Magnetization of high-temperature superconductors (abstract)

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Superconducting Ba (X=Y, Nd, Sm, Gd, Dy, Er, Yb) CuO were made independently by the solid reaction method via mixing barium carbonate, (X) oxides, and copper oxide with atomic ion ratio Ba:X:Cu=R:1:R+1 with R=1−4, and sintering at 900–1000 °C for 24–72 h. (1) The best superconductor was found to be formed via R=2 and sintering near 950 °C. All samples studied were found to have layered perovskite x-ray diffraction patterns. Samples via R=1,3, and 4 were also found to have structures similar to those via R=2, and found to have two phases by the x-ray diffraction. The temperature-dependent magnetizations M(T) were measured by using the BTi SQUID susceptometer. The transition temperature Tc was determined by extrapolating M(T) and was found to be sensitive to the cooling rate of the sample during its synthesis. The Meissner effect was observed up to 78–92 K, and the fluxoids were found to be trapped while the samples cooled in a finite field at all temperatures below Tc. M(T) was found to be of three types: type 1 being similar to that of Pb, type 2 having a knee perhaps due to the two crystal structures, and type 3 having zero slope near Tc, perhaps due to the sample crystal sizes being less than the effective penetration depth. Samples with large crystal sizes were found to be of type 1 or 2. The measured M(T) can be understood in the framework of the BCS theory, and 2Δ(0)/k Tc were deduced to be in the range of 3–4.4. However, the magnetic hysteresis was found to be present at even low field 30 G at 10 K, and M(T) near Tc was found to be sensitive to thermal fluctuations. Various mechanisms for superconductivity in these systems will also be discussed.