水合钴氧化物超导体的合成和表征

最近发现的水合钴氧化物超导体引起了超导领域的强烈兴趣. 由于该超导体非常不稳定,因此有关该超导体的合成和表征是个重要的基本课题. 本文报道几种典型的合成方法,并对所得到的超导体进行表征. 我们将着重介绍利用歧化反应来制备含有很低的钠离子浓度和很高的水合氢离子浓度的钴氧化物超导体的新方法.     

Na0．15（H3O）0．2CoO2·ωH2O的超导电性

最近发现的水合钴氧化物超导体在某些方面具有与高温超导体十分相似的性质．本文报道含有高浓度的水合氢离子的钴氧化物超导体的超导电性磁测量结果．通过对样品的M（H，T）的测量，得到了大致的H～T相图．最后还讨论了系统中可能存在的2D-3D转变．     

Na0.15(H3O)0.2CoO2·wH2O的超导电性

最近发现的水合钴氧化物超导体在某些方面具有与高温超导体十分相似的性质.本文报道含有高浓度的水合氢离子的钴氧化物超导体的超导电性磁测量结果.通过对样品的M(H,T)的测量,得到了大致的H～T相图.最后还讨论了系统中可能存在的2D～3D转变.     

高临界温度氧化物超导体的超导特征参量：λξ和Δ

磁场穿透深度（λ）、相干长度（ξ）和能隙（Δ）是表征超导体处于超导态时其内禀特性的三个物理参量，是了解超导体的独特性质、探索超导机理和建立微观理论的依据和约束。本文在简要介绍传统超导体中有关λ、ξ和Δ的引入与测量方法的基础上，着重评述用于高TC氧化物超导体中有关λ、ξ和Δ的新测量方法，以及目前对各种类型高TC超导体所给出的实验结果。     

1987年以来我国高温超导研究进展

本文评述了自１９８７年以来我国高临界温度氧化物超导体研究工作的进展，包括超导体材料研究和制备，超导薄膜研究和制备，基础性研究以及超导量子干涉器件等应用方面的进展．     

高压下HgBa_2NcaCu_2NO_（6＋δ）超导体的制备及其超导电性

利用高温高压方法合成了ＨｇＢａ２ＣａＣｕ２Ｏ６＋δ超导体．Ｘ射线衍射结构分析显示该超导体具有四方结构，晶格常数ａ—０．３８６７ｕｒｎ，ｃ—１．２７４３ｕｒｎ．电阻和交流磁化率测量结果表明该超导体的超导起始转变温度为１２７Ｋ，本电阻温度为１２ｌＫ，抗磁转变温度为１１８Ｋ．对合成后的样品做了氧气氛中的退火处理．研究了２７ｋｂａｒ压力下，不同烧结温度对合成作品的超导电性的影响，结果表明该压力下ＨｇＢａＣａＣｕＯ６＋δ超导体的最佳合成温度为８２０～８３０℃．     

用“持续电流法”测量氧化物超导体的电阻率和临界电流

氧化物超导体发现后，弄清它的直流电阻率到底是多少成为一个极待解决的问题．我们用“持续电流法”首先测量了钇、钡、铜氧比物超导体的直流电阻率，还用同样的实验装置测量了这种超导体的临界电流及其随温度的变化．本文介绍这种测量方法的原理和实验装置，并给出了我们的主要实验结果．     

对Ⅱ类超导体混合态磁阻起因的重新认识

本指出常规和基于Ａｂｒｉｋｏｓｏｖ涡旋理论所提出的关于Ⅱ类超导体混合态磁阻起因的解释存在着某些不足之外，特别对新发现的高温氧化物超导体，该解释同实验有明显地冲突。为描述Ⅱ类超导体混合态磁阻，我们认为最好的理论基础是约瑟夫逊相滑移理论。本就此进行讨论并试图说明在此基础上可以导出常规的关于Ⅱ类超导体混合态磁阻的表达式。     

用Cu_2O合成Y－123超导体的研究

用Ｃｕ_２Ｏ合成Ｙ－１２３超导体的研究兰玉成中国科学院物理研究所车广灿，吴非，陈红，贾顺莲，董成，赵忠贤中国科学院物理所超导实验室，北京１０００８０１９９５年１１月２７日收到一、引言众所周知，在高Ｔｃ氧化物超导体中，［ＣＵ－Ｏ］ｐ层或［Ｃｕ－Ｏ］ｐ链?..     

高压下HgBa2CaCu2O6＋δ超导体的制备及其超导电性

利用高温高压方法合成了ＨｇＢａ２ＣａＣｕ２Ｏ６＋δ超导体。Ｘ射线衍射线结构分析显示该超导体具有四方结构，晶格常数ａ＝０．３８６７ｎｍ，ｃ＝１．２７４３ｎｍ．电阻和交流磁化率测量结果表明该超导体的超导起始转变温度为１２７Ｋ，零电阻温度为１２１Ｋ，抗磁转变温度为１１８Ｋ。对合成后的样品做了氧气氛中的退火处理，研究了２７ｋｂａｒ压力下，不同烧结温度对合成样品的超导电性的影响，结果表明该压力下ＨａＢａ２Ｃ     

Survival of the d-wave superconducting state near the edge of antiferromagnetism in the cuprate phase diagram

In the cuprate superconductor YBa2Cu3O6+x, hole doping in the CuO2 layers is controlled by both oxygen content and the degree of oxygen ordering. At the composition YBa2Cu3O6.35, the ordering can occur at room temperature, thereby tuning the hole doping so that the superconducting critical temperature gradually rises from 0 to 20 K. Here we exploit this to study the c-axis penetration depth as a function of temperature and doping. The temperature dependence shows the d-wave superconductor surviving to very low doping, with no sign of another ordered phase interfering with the nodal quasiparticles. The only apparent doping dependence is a smooth decline of superfluid density as T(c) decreases.     

