超导体的交流磁化率测量

本文探讨利用交流磁化率研究超导电性.测量了MgB2样品的若干超导电参量,包括临界温度Tc,上临界磁场Hc2,临界电流密度Jc,不可逆场Hirr等与温度的关系,交流损耗与频率和温度的关系等.此外还简要介绍了交流磁化率在磁通动力学和非线性性质方面的研究方法.     

在小型制冷机中测量薄片交流磁化率以决定超导转变温度的装置

本文介绍了一种利用电磁感应原理和超导磁效应,在小型制冷机中测量超导体转变温度的装置.本装置包括密闭的真空室、压缩制冷机、真空泵、真空计、锁相放大器、温控仪、计算机、线圈绕组.其中,线圈绕组置于真空室内,由初级线圈和次级线圈组成,初级线圈和次级线圈分别绕制在两个线圈骨架上;被测超导薄片材料放置于初级线圈和次级线圈之间;压缩制冷机用来为超导材料制冷;真空泵用来对真空室抽真空;温控仪用来测量和控制真空室内的温度;锁相放大器为初级线圈提供交流电压信号,并测量次级线圈的电信号以得到交流磁化率值;计算机记录温控仪测得的温度数据和锁相放大器测得的次级线圈的电压信号,并显示锁相放大器测得的次级线圈的电压信号随温度变化的曲线.实验证明该装置可以通过测量超导体交流磁化率的变化测得超导转变温度,具有自动化测量及测试成本低等特点.     

超导混合态椭球的交流磁化率

本文研究了处于混合态非均匀磁化的高温超导椭球在线性响应区的交流磁化率，给出了不同离心率的非均匀磁化椭球在直流磁场和小交流磁场中的交流磁化率的解析解析阐明了样品几何形状（退磁因子）对交流磁化率的影响文中研究了处于超地椭球的交流磁化率。     

纳米尺度下超导体和拓扑绝缘体的电输运特性

超导体和拓扑绝缘体研究是当前凝聚态物理领域中的重大课题.文章重点介绍了作者所在实验室在纳米超导和拓扑绝缘体电输运领域的实验进展,其中包括金属和铁磁纳米线中的超导近邻效应、半金属纳米线中的新奇超导特性、拓扑绝缘体薄膜中的量子输运以及超导态-拓扑量子态的相互作用等,并对该领域的进一步发展进行了展望.     

提高锁相放大器测量交流磁化率精度的方法

在交流磁化率的测量实验中,互感法是常用方法,通常是利用线圈绕组和锁相放大器组合来测量样品在线圈中引起的电感变化.但这种方法其背景信号随绕组的增加而增大.本文采用2对线圈绕组并把两初级线圈反绕、两次级线圈同向绕的设计,减小背景信号,实际使用证明,经改进后测量灵敏度和测试精度都有很大提高.     

熔融织构的YBCO超导体的交流磁化率频率响应

在涡漩玻璃理论的基础上，我们推导出了交流磁化率的虚部峰值温度和频率的标度方程，可表示为Tp=Cf^1/（v（z-1））．实验结果表明我们的标度方程和实验符合得很好．利用标度方程可以确定样品在磁场下的涡漩玻璃转变温度Tg和指数v（z-1）的数值．     

熔融织构的YBCO超导体的交流磁化率频率响应

在涡漩玻璃理论的基础上,我们推导出了交流磁化率的虚部峰值温度和频率的标度方程,可表示为Tp=Cf1/(v(z-1)).实验结果表明我们的标度方程和实验符合得很好.利用标度方程可以确定样品在磁场下的涡漩玻璃转变温度Tg和指数v(z-1)的数值.     

TFA-MOD法YBa2Cu3O（7-δ）超导薄膜的交流磁化率研究

通过三氟乙酸盐-金属有机沉积(TFA-MOD)方法在LaAlO3单晶基片上制备了YBa2Cu3O7-δ(YBCO)高温超导薄膜,并测定在垂直于a-b面的外加交变磁场下薄膜样品的交流磁化率随温度的变化关系,在频率543.2 Hz、振幅1 V条件下,样品的超导转变发生在91 K附近.交流磁化率虚部峰值温度Tp随着磁场振幅的变大而向低温区移动,随着频率的增大而向高温区移动,这些现象与磁通蠕动模型相一致,峰值温度TP与外交变磁场的振幅Hac满足Hcaε(1-Tp/Tct)n的关系,其n～1.55;而与频率之间满足1/Tp与lnf的线性关系,可能是由于TAF-MOD薄膜样品中较多孔洞和非位错缺陷的出现使其偏离一般物理气相沉积薄膜样品具有的SNS型连接(n～2).根据频率f与温度T的Arrhenius公式以及二维简约的Bean模型,得到当前薄膜样品的激活能U与电流密度J、以及J与温度T的标度关系.     

磁场下Tl2Ba2Ca2Cu3Ox单相超导体交流磁化率的研究

测量了 Tl_2Ba_2Ca_2Cu_3O_z 单相超导体在不同外磁场及不同测量频率下的交流磁化率实部和虚部随温度的变化关系,发现实部曲线的起始转变温度 T_(onset)和虚部曲线的峰值温度 T_m,在一定测量频率下都与外磁场的2/3次方成线性关系,在一定外磁场下 T_(onset)与测量频率的对数成线性关系.并且我们认为 T_(onset)和 T_m 都不是一般意义的 T_c,而是受磁通格子熔化影响较大的特殊温度点,所以对 T_(onset)及 T_m 的研究可以反映超导体的磁通格子熔化特性,很值得进一步的研究.     

