高温超导混合配对态与磁通涡旋格子

建立在对基于Gor’kov方法而导出的微观GinzburgLandau方程的分析揭示了高温超导体YBa2Cu3O7配对态对称性和磁通涡旋格子结构.分析指出,存在一个格子转变温度T,当温度高于T时,超导基态显示dx2-y2波对称性特征;低于该温度s波沟道幅值成为可观的量级;超导基态为混合s-dx2-y2态.对应单分量波函数磁通涡旋格子为三角的结构;而稳定的斜格子反映出混合波特征.s与d沟道间耦合约束了磁场下dx2-x2波对称性自由度,而对高温超导反常输运行为如上临界磁场温度曲线上翘现象所负责 关键词： 高温超导 GinzburgLandau理论 磁通涡旋     

Experimental demonstration of vortex pancake in high temperature superconductor

In order to demonstrate the existence of the vortex pancake in high temperature superconductor experimentally, a configuration in which the current and voltage electrodes lies separately on the top and bottom surface is used. The E-j relation obtained with this electrodes spatial configuration is different from the expected E-j behavior of the stiff vortex line model. Thus, the current results support the existence of the vortex pancake in high temperature superconductor.     

Investigating the vortex melting phenomenon in BSCCO crystals using magneto-optical imaging technique

Using a novel differential magneto-optical imaging technique we investigate the phenomenon of vortex lattice melting in crystals of Bi₂Sr₂CaCu₂O₈ (BSCCO). The images of melting reveal complex patterns in the formation and evolution of the vortex solid-liquid interface with varying field (H)/temperature (T). We believe that the complex melting patterns are due to a random distribution of material disorder/inhomogeneities across the sample, which create fluctuations in the local melting temperature or field value. To study the fluctuations in the local melting temperature/field, we have constructed maps of the melting landscape T _m(H, r), viz., the melting temperature (T _m) at a given location (r) in the sample at a given field (H). A study of these melting landscapes reveals an unexpected feature: the melting landscape is not fixed, but changes rather dramatically with varying field and temperature along the melting line. It is concluded that the changes in both the scale and shape of the landscape result from the competing contributions of different types of quenched disorder which have opposite effects on the local melting transition.     

Theory of specific heat of vortex liquid of high T_c superconductors

Superconducting thermal fluctuation(STF) plays an important role in both thermodynamic and transport properties in the vortex liquid phase of high T_c superconductors.It was widely observed in the vicinity of the critical transition temperature.In the framework of Ginz burg-Landau-La wrence-Doniach theory in magnetic field,a self-consistent analysis of STF including all Landau levels is given.Besides that,we calculate the contribution of STF to specific heat in vortex liquid phase for high T_c cuprate superconductors,and the fitting results are in good agreement with experimental data.     

Melting of heterogeneous vortex matter: The vortex ‘nanoliquid’

Disorder and porosity are parameters that strongly influence the physical behavior of materials, including their mechanical, electrical, magnetic and optical properties. Vortices in superconductors can provide important insight into the effects of disorder because their size is comparable to characteristic sizes of nanofabricated structures. Here we present experimental evidence for a novel form of vortex matter that consists of inter-connected nanodroplets of vortex liquid caged in the pores of a solid vortex structure, like a liquid permeated into a nanoporous solid skeleton. Our nanoporous skeleton is formed by vortices pinned by correlated disorder created by high-energy heavy ion irradiation. By sweeping the applied magnetic field, the number of vortices in the nanodroplets is varied continuously from a few to several hundred. Upon cooling, the caged nanodroplets freeze into ordered nanocrystals through either a first-order or a continuous transition, whereas at high temperatures a uniform liquid phase is formed upon delocalization-induced melting of the solid skeleton. This new vortex nanoliquid displays unique properties and symmetries that are distinct from both solid and liquid phases.     

Charged vortex structure in high-temperature superconductors

By using a model Hamiltonian with competing antiferromagnetic (AFM) spin density wave (SDW) and d-wave superconductivity orders, the effect of next-nearest-neighbour (nnn) hopping on spin and charge structures in high-temperature superconductors is investigated at finite temperatures. For an optimally doped sample, we find that the AFM order magnitude in the vortex core is firstly enhanced and then suppressed, accompanied with a ``positively → negatively → positively" charged vortex structure transition with increasing nnn hopping strength, which implies that the AFM order is unnecessarily bounded to an electron-rich vortex core. In addition, a charge ordering pattern with four negatively charged peaks localized in a small region is also found around the vortex core centre without net charge. Recent scanning-tunneling-microscopy experimental observations of the checkerboard structure are hopefully understood.     

