IRREVERSIBILITY LINE AND THERMAL DEPINNING IN YBa2Cu3O7-δ

By measuring magnetic torque moment in a field-sweeping process, the temperature and field dependence of the critical current density j (with a criterion of electric field) and the normalized relaxation rate Q = d lnj/d ln E of a YBa₂Cu₃O_7-δ thin film were obtained. With a minimum current density (j_min = 10A·cm^-2) the irreversibility lines at different sweeping rates were determined. It was found that these irreversibility lines cannot be fitted to either the melting line or the vortex-glass transition. All the data can be interpreted by the thermally-assisted-flux-flow model. Further investigation shows that, at irreversible tem-perature and field, U_c is much smaller than k_BT, which indicates that the thermal depinning is the real origin of the irreversibility line.     

STUDY OF HALOGEN DOPING IN POLYCRYSTALLINE YBa2Cu3O7-δ

This paper presents systematically the doping effects of halogens in YBa₂Cu₃O_7-δ polycrystalline samples, with a series of concentrations of fluorine, chlorine, bromine or iodine. All samples were prepared by chemical method of citrate pyrolysis, which makes the halogen concentration in the samples more uniform. Samples doped with F have T_c as high as 94.5K, Cl, Br and I dopings also raise T_c to a certain extent. In addition, the influence of halogen doping on critical current density, morphology, constitution and so forth is discussed.     

DISLOCATION DISSOCIATION ON CLIMB PLANE IN TEXTURED YBa2Cu3O7-δ SUPERCONDUCTOR

Using high resolution electron microscopy, the structure of [100] tilt boundary with misorientation of 1.2° in YBa₂Cu₃O_7-δ has been determined. The boundary is shown to consist of an array of partial dis-locations, These partials result from the dissociation of a perfect dislocation with a Burgers vector [0, 0, c] on its climb plane according to the reaction, b→b₁+b₂+b₁+b₂+b₁+b₂. Here b=[00c], b₁=[O,b/2, c/6} (or [a/2, 0, c/6] ). b₂=[0, -b/2. c/6] (or[ -a/2, 0, c/6] ). The spacing between the perfect dislocations satisfies the Read-Shockley formula. There exist no amorphous phases in the boundary region.     

YBa2Cu3O7-δ薄膜热激发磁通蠕动研究

本文选择两块零电阻温度分别为82K和71K超导性能不同的薄膜样品,同步进行了电阻在外磁场中展宽的测量,并用Palstra所提出的Arrhenius定律研究两样品热激活能的差别。实验发现样品热激活能随磁场增加线性下降。由测量结果推出的不可逆线仍有B^*-(1-T^*/T_c)^3/2形式。 关键词：     

YBa2Cu3O7-δ外延膜中的非线性光响应

实验结果表明，在连续波He-Ne激光(λ=0.63μm)照射下，YBa₂Cu₃O_7-δ(YBCO)外延膜在超导转变温度T_c以下附近温区的光响应行为，表现为辐射热效应和非辐射热效应同时存在，尤其当样品的R-T变化率dR/dT→0温区明显地表现为一种非平衡光响应行为，且随电流的变化呈非线性关系.基于上述实验事实，在光致准粒子激发假设下，对T_c附近YBCO薄膜中的非线性光响应机制进行了讨论；给出了光辐射 关键词：     

ANISOTROPY OF CRITICAL CURRENT DENSITY IN c-AXIS ORIENTED EPITAXIAL YBa2Cu3O7-δ FILMS

The dependence of critical current density J_c on the angle α between the directions of the applied magnetic field H (which was rotated in the c-axis-I plane) and the in-plane current I was measured on a c-axis oriented epitaxial YBa₂Cu₃O_7-δ films at 81 K, with the magnetic field strength up to 6T. Analysis of the experimental results on the basis of the classical scaling law of pinning force shows that there exist simultaneously planar-pinning and volume-pinning mechanisms, and the contribution of volume pinning increases wish decreasing while that of the planar pinning decreases, We propose that the decrease of Lorentz-force-independent critical current density with increasing H for H∥I results from the suppression of superconductivity by the magnetic field, The fact that the contribution of volume pinning increases with decreasing α also arises from the suppression of superconductivity in CuO₂ plane by the magnetic field.     

