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.     

Y1—xPrxBaxCu3O7—δ超导体正常态异常的正电子湮没研究

在７７－３００Ｋ温度范围内用正电子湮没方法测量了Ｙ１－ｘＰｒｘＢａＣｕ３Ｏ７－δ（ｘ＝０．２，０．１，０．０）三个超导样品中正电子湮没寿命随温度的变化，细致地研究了正常态异常现象，ｘ为０．０和０．１的二个样品分别在较Ｔｃ高４０Ｋ和３０Ｋ处观察到了正常态异常现象．ｘ＝０．２样品没有观察到正常态异常现象，实验观察到的正常态异常现象可能是发生超导转的前秦，由结构不稳定产生的类相变所致。     

常压和高压合成MgB2的低温比热及两个超导能隙研究

测量了常压和高压合成MgB2超导样品6K—80K的比热,并在数据拟合中考虑了爱因斯坦振动模的贡献,得到其电子比热数据.解释了超声测量得到的德拜温度和低温比热拟合结果之间的巨大差异.并且在12K附近观察到了异常的电子比热,此异常来源于MgB2的第二个能隙.此外,对高压和常压合成对样品的影响做了初步分析. 关键词： 低温比热 MgB2 能隙     

巨磁阻Mn氧化物中小极化子的直接观察

巨磁阻系列材料Ｌａ（Ｃａ）ＭｎＯ３和Ｐｒ（Ｃａ）ＭｎＯ３在低温下进入电荷有序态．利用低温电子显微镜观察,对定域化载流子（空穴）引起的结构转变进行了特征化,发现了一系列复杂的微结构现象．特别重要的是,在低温下出现的一个调制结构,它表现出一些奇特的温度变化性质．进一步分析表明,这个调制结构和小极化子有序化状态相对应     

INTERNAL FRICTION AND ULTRASONIC ATTENUATION RELATED TO CARRIERS IN HIGH Tc SUPERCONDUCTORS

For both BiSrCaCuO and TlBaCaCuO samples, internal frictions (Q^-1) in the kHz range reveal a plateau (Q^-1_p) above T_c and a rapid drop below T_c with the turning points located just at T_c for various samples with different T_c. This anomaly cannot be observed in non-superconducting samples. The ultrasonic attenuation (α) in TlBaCaCuO displays similar results to the internal friction. Moreover, it is discovered that the Q^-1_p is nearly proportional to the carrier density for Y(Pr)BaCuO and Gd(Pr)BaCuO with different Pr contents. These results show that the drop of Q^-1 and a below T_c is closely related to superconducting condensation. The Q^-1_p and α_p can be explained using coupling-model of carriers with local dynamic distortion because high T_c superconductors are strong-correlation systems. Furthermore, by taking account of the smearing of superconducting gap structure resulting from the recombination of quasi-particles and by modifying the BCS relative jump rate as S(E, E', Γ) = Re{1-Δ²/[(E-iΓ)(E'-iΓ)]}, the calculated results of internal friction and ultrasonic attenuation below T_c are in good agreement with the experimental data. The superconducting gap Δ and the damping rate Γ for both BiSrCaCuO and TlBaCaCuO have also been obtained, they are in accordance with those got by tunneling spectrum and NMR methods, etc.     

颗粒状NbC,NbN转变温度T_c提高的探讨

本文分析了表面效应对NbC, NbN 的声子谱及电子态密度的影响. 预言颗粒状NbC, NbN 化合物其转变温度T_c将会提高. T_cmax 将在20K 以上. 关键词：     

多晶MgC1-xNi3的微结构弛豫谱研究

我们测量了千赫兹范围内超导及非超导MgC1-xNi3样品从液氮温度至室温的微结构弛豫谱.对于超导样品,发现了两个内耗峰:P1(300K)和P2(125K).而对于非超导样品,P1峰位移到了250K,而P2峰几乎被完全抑制.变频实验发现随着测量频率升高,P2峰移向高温,计算所得激活能为0.13eV.我们认为P2峰可能与C原子在偏心位置的跳跃有关.进一步我们可以猜测C原子的这种行为与许多实验中观察到的超导样品在150K和50K正常态度越行为相对应.     

插层FeSe高温超导体的高压研究进展

通过对FeSe进行化学插层可以将其超导转变温度（T_c）从约8 K提高到40 K以上,实现高温超导电性.最近,我们对两种插层FeSe高温超导材料（Li_0.84Fe_0.16）OHFe_0.98Se和Li_0.36（NH₃）_yFe₂Se₂开展了高压调控研究,发现压力会首先抑制高温超导相（称为SC-I相）,然后在临界压力P_c以上诱导出第二个高温超导相（称为SC-Ⅱ相）,呈现出双拱形T-P超导相图.这两个体系的P_c分别约为5和2 GPa,两个体系SC-Ⅱ相的最高T_c分别可以达到约52和55 K,比相应SC-I相的初始T_c提高了10 K.对（Li_0.84Fe_0.16）OHFe_0.98Se的正常态电输运性质分析表明,SC-I和SC-Ⅱ相的正常态分别具有费米液体和非费米液体行为,意味着这两个超导相可能存在显著差异.此外,还发现这两个体系的SC-Ⅱ相的T_c与霍尔系数倒数1/R_H（∝载流子浓度n_e）具有很好的线性依赖关系.对（Li_0.84Fe_0.16）OHFe_0.98Se的高压X射线衍射测量排除了其在10 GPa以内发生结构相变的可能,因此P_c以上SC-Ⅱ相的出现和载流子浓度的增加很可能起源于压力导致的费米面重构.     

Bi2Sr2CaCu2O8的晶体结构与超导性能

本文研究了Bi-Sr-Ca-Cu-O体系中理想成份为Bi₂Sr₂CaCu₂O₈化合物的超导性能和晶体结构。名义成份为BiSrCaCu₂O_5.5零电阻超导转变温度T_c(0)=81.5K。用X射线粉末衍射方法测定了Bi₂Sr₂CaCu₂O₈的晶体结构,其基本结构属体心四方晶系,空间群为D_4h¹⁷-l4/mmm,点阵常数a=3.825?,c=30.82?。每单胞化合式单位为2.2Ca占据2(a)等效点系,4Sr,4Bi和4Cu占据三组4(e)位置,其原子参数z分别为0.110,0.302和0.445,16O分别占据8(g),z=0.445和二组4(e),z=0.210和0.380。Bi₂Sr₂CaCu₂O₈晶体结构可认为是阳离子沿z轴的(00z)和((1/2)(1/2)z)交错排列,由Aurivillius相导生出来的。讨论了在Bi-Sr-Ca-Cu-O体系中可能存在的其它沿z轴不同堆垛层数的超导相。 关键词：