高温超导体磁通钉扎和磁通动力学研究简介

ψ=ψ0eiφ行为用统一的波函数进行描述,其相位φ在宏观尺度上是相同的.当磁场低于一定值的时候,在超Φ0=h/2e保证最大的界面面积,降低系统能量.该最小的磁通束被称为磁通量子,其磁通量是(h为普朗克超导态是一个宏观量子相干态,其载流子是库珀对.在没有外加磁场和电流的时候,这些库珀对的运动导体的边界处穿透深度内会出现一...     

Nuclear magnetic resonance studies of vortices in high temperature superconductors

The distinct distribution of local magnetic fields due to superconducting vortices can be detected with nuclear magnetic resonance (NMR) and used to investigate vortices and related physical properties of extreme type II superconductivity. This review summarizes work on high temperature superconductors (HTS) including cuprates and pnictide materials. Recent experimental results are presented which reveal the nature of vortex matter and novel electronic states. For example, the NMR spectrum has been found to provide a sharp indication of the vortex melting transition. In the vortex solid a frequency dependent spin–lattice relaxation has been reported in cuprates, including YBa₂Cu₃O_7-x, Bi₂SrCa₂Cu₂O_8+δ, and Tl₂Ba₂CuO_6+δ. These results have initiated a new spectroscopy via Doppler shifted nodal quasiparticles for the investigation of vortices. At very high magnetic fields this approach is a promising method for the study of vortex core excitations. These measurements have been used to quantify an induced spin density wave near the vortex cores in Bi₂SrCa₂Cu₂O_8+δ. Although the cuprates have a different superconducting order parameter than the iron arsenide superconductors there are, nonetheless, some striking similarities between them regarding vortex dynamics and frequency dependent relaxation.     

欠掺杂高温超导体中的涡旋电荷结构相变

利用平均场t-t′-U-V-V_c模型,通过自洽求解Bogoliubov-de Gennes方程,研究了高温超导体中涡旋结构的相变.发现增大原位排斥势U,自旋密度波、电荷密度波以及d波序参量由棋盘结构转变为条纹结构.模型哈密顿量中引入合适强度的长程库仑势后,欠掺杂高温超导体样品中也可以出现二维或者棋盘结构,结果与文献报道的扫描隧道显微镜实验结果一致. 关键词： 高温超导 涡旋结构 长程库仑势     

Application of longitudinal generalized magneto-optical ellipsometry in magnetic ultrathin films

The longitudinal generalized magneto-optical ellipsometry(GME) method is extended to the measurement of threelayer ultrathin magnetic films. In this work, the theory of the reflection matrix is introduced into the GME measurement to obtain the reflective matrix parameters of ultrathin multilayer magnetic films with different thicknesses. After that, a spectroscopic ellipsometry is used to determine the optical parameter and the thickness of every layer of these samples, then the magneto-optical coupling constant of the multilayer magnetic ultrathin film can be obtained. After measurements of a series of ultrathin Fe films, the results show that the magneto-optical coupling constant Q is independent of the thickness of the magnetic film. The magneto-optical Kerr rotations and ellipticity are measured to confirm the validity of this experiment. Combined with the optical constants and the Q constant, the Kerr rotations and ellipticity are calculated in theory. The results show that the theoretical curve fits very well with the experimental data.     

高温超导体磁通动力学和混合态相图(II)

文章简要介绍了高温超导体磁通动力学和混合态物理在过去十余年的发展.高温超导体由于其自身的一些特点,使得它与常规超导体相比较拥有极其丰富的相图,磁通动力学也表现出了非常丰富的研究内容,很多新的概念被提出,新的现象被观察到.比如说涡旋玻璃态,集体钉扎和蠕动,磁通格子的一级和二级熔化相变,布拉格玻璃,峰值效应,二维涡旋饼态,Josephson 磁通运动等等,均是在高温超导体发现之后提出来的新的概念或新发现的现象.有些研究结果目前尚无定论,如关于涡旋玻璃态存在与否的争论至今仍然在进行,但是这些研究内容无疑会大大促进超导物理的发展.高温超导体磁通动力学纷繁复杂的研究内容可以归结为三个相互关联的数字：Ginzburg数(Tc/H2cεζ3)2/2,量子电阻Qu=(e2/)(ρn/εζ),和临界电流的比值jc/j0,这里ζ是相干长度,Hc是热力学临界磁场,ε是有效质量的各向异性度,ρn是正常态电阻率,jc是零温临界电流,j0是拆对临界电流.对于高温超导体前两个数值（Ginzburg数和量子电阻）很大,而临界电流比值较小,因此导致有强的热涨落和量子涨落,以及很强的磁通运动行为（对应小的实测临界电流）.磁通动力学的研究从更深层次影响超导体的临界电流问题和强电应用的发展,最后简要地介绍了这方面的情况.     