YBa2Cu3O7-δ中正交-四方相变的级次

研究了YBa₂Cu₃O_7-δ正交相和四方相中氧在Cu(1)-O基平面上跳动引起的两低频内耗峰;研究了在正交-四方相变过程中它们随氧含量的变化规律,并据此分析了YBCO中正交-四方相变的级次,认为在～200℃该相变实际上可能是一个一级相变。 关键词：     

Fir josephson photoresponse of tilted c-axis epitaxial YBa2Cu3O7-δ

YBa₂Cu₃O_7- thin films may be epitaxially grown, on suitably prepared substrates, with a non-zero tilt angle between the film's c-axis and its surface normal. These films are fast, room-temperature, thermoelectric detectors with a response extending from UV to FIR wavelengths. We have found that tilted c-axis films, when cooled below T_c, also exhibit a fast and sensitive Josphson photoresponse to 250 µm and 496 µm radiation, in contrast to the nil response of c-axis normal films. It is believed that the non-zero component of the radiation electric field along the c-axis of tilted films modulates relatively weak Josephson critical currents in this direction, generating a photoresponse from current biased samples.     

YBa2Cu3O7-δ熔融织构样品的磁响应研究

对YBa₂Cu₃O_7-δ熔融织构样品在不同温度、不同磁场不同外场扫描速率下进行了磁滞回线测量,获得了电流密度J(T,B),lnE-lnJ曲线以及热激活能U₀(T,B)随温度、磁场的变化关系,从而分析了该样品磁响应特性,特别是第二峰(简称峰)受外场扫描速率的影响,探讨了峰形成的真正原因,以及影响峰位置、大小的各种因素 关键词： 磁响应 熔融织构 峰效应 临界电流密度     

YBa2Cu3O7-δ外延薄膜的有效钉扎势

在0—7T磁场范围内，测量了不同测量电流密度下YBa₂Cu₃O_7-δ外延薄膜的电阻温度关系.实验结果表明，临界温度以下，混合态的耗散电阻率能很好地用热激活磁通蠕动描述.有效钉扎势的电流密度关系遵守Zeldov等人提出的对数关系，有效钉扎势的温度和磁场关系遵守U∝(1-T/T_c)H^-α关系，其中α=0.63，与热激活磁通点阵位错运动模型相一致，表明样品具有2D涡旋性质. 关键词：     

COHERENT OPTICAL PHONON EXCITATION BY FEMTOSECOND LASER PULSE IN YBa2Cu3O7-δ THIN FILMS

With a correlation of nonequilibrium carriers relaxation and coherent phonons displacive excitation, the coherent optical phonon oscillations in YBa₂Cu₃O_7-δ thin films excited by femtosecond laser pulse are simulated theoretically. It is revealed that as the oxygen concentration decreases, the coherent phonon oscillations become easier to be observed due to the decrease of the local coupling between the carriers and the lattice vibrations in the CuO₂ plane.     

YBa2-xPrxCu3O7-δ体系中的Pr替代效应

报道了YBa_(2-x)Pr_xCu_3O_(7-δ)(0≤x≤0.15)体系的超导电性和正常态输运性质的测量。实验结果表明随着Pr在Ba晶位替代量的增加,超导转变临界温度T_c和由Hall系数推算的荷电载流子浓度P_H均单调下降。通过与Y_(2-x)Pr_xBa_2Cu_3O_(7-δ)和Nd_(1+x)Ba_(2-x)Cu_3O_(7-δ)体系的比较研究,作者发现上述三个体系的T_c变化均与载流子浓度的变化相关联。作者进而提出,在YBa_(2-x)Pr_xCu_3O_(7-δ)体系中Pr的空穴填充效应 关键词：     

