Spin correlations in amorphous Bi2Sr2CaCu2Ox

Zero and longitudinal field SR has been used to probe the magnetic state of amorphous Bi₂Sr₂CaCu₂O_x prepared by rapid quenching from the melt. The results suggest quasi-static spin correlations between extremely dilute Cu²⁺ ions.Work performed while at the J J Thomson Physical Laboratory, University of Reading.     

Mechanism of Bi2Sr2CaCu2Ox crystallization and superconducting properties for Bi2Sr2CaCu2Ox/Ag tapes prepared using isothermal partial melting method

The mechanism of Bi₂Sr₂CaCu₂O_x (Bi-2212) crystallization from a partially molten state has been studied by means of thermal analysis, X-ray diffraction (XRD) measurement, and microstructural observation in Bi-2212/Ag tapes. Tapes have been prepared using the isothermal partial melting (IPM) method, under which partial melting and solidification are carried out at the same temperature in an nitrogen atmosphere and 20% oxygen partial pressure (p(O₂)), respectively. The Bi-2212 phase crystallizes by peritectic reaction between (Sr, Ca)CuO₂ (1:1 phase) and the liquid phase at processing temperatures (T_p)≥855°C. At the beginning of Bi-2212 crystallization, only the 1:1 and the liquid phases are present in tapes. The Bi-2212 nucleation occurs in the vicinity of 1:1 grains (homogeneous nucleation). This peritectic reaction progresses by recovery of oxygen in the liquid phase. In contrast, the nucleation of the Bi-2212 phase occurs heterogeneously at T_p≤850°C. The Bi-2212 phase co-exists with four Bi-free phases ((Sr, Ca)₁₄Cu₂₄O₄₁ (14:24 phase), (Sr, Ca)₂CuO₃ (2:1 phase), (Sr, Ca)O (1:0 phase), and the 1:1 phase) and the liquid phase in the early stage of the Bi-2212 crystallization step. Since the precipitation temperature of the Bi-2212 phase is higher than 850°C, the Bi-2212 crystallization progresses by direct precipitation. The crystallization paths affect microstructure and transport critical current density (J_c) of Bi-2212 tapes.     

Dynamic mixed state in micron-size bridges based on Bi2Sr2CaCu2Ox whiskers

Measurements of the current-voltage characteristics of micron-size bridges made of Bi₂Sr₂CaCu₂O_x single-crystal whiskers are carried out. It is found that, at temperatures below the superconducting transition temperature, the current-voltage characteristics exhibit quasi-periodic voltage jumps with segments of constant differential resistance whose value is proportional to the jump number. For the narrowest bridges (0.5–1 ¼m), up to ten voltage jumps are observed. The result of the experiment is explained by the formation of vortex lines under the current effect.     

Visualization of magnetic flux penetration and trapping in superconducting Bi2Sr2CaCu2Ox single crystals

Magnetic flux profiles in superconducting Bi₂Sr₂CaCu₂O_x single crystals have been observed using a magnetic garnet film as a magnetic field sensor. Measurements have been performed for the “zero-field-cooled” and “field-cooled” regimes as a function of temperature and magnetic field. An analysis of the observed patterns has revealed some peculiarities in the flux trapping effects. In particular, a nonuniform magnetic structure has been directly observed in the remanent state after “field-cooled” processes.