通过磁光效应研究磁性液体在外场下的颗粒成链现象

在对磁性液体的磁光效应的理论分析与实验测量的基础上讨论了其在磁场下的各向异性现象.首先,通过电磁理论对成链的各向异性介质模型的介电张量形式进行了一般性的分析,然后,基于磁性液体的微观理论模型讨论了磁性液体在外加磁场下出现凝聚成链的行为.基于上面的结果,我们将实验可观测量——两种传输模式的折射率差和不同颗粒数的链(长度不同的链)的比例以及磁场强度建立联系.最后,在实验上探测了磁性液体薄膜的透射光偏振状态变化随外磁场的变化,并对测得的数据进行计算机拟合分析,验证了不同长度的链的数目之间的比例关系.     

Nonequilibrium dynamics in type-II superconductors with inhomogeneous vortex pinning

We study numerically the dynamics relating to negative vortex motion in inhomogeneous pinning systems. We show that this dynamical phenomenon results from the internal field effect produced by the growing local barriers with decreasing temperature. We find that the negative motion is characterized by a peak of negative voltage or resistance in resistance–temperature transport measurements. We also demonstrate that the time window to observe the negative motion is determined by the magnitude of driving force in addition to the temperature scanning rate.     

高温超导体磁通动力学和混合态相图(Ⅰ)

文章简要介绍了高温超导体磁通动力学和混合态物理在过去十余年的发展。高温超导体由于其自身的一些特点，使得它与常规超导体相比较拥有极其丰富的相图，磁通动力学也表现出了非常丰富的研究内容，很多新的概念被提出，新的现象被观察到。比如说涡旋玻璃态，集体钉扎和蠕动，磁通格子的一级和二级熔化相变，布拉格玻璃，峰值效应，二维涡旋饼态，Josephson磁通运动等等，均是在高温超导体发现之后提出来的新的概念或新发现的现象。有些研究结果目前尚无定论，如关于涡旋玻璃态存在与否的争论至今仍然在进行，但是这些研究内容无疑会大大促进超导物理的发展。高温超导体磁通动力学纷繁复杂的研究内容可以归结为三个相互关联的数字：Ginzburg数（T/He^2εξ^3）^2／2，量子电阻Qu=（e^2／h）（ρn/／εξ），和临界电流的比值jc/jo,这里ξ是相干长度，Hc是热力学临界磁场，ε是有效质量的各向异性度,ρn是正常态电阻率，jc是零温临界电流，jo是拆对临界电流。对于高温超导体前两个数值（Ginzburg数和量子电阻）很大，而临界电流比值较小，因此导致有强的热涨落和量子涨落，以及很强的磁通运动行为（对应小的实测临界电流）。磁通动力学的研究从更深层次影响超导体的临界电流问题和强电应用的发展，最后简要地介绍了这方面的情况。     

Mechanisms of the pancake vortex and vortex line movement in the high Tc superconductors Bi2Sr2CaCu2O8+δand La1.93Sr0.07CuO4+δ

In the strict sense, it is not very clear why with magnetic field increasing, the normal-superconductive (NS) transition becomes broad for Bi2Sr2CaCu2O8+δ(Bi2212) while the NS transitions are almost parallel for La1.93Sr0.07CuO4+δ(La214). In this paper, R-T relations are measured by the six-probe method. We propose a moving mechanism of the pancake vortex and vortex line for Bi2212. The theoretical curves fit the experiment data well.     

Investigation of ultracold atoms and molecules in a dark magneto-optical trap

In this paper,ultracold atoms and molecules in a dark magneto-optical trap(MOT) are studied via depumping the cesium cold atoms into the dark hyperfine ground state.The collision rate is reduced to 0.45s-1 and the density of the atoms is increased to 5.6×1011cm-3 when the fractional population of the atoms in the bright hyperfine ground state is as low as 0.15.The vibrational spectra of the ultracold cesium molecules are also studied in a standard MOT and in a dark MOT separately.The experimental results are analyzed by using the perturbative quantum approach.     

Quasi-local action of curl-less vector potential on vortex dynamics in superconductors

Studies of the Abrikosov vortex motion in superconductors based on time-dependent Ginzburg–Landau equations reveal an opportunity to detect the values of the Aharonov–Bohm type curl-less vector potentials without closed-loop electron trajectories encompassing the magnetic flux.     

苦瓜中维生素C含量的测定

建立苦瓜中维生素C含量直接测定新方法。以10%盐酸溶液为溶剂,采用紫外分光光度法测定含量。维生素C在0—10μg/mL浓度范围内与吸光度呈良好的线性关系,回归方程为：y=0.04746C＋0.00109,r=0.9997。检出限为0.023μg/mL,加标回收率在94.8%—101.1%之间。该法操作简单,干扰离子少,结果准确,测量快速、准确度和灵敏度高,可应用于维生素C含量的测定。