in the low‐field regime at high temperature has temperature dependence in contrast to the case of matching effects.