New High T_C Superconducting Single Crystal Prepared By The Solvent Floating Zone Technique

It is well known that the hole concentration plays an important role in the normal and superconducting properties of high-Tc cuprate oxides. Thus, it is an important step of understanding the relationship between the superconductivity and the carrier concentration to elucidate the superconducting mechanism in the high-Tc cuprate systems. A number of studies have revealed the correlation between superconducting transition temperature (Tc) and oxygen content from the point of view of hole concentration in the two-dimensional Cu-O planes. In the Bi-2212 system, it was found that Tc could be varied over a large range by adjusting the carrier concentration. For the Bi-2212 system, controlling over carrier concentration could be achieved by the substituting cations on lattice sites and annealing samples in oxygen partial pressure atmosphere. The carrier for the Bi system are holes and the hole concentration in the CuO2 planes determines the critical temperature Tc of crystals, but the hole concentration depends on the oxygen content since it is assumed that an additional oxygen atom creates two holes.Numerous studies have reported that the oxygen content and distribution of Bi-2212 crystals have very important effect on the superconducting properties of them. In this paper, Bi-2212 single crystal have been grown by the solvent floating zone method (TSFZ) and studied the effects of annealing on the superconducting properties of Bi-2212 single crystals. TSFZ is an effective method to prepare high quality and large single crystals of Bi-2212 and then the single crystals cut from the as grown rods are used in the annealing experiments. In order to more fully study the effect of annealing on the structure and superconducting properties of Bi-2212 single crystals, the annealing experiments were carried out respectively under the different temperature and the different oxygen partial pressure.Firstly, annealing in air was carried out on crystals under different temperature ranging from 300? to 600?.The as-grown and post-annealed crystals were explored by SEM, XRD and resistivity and magnetization measurements. The results suggest that the annealing temperature have effect on the oxygen content and affect Tc of samples. XRD studies indicate that the crystals have orthorhombic symmetry. The refinement results for XRD date shows that c-axis contracts and Tc decreases with increasing annealing temperature.Secondly, annealing at 450? was carried out on crystals under different oxygen pressure, including 10 bar and 100 bar. The as-grown and post-annealed crystals were explored by SEM, XRD and resistivity and magnetization measurements. It was demonstrated that the over-doped crystals were achieved and the superconducting temperature Tc was found to affected strongly and and decreases with increasing annealing oxygen partial pressure.The refinement results for XRD date shows that c-axis contracts with increasing annealing toxygen partial pressure.The relation between Tc and c-axis lattice parameter suggests that the change of Tc is correlated with redistribution of holes between the Bi-0 layers and Cu-0 planes. The temperature dependence of the resistivity at different applied magnetic fields (parallel and perpendicular)for the crystals annealed under high oxygen pressure,and then the upper critical fields (?Hc2) are deduced from the figures.The results indicate the anisotropic character of Bi-2212 single crystals.The anisotropic character ?=?Hc2? (0)/?Hc2//(0) of samples annealed under different oxygen pressure is deduced, showing that ? decreases with increasing oxygen pressure.