Gd替代YBa_2Cu_3O_(7-δ)超导体的相结构与局域电子结构的研究

Phase structures and local electron structures of Gd-doped YBa_2Cu_3O_(7-δ) systems

利用正电子湮没实验,结合x射线衍射(XRD)结构分析,研究了具有混合稀土特征的(Y1-xGdx)Ba2Cu3O7-δ系列样品.XRD实验结果表明,半径较大的Gd离子Y位替代使得样品晶胞参数和晶胞体积增大,但所有样品仍保持与YBa2Cu3O7-δ(YBCO)样品相同的单相正交结构.正电子湮没实验表明,正电子各寿命参数表现出很强的Gd替代依赖关系.从正电子实验结果出发,计算了Cu-O链区局域电子密度ne的变化.结果表明,局域电子密度ne随Gd含量x的增加而减小,而超导转变温度Tc随局域电子密度ne的减小而增加,这种局域电子密度ne与超导电性的关联是与铜位替代完全不同的,且可能是近年来人们关于混合稀土铜氧化物体系具有较高临界电流密度的原因之一.该研究结果为铜氧化物超导体的应用和机理研究提供了相应的正电子实验资料.

The Gd substituted Y1-xGdxBa2Cu3O7-δ systems are studied systematically by positron annihilation and x-ray diffraction. The x-ray results show that the crystal parameters and crystal volume of the systems increase with the increase of Gd content. However, all the substituted samples maintain the single orthorhombic phase as YBa2Cu3O7-δ （YBCO） sample does. The positron annihilation results show that positron lifetimes depend on the Gd content strongly. According to the positron annihilation result, the local electronic density on Cu-O chain is calculated. The result shows the local electronic density decreases with the increase of the Gd content, while the Te increases with the decrease of the local electronic density. The relationship between the local electronic density and superconductivity is different from the substitution on Cu site, which may be one of the reasons why the mixed rare earth cuprate superconductor has a higher critical electric current density than the YBCO. These experimental results give the corresponding positron experiment data for the application of cuprate superconductors and study of the mechanism.