YBa2Cu3O7-x的金属-绝缘体转变

借助于局域化的标度理论,我们对层状结构系统引进一个由电导各向异性或层间耦合决定的有效维度,认为电导的减小和有效维度的降低是YBa₂Cu₃O_7-x和其它氧化物金属-绝缘体转变的原因,同时也讨论了有限温度的标度理论以解释正常态电阻率的温度行为。 关键词：     

YBa2Cu3O7-δ超导体氧缺陷的正电子寿命谱

系统研究了不同氧缺陷的YBa_2Cu_3O_(7-δ)(δ=0.06—0.68)超导体正常态(300K)和超导态(77K)下的正电子寿命谱,利用两态捕获模型对实验结果进行了分析,计算了相应的局域电子密度n_c和空位浓度C_v随氧缺陷δ的变化,给出了氧缺陷YBa_2Cu_3O_(7-δ)体系的正电子湮没特征,讨论了相应的正电子湮没机制以及与超导电性之间的关联。 关键词：     

YBa2Cu3O7-x高温导电性的研究

本文将以YBa₂Cu₃O_7-x的高温实验为基础,分析和讨论了氧对YBa₂Cu₃O_7-x高温导电性的影响以及一维Cu—0链的导电作用。还报道了一种新现象,即在高温下,一旦氮气被换成氧气,YBa₂Cu₃O_7-x就迅速地发生从半导体到金属的结构相变。 关键词：     

水腐蚀下YBa2Cu3O7-δ块体微观结构以及性能变化

本文系统性地研究了长期的水环境对YBa₂Cu₃O_7-δ(YBCO)超导体的影响.我们的研究发现,YBCO块体的水腐蚀从表面开始,发展到内部.测试中发现YBCO与水反应生成Y₂BaCuO₅、毒性BaCO₃、CuO等,而这个腐蚀行为似乎是从晶粒中提取Ba和Cu,并将Y浓缩到晶粒中心.在77 K温度下,未与水接触的样品的临界电流密度达到22500 A/cm²,而浸入水中50小时的样品仅为6000 A/cm²(甚至低于前者的三分之一).不仅如此,受腐蚀过后的YBCO块的磁化率也严重减弱.虽然临界转变温度并未有较明显的改变,但水腐蚀对YBCO超导的影响无疑是非常致命的,因此,我们建议在相关设备的使用或储存过程中应注意水的来源并杜绝与之接触.     

用穆斯堡尔效应研究YBa2Cu3O7-δ中的磁有序

以⁵⁷Fe作为探针,用穆斯堡尔效应研究YBa₂Cu₃O_7-δ的磁有序与超导电性的关系。实验结果可以用非声子超导机制进行定性解释。 关键词：     

YBa2Cu3O7-δ薄膜微结构的同步辐射三维倒空间扫描研究

高温超导薄膜因其微波表面电阻低,可用于尖端高温超导微波器件的制作.然而由于高温超导材料特殊的二维超导机制和极短的超导相干长度,高温超导材料的微波表面电阻对微结构特别敏感.为了探究高温超导材料微结构和微波电阻的联系,采用脉冲激光沉积(PLD)技术在(001)取向的MgO单晶衬底上生长了不同厚度的YBa₂Cu₃O_7-δ(YBCO)薄膜.电学测量发现不同厚度的样品超导转变温度、常温电阻差别不大,但超导态的微波表面电阻差异很大.同步辐射三维倒空间扫描(3D-RSM)技术对YBCO薄膜微结构的表征表明:CuO₂面平行于表面晶粒(c晶)的多寡、晶粒取向的一致性是造成超导态微波表面电阻差异的主要原因.     

正交和四方YBa2Cu3O7-x的热膨胀系数

用电容法测量了正交和四方YBa₂Cu₃O_7-x的热膨胀系数与温度的关系。结果表明,正交样品在205K附近,四方样品在92—130K之间热膨胀系数有异常。估计前者可能与晶格的不稳定性有关,说明具有较强的电-声子相互作用,后者可能与结构转变有关,因此破坏了高温超导电性。同时也发现正交样品在92K热膨胀系数有微小跳跃。由此估计了压力效应 关键词：