Quantum Phase in Nanoscopic Superconductors

Using the pseudospin representation and the SU(2) phase operators we introduce a complex parameter to characterize both infinite and finite superconducting systems. While in the bulk limit the parameter becomes identical to the conventional order parameter, in the nanoscopic limit its modulus reduces to the number parity effect parameter and its phase takes discrete values. We evaluate the Josephson coupling energy and show that in bulk superconductor it reproduces the conventional expression and in the nanoscopic limit it leads to quantized Josephson effect. Finally, we study the phase flow or dual Josephson effect in a superconductor with fixed number of electrons.     

Vortex Self-Organization in the Presence of Magnetic Pinning Arrays

Lateral nanostructuring is an efficient tool to control vortex confinement in superconductors. This will be illustrated by studying pinning phenomena in type-II superconducting Pb films with a lattice of submicron magnetic dots. We consider rectangular Co dots with in-plane magnetization and circular Co/Pt dots with out-of-plane magnetization. The domain structure of the Co dots can be changed from multi- to single-domain, resulting in an enhancement of their stray field. After covering this Co dot array with a Pb film, we demonstrate the influence of the local magnetic stray field of the dots on their flux pinning efficiency. The Co/Pt dots have a single-domain structure with their magnetic moment out of plane. Depending on the magnetic history, the magnetic moment of all dots can be aligned in positive or negative direction, or a random distribution of positive and negative magnetic moments of the dots can be achieved. For a Pb film covering this Co/Pt dot array, we observe an asymmetric magnetization loop due to the magnetic interactions between the vortices and the magnetic dots.     

Synthesis and Physical Properties of the Hg-Based Superconductors with the 1212 Structure: Hg0.7 V0.3Sr2Ca1-xYxCu2O6+gd

A partial substitution of vanadium at the Hg site helps to stabilize the Sr analogue of the Hg-1212 phase. Simultaneous replacement of Ca by a rare earth ion is found necessary to form single-phase materials. Crystalline and nearly single-phase compounds were synthesized for x≥0.4 in the system Hg_0.7V_0.3Sr₂Ca{i−x}Y _x Cu₂O_6+δ. The lattice parameters show an increase in thea axis and a significant decrease in thec axis with increase inx. A maximumT _c of 85K was obtained for x= 0.4.T _c decreases with increase in Y concentration. However, all the compositions in the single-phase region show a broad superconducting transition. Annealing in vacuum and re-annealing in oxygen helps to improve the diamagnetic volume fraction, but the nature of the transition remained unaffected. Studies on the magnetizationhysteresis loops suggest a poor flux pinning nature of the compounds. Although the compounds have a large decrease in anisotropy along thec axis, partial occupancy of Y³⁺ in the Ca²⁺ layer adjacent to the CuO₂ conduction plane is detrimental to the flux pinning behavior. This study suggests that the presenceof defects in the proximity of the conduction plane in the high-T _c copper oxide system has an adverse effect on the superconducting properties.     

Fluctuations of the Order Parameter''s Phase in Layered Superconductors

The influence of the quantum fluctuations of the order parameter's phase on the critical temperature T _c is studied for a Josephson coupled layered superconductor. Two characteristic critical temperatures exist for a system, namely the superconducting critical temperature T ⁽²⁾ _c for a single layer estimated by the mean-field theory and the transition temperature for the outset or the superconducting phase coherence T* _c . The true critical temperature T _c is shown to vary inside the intervals T* _c T _c T ⁽²⁾ _c . For a strong quantum phase fluctuation limit, the superconducting layers become decoupled.     

Pressure Dependence of Tc in High Temperature Superconductors: Role of Interlayer Interactions

We have studied the role of interlayer interactions (W) in the pressure dependence of T _c of layered superconductors. The expressions for dT _c /dP are obtained by including the effects of layered structure within the framework of two different proposed models, namely the negative-U Hubbard model and the Hirsch model. We observe that the inclusion of interlayer interaction provides better explanation of pressure dependence of T _c . Our numerical results show that the systems having one CuO₂ layer per unit cell may be well described by small values of W while the larger values of W accounts for the systems having two or more superconducting layers in a unit cell. The calculated values of dT _c /dP vs. W are found to be in good agreement with those of experimental results obtained for various high T _c superconductors of cuprate family.     

Power Handling of High Temperature Superconductors Measured in an HTS/Sapphire Resonator: A Power Reference Film

The power handling of HTS thin films was accurately characterized in an 8.15 GHz HTS/Sapphire resonator as a function of temperature. The surface resistance of a superconductor depends upon the level of the surface magnetic field especially if the value exceeds the lower critical magnetic field. A four-film power round robin was performed, which allowed for both the power handling of each of the HTS films and the statistics about the measurement repeatability to be calculated. A highly nonlinear empirically derived formula was identified and used to interpolate the measurement data. These four HTS films can now be used as power-qualified reference films for future HTS/Sapphire resonator measurements.     