Heat conduction in the vortex state of NbSe2: evidence for multiband superconductivity

The thermal conductivity kappa of the layered s-wave superconductor NbSe2 was measured down to T(c)/100 throughout the vortex state. With increasing field, we identify two regimes: one with localized states at fields very near H(c1) and one with highly delocalized quasiparticle excitations at higher fields. The two associated length scales are naturally explained as multiband superconductivity, with distinct small and large superconducting gaps on different sheets of the Fermi surface. This behavior is compared to that of the multiband superconductor MgB2 and the conventional superconductor V3Si.     

Transmission properties of Fibonacci quasi-periodic one-dimensional superconducting photonic crystals

The transmission properties of Fibonacci quasi-periodic one-dimensional photonic crystals (1DPCs) containing superconducting material are theoretically investigated based on the transfer matrix method. It is shown that the 1DPCs can possess a same photonic band gap property as the periodic structure superconducting PC. The results of transmittance spectra show that the cutoff frequency can be manipulated through the thicknesses of the superconductor and dielectric layers as well as the ambient temperature of the system. It is observed that the shift of cutoff frequency becomes more noticeable by adjusting the thickness of the superconductor layer than that of the dielectric one. Furthermore, the cutoff frequency becomes very sensitive when the system temperature is tuned to close vicinity of the critical temperature of the superconductor.     

Thermodynamic behaviour of type II superconductors at low induction for arbitrary temperature and impurity content

Using Eilenberger's equations in vortex symmetry, we calculate numerically the structure of an isolated flux line and the lower critical field of a type II superconductor as functions of temperature and impurity content. In order to describe in addition the low-induction regime of the mixed state, a circular-cell approximation is introduced and compared with recent exact results of the Eilenberger theory. The agreement is excellent, if the flux-line overlap is weak. Within this approach it is shown that the nature of the phase transition at the lower critical field depends very sensitively on anisotropies of the Fermi surface.     

Adiabatic quantum pumping at the Josephson frequency

We analyze theoretically adiabatic quantum pumping through a normal conductor that couples the normal regions of two superconductor - normal-metal - superconductor Josephson junctions. By using the phases of the superconducting order parameter in the superconducting contacts as pumping parameters, we demonstrate that a nonzero pumped charge can flow through the device. The device exploits the evolution of the superconducting phases due to the ac Josephson effect, and can therefore be operated at very high frequency, resulting in a pumped current as large as a few nanoamperes. The experimental relevance of our calculations is discussed.     

N/I/d波超导体c轴隧道结的微分电导

以方势垒描述绝缘层,对N/I/d波超导体c轴隧道结的微分电导进行了研究.结果表明:在N/I/d波超导体c轴隧道结的隧道谱中存在V型结构、能隙外的凹陷和小的零偏压电导峰.这一结果能很好的解释相关的实验现象.     

Nearly free electrons in the layered oxide superconductor Ag5Pb2O6

We present first measurements of quantum oscillations in the layered oxide superconductor Ag5Pb2O6. From a detailed angular and temperature dependent study of the de Haas-van Alphen effect we determine the electronic structure and demonstrate that the electron masses are very light, m* approximately 1.2me. The Fermi surface we observe is essentially that expected of nearly free electrons--establishing Ag5Pb2O6 as the first known example of a monovalent, nearly free electron superconductor at ambient pressure.     

Trivial and non-trivial superconductivity in dsDNA

A double-stranded DNA (dsDNA) is modeled by two coupled one-dimensional Kitaev's chain and the topological superconductivity is studied. It is shown that the zero energy mode exists under some specific conditions. The wave function of zero mode is calculated and it is shown that the Majorana quasi-particles exist on the ends of each strand. By calculating the winding number, we show that the topological phase transition can happen if the hopping integral between two strands is very smaller than the pairing potential between the Cooper pairs. It means that the dsDNA behaves as a trivial superconductor, commonly, but single-stranded DNA (or two coupled ssDNA with very small hopping between them) may behave as a non-trivial superconductor. Finally, we suggest an experimental setup for probable detection of Majorana quasi-particle in DNA.     

MgB_2超导体块材的高压合成

利用高压烧结的方法成功地合成出块状MgB2 超导体。所获得的样品具有良好的单相性。低温磁性测量实验和电阻测量实验均表明样品具有高于 39K的超导转变温度。实验结果表明 ,高压合成是制备和研究MgB2 新型金属间化合物超导体的一种非常有效的手段