PREPARATIVE AND STRUCTURAL STUDIES ON THE SUPERCONDUCTING PHASE YBa_2Cu_3O_(7-d)

Reasonably homogeneous samples were obtained by following the formula Y_(0.334)Ba_(0.666)CuO_(3-y) andusing citric acid as a complex-forming agent to improve the dispersion of the metal oxides. We havealso found that the variant condition in preparative procedure leads to variant deviation from tetrago-nality and that more pronounced deviation gives better superconducting properties. The crystal structure determined from X-ray powder diffraction diagrams confirms the stoichiom-etry of the metal ions and gives the positions of the oxygen atoms. The orthorhombic or pseudo-tetragonal unit cell with a=3.893, b= 3.813 and c= 11.687 A contains YBa_2Cu_3O_7. It is composed ofthree perovskite cubes with three copper atoms at their corners, the two barium atoms at the centers ofthe top and bottom cubes, and the yttrium at the center of the unit cell. The oxygen atoms sit on centersof cube edges. The positions (0 0 1/2) are vacant. The vacancies represented by (0(1/2)0) cause the ar-rangement of atoms to lose tetragonal symmetry, and have far-reaching effect for this phase. The yttriumand barium atoms are coordinated in a cuboctahedron respectively with four equatoria oxygen atoms at(0 0 1/2) and two at (0 1/2 0) removed. In the light of bond valence theory we can infer from the in-teratomic distances Y-O, Ba-O and Cu-O that the occupancy of the oxygen atoms at (1/2 0 0) is wellbelow unity while the positions (0 0 1/2) are vacant, the formula of the phase is YBa_2Cu_3O_(7-d), thefraction of Cu(Ⅱ) and (Ⅲ) for the set of copper at (0 0 0) is respectively d/0.5 and (1-d/0.5), and0.2相似文献   

金属薄膜在云母与富勒烯衬底上的电阻弛豫现象

超高真空环境中，在云母和富勒烯衬底上蒸镀半连续的Nb和Ag膜，同时对其进行了原位电阻测量．在人为地突然中断蒸镀之后的10min内，仔细研究了样品电阻随时间的自动缓变过程（弛豫）．实验发现，Nb/云母和Ag/C₆₀样品的电阻随时间缓慢增加，Ag/云母样品的电阻随时间逐渐减小．样品弛豫的强度与衬底温度和金属膜厚度密切相关．分析表明，衬底上的金属原子徙动会引起金属岛的边际形变以及岛间融合，这是薄膜电阻弛豫的主要原因．电阻弛豫的方向与强度反映了金属膜/衬底系统的界面活性．衬底表面的缺陷也会影响这一弛豫过程．从微观的原子运动和宏观的热力学平衡这两方面描述了弛豫过程 关键词：     

固体C60/Si异质结的电学表征——整流特性、能带模型与温偏效应

研究了非掺杂固体C₆₀与n-Si和与p-Si接触的电学性质,电流-电压(J-V)特性测量表明两种接触的导电极性相反,且都具有很强的整流作用,表明在两种接触界面附近存在着阻挡载流子输运的、性质不同的势垒.电流-温度(J-T)测量表明,电流与温度的倒数呈指数依赖关系,从中估算出C₆₀/n-Si和C₆₀/p-Si异质结的有效势垒高度分别为0.30和0.48eV.引进异质结的能带模型,成功地解释了上述测量结果,由能带模型和测量数据估算出以硅为衬底的 关键词：     

Ce－C_（60）多层膜电阻的奇异化

通过对Ｃｅ－Ｃ（６０）多层膜电阻的原位观测，我们发现在Ｃｅ层上沉积Ｃ（６０）时，样品电阻有明显上升．这与以往研究过的其它金属（如Ｓｎ，Ａｇ，Ｃｕ等）－Ｃ（６０）体系的结果有很大差异．我们认为稀土元素Ｃｅ与Ｃ（６０）间可能有某种新类型的相互作用．