非晶态Zr-Ni和Zr-Co合金结晶化过程的研究

采用Melt-Spining方法制备了一系列非晶态合金Zr_(100-x)Ni_x(23.9≤x≤63.8)和Zr_(100-y)Co_y(22≤y≤53.4)。通过20—700℃温度范围内的电阻和差分比热的测量,发现:随着结晶化过程的进展,R(T)曲线呈现多阶梯状的结构。对于某些组分的非晶合金,还观察到在结晶化开始出现时,其电阻值反常地升高,-aR/dR(T)和DSC(T)曲线上的峰的位置和形状往往十分一致,只是有时第一峰是相互反向的。结合非晶合金化的动力学过程进行了讨论.给出了结晶化温度T_g与浓度的关系。     

Y1-xCaxBa2Cu3-xMxO7-δ (M=Fe,Ni)体系的超导电性

本文报道,通过对Y_1-xCa_xBa₂Cu_3-xMxO_7-δ(M=Fe,Ni)体系样品的晶体结构、氧含量、正常态电阻率与温度的关系,以及超导转变温度等测量,并与YBa₂Cu_3-xMxO_7-δ(M=Fe,Ni)体系进行比较,发现Y_1-xCa_xBa₂Cu_3-xFe_xO_7-δ体系的T_c显著地高于相应x值的YBa₂Cu_3-xFe_xO_7-δ体系,而Y_1-xCa_xBa₂Cu_3-xNi_xO_7-δ体系则相反,T_c低于仅Ni替代的体系,表明Ca和Fe同时替代时两者引起的载流子浓度(n_H)变化相互补偿,抑制了仅Fe替代时引起的n_H和T_c急剧下降;而作Ca和Ni同时替代时主要的不是两者引起载流子浓度变化的相互补偿,Ca和Ni替代效应之间的关联较弱。作者认为,对Y_1-xCa_xBa₂Cu_3-xFe_xO_7-δ体系属于CuO₂平面外的元素替代,这时载流子浓度是决定Tc的主要因素;而对Y_1-xCa_xBa₂Cu_3-xNi_xO_7-δ体系,由于Ni²⁺离子主要占据Cu(Ⅱ)位,它导致磁拆对效应,Ni²⁺离子的拆对效应是引起T_c下降的直接原因。 关键词：     

超导金属玻璃Zr_(78)CO_(22)在横向磁场中超导-正常转变的阶梯结构

在横向磁场中,测量了超导金属玻璃 Zr_(78)Co_(22)的 V-H 转变曲线.观察到除存在磁通流动外,还存在着两种类型的阶梯式有阻态.一类是由序参量的热力学涨落产生的相位滑移中心所致,另一类则相应于热斑的存在.同时还观察到相位滑移中心与热斑共存的现象。对磁通蠕动.磁通流动和磁通跳跃对阶梯结构的影响作了讨论.     

三组元超导复合线绕磁体的快速励磁特性

采用强迫励磁和在额定电流下突然合闸两种励磁方式,研究了国产Nb-Ti-Cu-Cu-Ni三组元超导复合线绕磁体的快速励磁特性.得到了在临界电流不发生明显退化时的最大励磁速率.提出了改善材料稳定性的建议.     

氧缺位对单相YBa_2Cu_3O_(7-x)热电势率的影响

研究了氧缺位对单相YBa_2Cu_3O_(7-x)体系超导电性及热电势率的影响。结果表明,超导转变温度随氧缺位的增加而下降,热电势率随氧缺位的增加而增大。由实验结果推断,单相YBa_2Cu_3O_(7-x)的多数载流子为空穴。存在较大的声子曳引热电势表明。在YBa_2Cu_3O_(7-x)超导体中存在较强的电-声子相互作用。     

超导金属玻璃Zr_(66.7)Ni_(33.3)的结构弛豫、结晶化和上临界磁场

测量了一系列作等时热处理的超导金属玻璃Zr(66.7)Ni_(33.33)的上临界磁场H_(c_2)(T)及其转变宽度△H_(c2)(T)。发现:在结构弛豫阶段,H_(c2)(T)、△H_(cz)均随热处理温度T_a的增高而线性地下降;在结晶化阶段,从_(c2)与T_α则里指数规律变化。作者改进并推广了Gibbs的结构弛豫理论,使之适用于包括结构弛豫和结晶化的全过程,理论与实验结果符合很好。     

超导金属玻璃Zr_(78)Co_(22)中局域的有阻态

本文系统地研究了在无外磁场时,超导金属玻璃Zr_(78)Co_(22)的I-V特性。观察到电流引起的由超导态向正常态的转变是以阶梯跃变的方式完成的。阶梯的出现是局域的,且与电流扫描速率有关。分析表明,有阻态阶梯的出现,是由于材料本身的不均匀性而导致热斑出现的结果。一维热流方程可以用来描述金属玻璃Zr_(78)Co_(22)局域的有阻态,但必须计及各热斑间的相互作用和样品本身的自吸热等效应。     

颗粒超导体Ba_2YCu_3O_(7-δ)的低温电阻特征

本文中研究了单相Ba_2YCU_3O_(7-6)样品的低温电阻与外磁场、温度以及测量电流之间的关系。结果表明,外磁场的大小、方向,以及测量电流的大小,显著地改变着样品的低温电阻行为。从样品的颗粒结构特征对结果进行了讨论。     

POSITRON LIFETIME STUDIES OF Y1-xCaxBa2Cu3O7-δ AND Y1-xCaxBa2Cu3-xFexO7-δ

In this paper it is reported that the measurement of the bulk positron lifetime as a function of substitution content x in the temperature range from 70 to 220K was performed in two high-T_c superconducting systems, Y_1-xCa_xBa₂Cu_3-xFe_xO_7-δ and Y_1-xCa_xBa₂Cu₃O_7-δ. It was found that τ_B of both systems decreases significantly with x and the temperature dependence of τ_B is very weak in normal state. In lower temperature region (Tc), a dramatic x-dependent temperature variation of τ_B was observed in the Ca-substituted system: from a decrease of τ_B with decreasing T to an increase of τ_B. With increasing x, the temperature dependence of τ_B remains weak in the Ca- and Fe- substituted systems. Compared with the experimental data of positron lifetime in other substituted systems and the calculation of the positron density distribution, the authors suggest that positron bulk lifetime spectra behaviour can be interpreted by the physical model based on the transfer of electron density between the CuO₂ planes and Cu-O chains. Therefore, the study of positron lifetime spectra provides a useful means to detect the local charge density and to study the correlation between the electronic structure and the high-T_c superconductivity.     

Effect of magnetic field on the spin-Peierls transition in single-crystal CuGeO

This paper reports that high quality CuGeO3 single crystals were successfully grown by floating-zone technique and the magnetic property was studied. The temperature dependence of magnetic susceptibility below the spin-Peierls （SP） transition temperature （Tsp） under magnetic fields applying along both the a- and c-axis direction can be fitted well by a model of noninteracting dimmers. The spin gap derived from the fitting is consistent with other reports. There is a very weak anisotropy in the fitting parameters for different directions, which should be expected from a SP system. A small upturn in susceptibility at low temperature due to paramagnetic impurities and/or defects can be observed. A suppression of the upturn by magnetic field is first discovered in this system and the possible origins for this suppression are discussed.