Thermal expansion of a YBa2Cu3O7 superconducting ceramic

The thermal expansion of the High T_c superconducting ceramic YBa₂Cu₃O_7-δ has been measured from 50 K up to room temperature by means of a capacitance dilatometer. No detectable anomalous change in lenght is observed at the critical temperature T_c, within the resolution of our experimental set up: Δl/l = 5 10⁻⁸. This indicates a weak dependence of T_c on pressure, contrarily to the one measured on (LaBa)CuO lower T_c superconducting ceramics. The Debye temperature is also estimated.     

Increase of YBa2Cu3O7 critical currents by Mo substitution and high-pressure oxygen annealing

We have previously shown for powders that Mo substitution into the CuO chains of YBa₂Cu₃O₇ can create effective pinning centres which significantly increase the critical current density (j_c) in 7 T field by a factor of 4 and 10 at 50 and 60 K, respectively (Rogacki et al., 2006) [1]. The present work reports on the influence of the Mo substitution and high-pressure oxygen annealing on the pinning properties and critical currents of YBa₂Cu_3-xMo_xO_7-d by comparing pure (x = 0, d > 0) and substituted (x = 0.03, d < 0) single crystals. Pinning properties have been investigated by measurements of magnetization loops and calculations of j_c in the ab-plane, in the temperature range from 2 to 90 K and in fields up to 14 T. Depending on the Mo substitution and the oxygen treatment, several types of pinning centres increasing j_c have been revealed and analysed in the frame of Dew-Hughes’ and Kramer's models.     

退火氧压对YBa2 Cu3O7-x薄膜中的激光感生热电电压效应的影响

测量了在不同氧压下退火生长的YBa2 Cu3O7-x(YBCO)薄膜中的激光感生热电电压(LITV)信号,发现随退火氧压的增大可使LITV信号的峰值有2-4倍的增强,并且变化趋势与薄膜热电势的各向异性随氧含量的变化规律相同.波长在473-808 nm范围内的连续激光辐照,在5000 Pa的氧压下退火生长的YBCO薄膜中探测到的LITV信号最大;而紫外脉冲激光辐照时,IJTV信号的最大值出现在退火氧压为105 Pa的薄膜中.LITV信号的光响应时间随退火氧压的增大而减小.理论分析表明,这一结果是由于退火氧压的提高导致薄膜材料的热扩散系数增大所致.在退火氧压为5000 Pa的YBCO薄膜中探测到响应最快的LITV信号,对紫外脉冲激光的响应时间为78 ns,并且信号的上升时间只有29 ns,与激光的脉冲宽度(约为28 ns)相当.     

Interface and grain boundary structures in YBa2Cu3O7-x and YBa2Cu4O8 materials

Y₂BaCuO₅YBa₂Cu₃O_7-x (Y211/Y123) interfaces in melt-processed YBa₂Cu₃O_7-x were studied by high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. Yttrium enrichment and barium depletion were observed locally at the Y211/Y123 interfaces where Y123 (001) facets were present. This effect may be interpreted as the result of lattice substitution of Ba by Y near these interfaces. Cation nonstoichiometry was found near Y211/Y123 interfaces where liquid phases (Cu-Ba-O) were present. This chemical disorder introduces numerous point defects in the Y123, and these defects may act as additional pinning sites alongwith stacking faults. A comparison of grain boundary (GB) chemical composition in polycrystalline YBa₂Cu₃O_7-x and YBa₂Cu₄O₈(Y124), studied using nanoprobe parallel-detection electron energy-loss spectroscopy (EELS), is presented. The studies of Y124 show that stoichiometric grain boundaries can also form weak links between superconducting grains. It is suggested that weak-link behavior is determined largely by misorientation at grain boundaries.     

Anisotropy of the superconducting properties of YBa2Cu3O7−x single crystals with reduced oxygen content

In an investigation of the resistivity anisotropy of YBa₂Cu₃O_7−x single crystals with suboptimal oxygen content it is observed that the superconducting transition for the component ρc of the resistivity tensor is shifted to lower temperatures with respect to the transition for the component ρab. A similar shift is also observed for the transition in the temperature dependence of the dynamic magnetic susceptibility. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 309–313 (25 August 1998)