Phase diagram, crystal growth and superconductivity of CaLaBaCu3O7-y crystal

CaLaBaCu3O7-y (CLBCO) is a high T-c superconductor with unique tetragonal structure and cross-cationic substitution sites. In this paper, we present our study on the BaO-CuO-CLBCO phase diagram and its application to the crystal growth of CLBCO. The eutectic composition of BaO-CuO system is confirmed by the improved thermogravimetric analysis (ITGA) and is used for the crystal growth as a self-flux system. A pseudo-phase diagram of CLBCO-eutectic flux system is established by differential thermal analysis (DTA), which enables the selection of a suitable starting composition and temperature program for the crystal growth. We explore the relation between crystal growth, cell parameters and the main superconducting transition temperatures (T-c). The T-c of CLBCO crystals depends strongly on the Ca/La ratio in the starting materials. As for YBa2Cu3O7-y (YBCO), a small amount of BaF2 additive greatly increases the main transition temperature of CLBCO crystals. This can be understood by the cluster model we proposed previously for YBCO crystal growth. Based on the research on the phase diagram, crystal growth and the superconductivity, high T-c crystals of CLBCO with size up to 6.5 x 6.5 x 0.4 mm(3) have been grown from the eutectic self-flux system with BaF2 as additive. T-c was determined by an a.c. susceptibility measurement, which shows the main transition temperature up to 60 K for the as-grown crystals. After annealing in flowing oxygen at 600 to 400 degrees C for two days, T-c increased to 73 K

Crystal growth and superconducting properties of (Y,Ca)Ba2Cu3O7-delta with BaF2 additive

High-quality single crystals of YBCO:Ca up to 2x2cm in size have been grown in non-contaminating yttria or BaZrO3 crucibles from the eutectic self-flux system with BaF2 additive. A cluster model is proposed to describe the function of F- ions in the high temperature solution. The distribution coefficient of Ca between the solid and the liquid phases is much less than1, which indicates that crystals with a Ca doping level > 0.22 are thermodynamically unfavourable. The preference of six-fold co-ordination for Ca substituted in Y-sites at low doping concentration has little effect on the lattice parameters, but it distorts the conductive Cu-O layers. At high doping concentration, a small fraction of Ca also substitutes into Ba-sites near the Cu-O chains. We propose a modified model for estimating the effective hole concentration which fits better the universal parabolic variation of T-C

Anisotropic magnetic penetration depth of grain-aligned HgBa2Ca2Cu3O8+ delta.

The temperature (T) dependence of the anisotropic magnetic penetration depth Λ(T) of magnetically aligned powders of crystalline HgBa2Ca2Cu3O8+δ is reported. Measurements were performed in the Meissner state using the ac-susceptibility technique. The temperature dependences of the in-plane, Λab(T), and out-of-plane, Λc(T), penetration depths are markedly different. This is believed to arise from the large anisotropy ratio γ=[Λc(0)/Λab(0)]≃30. The behavior of Λab(T) is indicative of d-wave superconductivity while Λc(T) is similar to the behavior expected for a superconductor with intrinsic Josephson coupling between the CuO2 planes. Similar measurements were performed on Ba0.6K0.4BiO3 powders for comparison

Anisotropic penetration depth measurements of high-T-c superconductors

A systematic study of the anisotropic magnetic penetration depth λ for several high-Tc superconductors is presented. It is shown that the existence of the linear term in the low temperature in-plane penetration depth is independent of the number of CuO2 planes per unit cell, the carrier concentration and the anisotropy. The anisotropy, however, is found to have strong effects in the out-of-plane component. The effects of Zn substitution on λ and the importance of the CuO2 planes for dx2_y2 pairing are discussed