各向异性介质二维稳态热传导问题的分析解

本文利用双变量函数及其偏导数在闭区间上完整的双重付里叶级数表达式求得矩形区域中各向异性介质稳态热传导问题的分析解。文中例子给出各向异性介质内的温度场和热流密度的数字结果,并讨论介质的各向异性效应。     

A Study of the Thallium- and Bismuth-based High-temperature Superconductors in the Framework of the Generalized BCS Equations

Presented here in the framework of the generalized BCS equations??which are based on multiple phonon exchanges for the formation of Cooper pairs (CPs)??is continuation of a study carried out earlier for MgB₂ Nb₃Sn and YBCO. Addressed now are the principal members of the Tl- and Bi-based high-T _c superconductors. For any of these, with the input of any two parameters from the set {critical temperature, two gaps}, and the Debye temperature, it is shown that one can calculate the remaining parameter. This is similar to what normal BCS equations achieve for a simple superconductor. Further, we show that these equations lead to upper bounds on the gap- and the T _c -values of a HTSC, beyond which one must invoke 2/3-phonon mechanism for the formation of CPs. Our considerations suggest the interesting possibility that senior members of the said families of cuprates may be three-gap SCs.     

Microwave Spectroscopy in YBCO Superconductors: Influence of Neutron Irradiation on the 123 Phase

The high-T _c superconducting system YBa₂Cu₃O:Li (123) was investigated by microwave absorption. The effect of neutron irradiation on the 123 phase as well as the dependence of the resulting modification both on the neutron fluence and on the doping behavior with Li are discussed. The results show a significant improvement of the superconducting characteristics: critical temperature, phase homogeneity, and critical current densities for a certain neutron fluence. We have found that the optinum neutron fluence for irradiation is 4.98×10²¹ m^–2.     

Impurities in d-Wave Superconductors: Effect of Singular Density of States

We analyzed the effect of nonmagnetic impurities in d-wave superconductors with an energy dependent density of states. Two singular forms for the density of states have been considered. The strength of singularity affect the degradation of the critical temperature and reduces the pair-breaking effect.     

Phenomenological Anisotropic Study of Surface Finish in Pack Rolling

A phenomenological anisotropic model has been presented for the surface roughness modeling of pack rolling. The model is an assembly of grains in different orientations and sizes. The grain size is assumed to be in log-normal distribution. To model the macro anisotropic mechanical behavior of the grains induced by the slip deformation, the grains are assumed as isolated anisotropic units. The units have different mechanic behavior, and depend on the crystallographic orientations and the external loading as well as the interaction of the adjunctive grains. In the paper, the material properties of the grains are assumed as uniform distributions. The roughness of the contact surfaces depends on the distribution types and the scatters of the distributions. It is found that the initial roughness of the contact surfaces has a little influence on the surface roughness when the rolling deformation is large. The comparison between the phenomenological model and crystallographic model shows that the phenomenological model can also give out a reasonable result, while it only takes much less CPU time. The agreement between the single sheet model and the pack rolling model shows that in a certain degree the pack rolling model can be replaced by the single sheet model to decrease the CPU time.     

反向回转双激振器振动机控制同步的理论研究

本文对两激振器作反向回转的振动机控制同步进行了理论研究和计算机仿真。推导出了考虑质心偏移时两主轴的转矩差方程，在此基础上建立了控制系统的数学模型，设计了具有鲁棒性的非线性变结构滑模控制器．仿真结果表明，控制同步不仅可以消除由质心偏移、电机特性差等原因引起的稳态相位差，同时还能大大降低瞬态相位差，从而使机体的振动方向保持不变，减小起、制动过程中机体的摇摆振动．     

微纳米尺度单晶铜各向异性表面切削特性实验研究

利用纳米压痕仪和原子力显微镜对微纳米尺度下单晶铜各向异性表面在不同载荷和刻划速度下的切削特性进行实验研究。结果表明:单晶铜各晶面表面在较低载荷下,划痕细小且不明显。随着载荷的逐渐增大,划痕深度和宽度逐渐变大,并形成明显的沟槽,在沟槽的两侧出现明显的侧流现象,探针前方出现切屑堆积,尤其单晶铜Cu(100)切屑堆积较明显;单晶铜Cu(100)在刻划速度为10μm/s、50μm/s时,切削力无明显变化规律,其余两晶向都是在同等载荷下,刻划速度越大,切削力越大。随着刻划速度的增大,切削力趋于稳定;载荷越大,切削力越大,其相应摩擦系数也增大。     

