Liangbin Wang

Phase decomposition and superconductivity in Bi2Sr2CaCu2Oy single crystals

The phase decomposition in high quality Bi2Sr2CaCu2Oy(2212) single crystals annealed in air at 400–750 °C is studied by x‐ray diffraction and scanning electron microscopy. It is found that the Bi2Sr2CaCu2Oy phase of the as‐grown single crystals readily decomposes even at temperatures as low as 450 °C. With the annealing temperature increasing, phases of bismuth oxides, strontium bismuth oxide, and the Bi2Sr2CuOx(2201) are segregated successively from the air annealed single crystals and are found to be temperature dependent. Combining with the enhancement of the superconducting onset temperatures and the very narrowness of the diamagnetic transitions observed for the annealed crystals, it is suggested that the structural change indicated by the surface phase decomposition may play an important role in the variations of superconductivity in Bi2Sr2CaCu2Oy single crystals.

(Hg, Mo)Sr2Y1−xCaxCu2O6+δ. A new series of mercury-based high-Tc superconductor

Abstract A new series of Hg-based superconductor Hg0.8Mo0.2Sr2Y1−xCaxCu2O6+δ with a maximum Tconset of 98 K (at x = 0.65) as determined by electrical resistance measurements has been synthesized for the composition range 0.4 ⩽ x ⩽ 0.65. X-ray diffraction and electron diffraction both give the tetragonal structure of 1212 cuprate with lattice parameters a = 3.825, c = 11.910 A for x = 0.5 samples and a space group of P4/mmm. Energy dispersive X-ray spectrometry analysis confirms that Hg and Mo are incorporated into the structure.

Oxygen loss effect under both reducing and oxidizing thermal treatments on superconductivity of single‐crystal Bi2Sr2CaCu2O8+y

After annealing at 450–550 °C, in either flowing nitrogen or flowing oxygen atmosphere, the Bi2Sr2CaCu2O8+y single crystals with as‐grown superconducting transition temperature Tc of 65–83 K all showed at Tc of ∼86 K with a transition width of 0.5–1 K. For the crystals annealed at 500 °C and in increasing oxygen pressure, however, the improvement of superconductivity was depressed. These results can be explained by oxygen loss under both reducing and oxidizing thermal treatments of the oxygen‐doped Bi2Sr2CaCu2O8+y samples.

Time effect of annealing on Bi2Sr2CaCu2Oy single crystals

Abstract Highly-oriented as-grown Bi 2 Sr 2 CaCu 2 O y single crystals have been air-annealed at 500 or 750°C for different times. The thermal treatments improved the superconductivity of the crystals relative to the as-grown T c and ΔT c distinctly in several minutes. The results have been explained by oxygen configuration and it is suggested that oxygen out-diffusion in the crystals is rather rapid at the beginning of the annealing.

TWO DIFFERENT MICROMODELS OF ZINC OXIDE VARISTORS

Micromorphology of ZnO varistors made with seed grains shows that there are two kinds of combinations of ZnO grains. Two different micromodels of ZnO varistors are put forward according to their micromorphology. The relation between ZnO grains’ mean diameter and the amount of seed grains has been deduced from the micromodels. The relation between the electric field E1mA/cm2, nonlinear coefficient, and breakdown energy density of ZnO varistors and the amount of ZnO seed grains has been explained by means of micromodels.

Effect of annealing on the structure and diamagnetic transition width of high‐quality crystal Bi2Sr2CaCu2Oy studied by x‐ray‐diffraction and ac susceptibility measurements

A large and highly oriented Bi2Sr2CaCu2Oy crystal with only a single domain was progressively air annealed at 500 and 750 °C for 4 and 8 h. Measurements of ac susceptibility showed that Tc (onset) and ΔTc (10%–90% of the complete transition) of the crystal were 74 and 3 K (as grown), 85 and 0.6 K (annealed at 500 °C for 4 h), and 89 and 2.5 K (annealed at 750 °C for 8 h), respectively. To explain this typical change of ΔTc with annealing temperature, a full structural characterization by x‐ray diffraction in the b*c* plane and by Laue photographs of the crystal as grown and after each annealing treatment was carried out. Apart from variations of the lattice parameters, it is noted that the smaller the ΔTc, the more highly oriented and structurally homogeneous the crystal. We explain the annealing effect on ΔTc by variations in crystallinity, structural homogeneity, and surface phase decomposition, and the changes in oxygen stoichiometry caused by the annealing process.

Distribution of the satellite reflections of the modulation and its thermal stability in single‐crystal Bi2Sr2CaCu2Oy

The satellite distribution around the main reflections of the incommensurate modulation in Bi2Sr2CaCu2Oy has been studied by means of x‐ray diffraction on a number of single crystals. The presence of the forbidden reflections which are incompatible with a B‐centered Bravais lattice is found to be correlated with the spread of the mosaic blocks in the crystal sheet and therefore spurious to the distribution of the satellite reflections in the reciprocal space. Diffraction data collected for a highly oriented crystal annealed in flowing nitrogen at 650 °C for 60 h have shown that the structural modulation is coherent with the Bi2Sr2CaCu2Oy phase and robust to the anneal. Relationships between superconductivity, structural modulation, and oxygen content in the compound are discussed.

Degradation of superconductivity in annealed irradiated Bi2Sr2CaCu2O8+δ single crystals during exposure to air

As for irradiated Bi2Sr2CaCu2O8+δ single crystals annealed in flowing N2 atmosphere, flowing O2 atmosphere, and air, their superconductivity was degraded during exposure to air. The degradation of superconducting transition temperature Tc is attributed to the doping of oxygen. The small recovery of Tc may be related to the redistribution of oxygen. The superconductivity of samples annealed in N2 atmosphere and air is more stable than that of the sample annealed in O2 atmosphere during exposure to air.

Anisotropic resistivity and Cu2+ localized spin moment in Bi2Sr2CaCu2O8+x crystals

Abstract The oxygen content effects on the anisotropic resistivity and Cu 2+ localized spin moment have been studied for Bi 2 Sr 2 CaCu 2 O 8+x single crystals. It was found that the in-plane and out-of-plane resistivity ( ϱ ab and ϱ c ) as well as the electron spin resonance (ESR) signals decrease with increasing oxygen contents from underdoping to overdoping states. We believe that ϱ ab upturn in an underdoped region is closely related to the localization of Cu 2+ spins. On the origin of ϱ c , we should take into account the scattering in the “barrier” layer between CuO 2 “cells” and scattering or fluctuations in the planes.

Phase decomposition of single crystals annealed in vacuum

The phase decomposition in high-quality (Bi2212) single crystals annealed in vacuum at temperatures 350 - 650 is studied by x-ray diffraction. It is found that the phase of the as-grown single crystals readily decomposes even at temperature as low at 400 in vacuum. With the annealing temperature increasing, is the first phase segregated from the vacuum-annealed single crystals, followed by phases of , oxide and (Bi2201) dependent on temperature. All the segregated phases are highly textured, as observed by using x-ray -scan rocking curves. The phase decomposition occurred at lower temperature and was more dramatic and complicated in vacuum-annealed single crystals than that in air-annealed single crystals reported previously and a possible reason for this is discussed.