Y1-xPrxBa2Cu3O7-δ单晶的生长与退火效应研究

采用顶部籽晶提拉法成功生长出了Y1-xPrxBa2Cu3O7-δ(x=0.07215,0.23960)单晶样品。利用X射线结构分析技术对生成单晶进行了表征,通过对样品进行了不同时间的退火处理,结合超导磁化特性测量,讨论了单晶样品超导转变温度Tc随退火温度和Pr替代含量的变化,并根据单晶样品磁化特性曲线,从低场下的布拉格玻璃态到高场下的磁通玻璃态的相变角度对尖峰效应进行了初步分析。     

Y0.928Pr0.072Ba2Cu3O7超导单晶的钉扎机制研究

研究了Y0.928 Pr0.072 Ba2 Cu3 O7(后文称YPBCO)单晶在不同温度下的磁特性,计算了不同温度下的钉扎力密度Fp对磁场的依赖关系.结果表明,在不同温度下的钉扎力密度可近似标度在同一条曲线上;标度函数和峰效应引起的最大Jc所对应的磁场与不可逆场的比值都表明,YPBCO单晶在磁通玻璃态的钉扎机制主要是样品中Pr的微观电子分布不均匀性引起的正常相面钉扎.     

Y0.8Pr0.2Ba2Cu3O7—δ单晶的热力学参数

用ＳＱＵＩＤ磁强计测得了Ｙ０．８Ｐｒ０．２Ｂａ２Ｃｕ３Ｏ７－δ单晶在外场平行于ｃ轴条件下的磁化强度随温度的变化。在超导转变下，除了磁化强度是可逆的以外，由于磁通涨落而导致的磁化强度交叉发生在超导转变温度以下约２Ｋ处。利用Ｈａｏ－Ｃｌｅｍ模型拟合可逆磁化强度得到了热力学临界场Ｈｃ（Ｔ）和金兹堡－朗道参数ｋ。其它超导参数由Ｈｃ（Ｔ）和ｋ推得，比较ＹＢａ２Ｃｕ３Ｏ７－δ的热力学参数，讨论了Ｐｒ离子的效应     

单晶Pr3Co的磁性随温度变化关系的研究

我们报告单晶Pr3Co在不同温度下样品的磁化情况．在低温下，磁化强度随磁场强度的变化关系曲线为回线，在高温时为直线．我们还估出了磁化过程中磁性转变（即磁矩变化）的大小．     

单晶Pr3Co的磁性随温度变化关系的研究

我们报告单晶Pr3Co在不同温度下样品的磁化情况.在低温下,磁化强度随磁场强度的变化关系曲线为回线,在高温时为直线.我们还估出了磁化过程中磁性转变(即磁矩变化)的大小.     

Pr替代YBCO体系的红外吸收谱研究

本文报道了对Y1-xPrxBa2Cu3O7-δ(x=0.1-1.0)体系红外吸收谱的研究结果，实验表明，在该体系中存在三个明显的吸收模式，分别位于560cm^-1(A1)、1435CM^-1(A2)、1631cm^-1(A3)。随Pr含量的增加，吸收峰A3的强度单调降低，并出现宽化趋势，结合作者正电子实验的结果，证明Pr替代导致Cu—0链区域氧空位缺陷的变化，可能是抑制超导电性的主要原因。     

纳米Pr6O11粉末掺杂MgB2超导电性研究

本文采用固相反应法制备了一系列纳米Pr6O11掺杂的MgB2超导块材,掺杂量分别为0,1,3,5,10wt.%.X射线衍射结果表明:随着Pr6O11掺杂量的增加,MgB2的晶格常数也逐渐增大,也就是说Pr原子部分替代了MgB2晶格中的Mg原子.磁测量结果显示,Pr6O11的掺杂对MgB2的超导转变温度(Tc)有很小的抑制.在低含量Pr6O11掺杂(1wt.%)时,MgB2的临界电流密度(Jc)和不可逆场(Hirr)均有明显的提高,但进一步提高Pr6O11的掺杂量时,会损害MgB2在高场下的性能.文中同时也讨论了Pr6O11掺杂影响MgB2的Tc和Jc性能的机理.     

各向异性样品临界电流密度的自场效应

本采用Ｎｅｗｔｏｎ迭代法解决了无限长各向异性ＨＴＳＣ导线临界电流密度的自场效应，设临界电流密度平行于ｘ轴，外场与ｙ轴成θ角。我们给出了零外场下，临界电流密度及其自场的空间分布，计算了平均临界电流密度与θ角和外场数值的关系，发现并讨论了自场尺寸效应中的极值问题。我们的方法将可以有效地推广到其他更复杂形状和性质的样品中。     

颗粒超导体临界电流密度的尺寸效应

假设颗粒超导体内某点临界电流密度是该处磁场强度平均值的某种函数,计算了其实测临界电流密度与样品尺寸的关系.发现对目前实验中所用的烧结样品,实验测得的 YBaCuo超导体的临界电流密度有显著的尺寸效应.     

Vortex lattice mobility and effective pinning potentials in the peak effect region in YBCO crystals

The peak effect (PE) in the critical current density in both low and high temperature superconductors has been the subject of a large amount of experimental and theoretical work in the last few/several years. In the case of YBCO, crucial discussions describing a dynamic or a static picture are not settled. In that region of field and temperature the mobility of the vortex lattice (VL) is found to be dependent on the dynamical history. Recently we reported evidence that the VL reorganizes and accesses to robust VL configurations (VLCs) with different effective pinning potential wells arising in response to different system histories. One of the keys to understand the nature of the PE is to investigate the VL behavior in the vicinity of the various VLCs in the region of the PE. The stability of these VLCs was investigated and it was found that they have distinct characteristic relaxation times, which may be related to elastic or plastic creep processes. In this paper we review some of these results and propose a scenario to describe the PE in YBCO crystals.     

Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

Peak effect studies in single crystals CeRu2 and 2H-NbS2

We have studied the peak effect (PE) phenomenon in single crystals of weakly pinned superconductors CeRu₂ and 2H-NbS₂. 2H-NbS₂ is iso-structural and iso-electronic to 2H-NbSe₂, whose similarity with CeRu₂ as regards the PE representing the order-to-disorder transformation of the flux line lattice was claimed some time ago. We report on the step change in equilibrium magnetization across the peak effect in CeRu₂. We also present the vortex phase diagram of 2H-NbS₂ obtained from the magnetization data, and compare the PE phenomenon in 2H-NbS₂ and 2H-NbSe₂.     

Investigation of flux pinning properties of YBCO:BaZrO3 composite superconductor from temperature dependent magnetization studies

Magnetization studies of YBCO:BaZrO₃ composite superconductor have been done over a wide range of temperature and applied magnetic field using MPMS SQUID VSM and the results are compared with that of pure YBCO. The analysis of the observed results indicate that there is considerable improvement in the values of critical current density (J_C) and pinning force density (F_p) of YBCO:BaZrO₃ composite samples as compared to pure YBCO sample in the entire investigated range of applied magnetic field and temperatures ranging from 4 to 77 K. The variation of J_C with reduced temperature t (=T/T_C) for the composite samples has been found to be similar to that of pure YBCO sample indicating similar nature of the vortex interaction with pinning centres in both pure and composite samples. The enhancement in the value of J_C and F_p in the composite samples as compared to pure YBCO sample has been attributed to the increased defect density in the composite samples due to the presence of BZO particles in YBCO matrix.     

Superconducting characteristics of (Bi, Pb)-2223 single crystals with controlled oxygen-content

Critical current density was measured for oxygen-controlled (Bi, Pb)-2223 single crystals before and after the irradiation with gold ions in a magnetic field parallel to the irradiation-induced defects along the c-axis. Eleven specimens prepared in different annealing conditions were measured. The condensation energy density of each specimen was evaluated from the observed critical current density by using the summation theory of pinning forces of columnar defects and the flux creep theory. It was found that the specimen heat-treated at 1 atm in oxygen atmosphere has the highest condensation energy density among all specimens. Hence, it is speculated that the optimum oxygen pressure for the anneal is around 1 atm.     

Field-induced pinning effect of Nd(Ba1−xNdx)2Cu3O7−δ single crystals grown by the traveling-solvent floating-zone method

We have studied the superconducting properties of Nd(Ba_1−xNd_x)₂Cu₃O_7−δ (Nd123, x ≈ 0.1) single crystals grown by the traveling-solvent floating-zone method under 0.1% O₂ in Ar atmosphere. The enhancement of the magnetization with increasing field is observed in the hysteresis (M-H) loop in fields both parallel and perpendicular to the c-axis of the Nd123 single crystals as well as in the bulk crystals prepared by the oxygen-controlled-melt-growth (OCMG) method. The composition variation of Ba/Nd is observed in the matrix of Nd123 crystals by an analytical TEM equipped with a cold field-emission gun. It turns out that the enhancement is due to the field-induced pinning effect ascribed to the weak superconducting Nd---Ba substitution regions in the Nd123 matrix.     

The distribution of transport critical current in high temperature superconductor ceramics

The temperature and size dependence of the transport critical current in Y-123 and Bi-2223 + Bi-2212 HTSC ceramic sample has been studied. It is shown that the critical current may be presented as power function of cross section area, the critical current density is homogeneous function of coordinates and a number of properties of the critical current density are reported.     

Electrical properties of molybdenite single crystals

Molybdenite crystals used in the present work were grown by direct vapour transport or sublimation method. The electrical resistivities and I-V characteristics were measured at different temperatures in the symmetry plane. The room temperature resistance of a specimen annealed for different periods has also been measured. These results are described and discussed.     

Zn doping of YBa2Cu3O7 in melt textured materials: peak effect and high trapped fields

The previously introduced modified melt crystallization process (MMCP) has been applied to prepare single grain YBCO bulk material with Zn partially substituting for Cu. Hole doping was controlled by an appropriate oxidizing treatment of the as-grown bulk. A field induced pinning was indicated by a well pronounced peak of the critical current density j_c in the j_c vs. H relationship for the maximal oxidized (overdoped) material containing Zn, whereas pure overdoped YBCO shows no peak effect. The peak effect for Zn-doped YBCO appearing for T=77 K at a relatively high field of about 3 T can be attributed to pair breaking by locally induced magnetic moments due to in plane Zn for Cu substitution. Besides high quality of the bulk YBCO, the peak effect is the reason for the trapped field as large as 1.12 T at 77 K in the cylinder of only 25 mm in diameter.     

High sensitive and ultrafast UV photodetector based on ZrO_2 single crystals

The fast-response ultraviolet (UV) photoelectric effect in ZrO2 single crystals with interdigitated electrodes has been investigated experimentally at room temperature. The photovoltage of ZrO2 single crystals exhibits a linear dependence on applied bias and light power density. The photocurrent responsivity to the UV light with a wavelength of 253.65 nm is 9.8 mA/W. For the photovoltaic pulse, a rise time of 501 ps and a full width at half maximum of 1.5 ns have been obtained, when the ZrO2 single crystal ...