新型折叠夹芯结构材料研究进展

折叠夹芯结构是一种具有轻质、高比强、高比刚度以及多功能潜力的新型三维结构材料,近几年受到国内外学者的极大关注.它将成为新一代结构功能一体化的理想结构材料.主要概述了折叠夹芯结构的发展历程,包括折叠芯材的拓扑构型设计、制备工艺研究以及折叠夹芯结构力学性能、失效方式、数值模拟和隔音、热力学等方面的工作.同时结合折叠夹芯结构的性能和优势,对其可能应用的领域进行预测,并对其未来研究方向进行了展望.     

Anisotropic nanowire growth via a self-confined amorphous template process: A reconsideration on the role of amorphous calcium carbonate

Calcium carbonate crystals with various morphologies have been found in a variety of biospecimens and artificially synthesized structures. Usually, the diversity in morphology can be attributed to different types of interactions between the specific crystal faces and the environment or the templates used for the growth of CaCO₃ crystals. On the other hand, isotropic amorphous calcium carbonate (ACC) has been recognized as the precursor of other crystalline calcium carbonate forms for both in vivo and in vitro systems. However, here we propose a self-confined amorphous template process leading to the anisotropic growth of single-crystalline calcite nanowires. Initiated by the assembly of precipitated nanoparticles, the calcite nanowires grew via the continuous precipitation of partly crystallized ACC nanodroplets onto their tips. Then, the crystalline domains in the tip, which were generated from the partly crystallized nanodroplets, coalesced in the interior of the nanowire to form a single-crystalline core. The ACC domains were left outside and spontaneously formed a protective shell to retard the precipitation of CaCO₃ onto the side surface of the nanowire and thus guided the highly anisotropic growth of nanowires as a template.

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A Hidden Order in the Cuprate Superconductors: The d-Density-Wave Phase

We study the electronic structure near impurities in the d-density-wave (DDW) state, a possible candidate phase for the pseudo-gap region of the high-temperature superconductors. We show that the density of states near a nonmagnetic impurity in the DDW state is qualitatively different from that in a superconductor with d_x ²-y² symmetry. Thus the electronic structure near impuritiescan provide insight into the nature of the two phases recently observed by scanning tunneling microscopy experiments in the superconducting state of underdoped Bi-2212 compounds.     

The Order-Parameter Symmetry and Fermi Surface Topology of 122 Fe-Based Superconductors: A Point-Contact Andreev-Reflection Study

We report on the results of directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe_1?x Co _x )₂As₂ single crystals and epitaxial c-axis oriented films with x=0.08 as well as in Ca(Fe_1?x Co _x )₂As₂ single crystals with x=0.06. The PCAR spectra are analyzed within the two-band 3D version of the Blonder–Tinkham–Klapwijk model for Andreev reflection we recently developed, and that makes use of an analytical expression for the Fermi surface that mimics the one calculated within the Density-Functional Theory (DFT). The spectra in Ca(Fe_0.94Co_0.06)₂As₂ unambiguously demonstrate the presence of nodes or zeros in the small gap. In Ba(Fe_0.92Co_0.08)₂As₂, the ab-plane spectra in single crystals can be fitted by assuming two nodeless gaps, but this model fails to fit the c-axis ones in epitaxial films. All these results are discussed in comparison with recent theoretical predictions about the occurrence of accidental 3D nodes and the presence of “hot spots” in the gaps of 122 compounds.     

Point contact spectroscopy in Fe-based superconductors: Recent advancements and future challenges

Iron-based superconductors (FeSC) present an unprecedented variety of features both in the superconducting and in the normal state. Different families differ in the value of the critical temperature, in the shape of the Fermi surface, in the existence or absence of quasi-nesting conditions, in the range of doping in which the antiferromagnetic (AFM) and the superconducting phase coexist and in the structure of the order parameter in the reciprocal space, and so on. In this paper the most important results of point-contact spectroscopy (PCS) in Fe-based superconductors are reviewed, and the most recent advances are described with the aim to discuss the future perspectives and challenges of this spectroscopic technique in the characterization of the superconducting properties of these complex compounds. One of the main challenges, faced so far only by a few researchers in the PCS field, is to fully explore the phase diagram of these materials, as a function of doping or pressure, to understand the interplay between superconductivity and magnetism, the effect of intrinsic or extrinsic inhomogeneities, the role of spin fluctuations (SFs) in the pairing, the symmetry and the structure of the order parameter(s).