Rb3C60超导薄膜的电输运性质

在成功制备Ｃ６０外延薄膜的基础上，我们又尝试了碱金属Ｒｂ对该膜的掺杂。结果表明，从室温至８０Ｋ左右，Ｒｂ３Ｃ６０薄膜的电阻温度系数大于０，并近似符合公式：ρ（Ｔ）＝ａ＋ｂＴ＾２，与单晶样品一致。但是，当温度低于８０Ｋ以后，出现了弱的对数局域。在５Ｋ、零场下，样品的Ｊｃ值一般为１０＾３－１０＾４Ａ／ｃｍ＾２，且有如下规律：Ｊｃ（Ｔ）＝Ｊｃ（０）（１－Ｔ／Ｔｃ）＾ａ，α＝１．３－２．０。此外，我们还测     

高温超导转变温度电输运测量的影响因素研究

介绍了一种基于小型制冷机的高温超导转变温度测量装置,并且探究了在这套装置基础上进行实验所需要的测试条件。实验包括三项内容:(1)比较了电流换向法、三点Delta法以及偏置欧姆法三种消除热电势的方法,并最终决定采用适合本装置的电流换向法进行测量;(2)对通入样品的测试电流大小对测试的影响进行了研究,进而确定了进行高温超导材料转变温度测试时所通入电流的范围;(3)对钇系和铊系两种高温铜氧化物超导体进行反复测量,确定最大重复性误差小于0.4K。     

Calculating transport AC losses in stacks of high temperature superconductor coated conductors with magnetic substrates using FEM

In this paper, the authors investigate the electromagnetic properties of stacks of high temperature superconductor (HTS) coated conductors with a particular focus on calculating the total transport AC loss. The cross-section of superconducting cables and coils is often modeled as a two-dimensional stack of coated conductors, and these stacks can be used to estimate the AC loss of a practical device. This paper uses a symmetric two dimensional (2D) finite element model based on the H formulation, and a detailed investigation into the effects of a magnetic substrate on the transport AC loss of a stack is presented. The number of coated conductors in each stack is varied from 1 to 150, and three types of substrate are compared: non-magnetic weakly magnetic and strongly magnetic. The non-magnetic substrate model is comparable with results from existing models for the limiting cases of a single tape (Norris) and an infinite stack (Clem). The presence of a magnetic substrate increases the total AC loss of the stack, due to an increased localized magnetic flux density, and the stronger the magnetic material, the further the flux penetrates into the stack overall. The AC loss is calculated for certain tapes within the stack, and the differences and similarities between the losses throughout the stack are explained using the magnetic flux penetration and current density distributions in those tapes. The ferromagnetic loss of the substrate itself is found to be negligible in most cases, except for small magnitudes of current. Applying these findings to practical applications, where AC transport current is involved, superconducting coils should be wound where possible using coated conductors with a non-magnetic substrate to reduce the total AC loss in the coil.     

AC susceptibility of magnetic markers in suspension for liquid phase immunoassay

AC susceptibility of magnetic markers in solution was studied for biosensor applications. First, frequency dependence of the susceptibility was measured, and size distribution of the markers was estimated by analyzing the experimental result with the so-called singular value decomposition (SVD) method. The size distribution estimated with the magnetic measurement agreed with that obtained from conventional optical measurement. Next, susceptibility measurement was applied to the liquid-phase immunoassay without bound/free (B/F) separation. We performed the detection of biotin-coated polymer beads in suspension using avidin-coated magnetic markers. Changes of the susceptibility and the size distribution caused by the binding reaction were shown.     

A model study of quark-number susceptibility at finite chemical potential and temperature

In this Letter, we try to give a direct method for calculating the quark-number susceptibility (QNS) at finite chemical potential and temperature. In our approach the QNS is given by a formula which solely involves (the dressed quark propagator at finite chemical potential μ and temperature T). From this the QNS at finite chemical potential and temperature is calculated in the framework of rainbow-ladder approximation of the Dyson–Schwinger approach using the meromorphic quark propagator proposed in [M.S. Bhagwat, M.A. Pichowsky, P.C. Tandy, Phys. Rev. D 67 (2003) 054019]. It is found that the QNS χ(μ,T) has a singularity when μ comes close to a critical value μ₀, and the susceptibility as a function of T becomes discontinuous at some values of T when μ is near μ₀. At high temperature the QNS approaches the ideal quark gas result, while at very small temperature (T<40 MeV) the susceptibility equals zero. For all values of μ we studied, the susceptibility shows a rapid increase near T=120–140 MeV, which could be regarded as the signal of a crossover.     

Numerical and theoretical evaluations of AC losses for single and infinite numbers of superconductor strips with direct and alternating transport currents in external AC magnetic field

AC losses in a superconductor strip are numerically evaluated by means of a finite element method formulated with a current vector potential. The expressions of AC losses in an infinite slab that corresponds to a simple model of infinitely stacked strips are also derived theoretically. It is assumed that the voltage-current characteristics of the superconductors are represented by Bean’s critical state model. The typical operation pattern of a Superconducting Magnetic Energy Storage (SMES) coil with direct and alternating transport currents in an external AC magnetic field is taken into account as the electromagnetic environment for both the single strip and the infinite slab. By using the obtained results of AC losses, the influences of the transport currents on the total losses are discussed quantitatively.     

Even harmonics of AC susceptibility in some sintered pellets of high temperature superconductors

The real (χ′ _n) and imaginary (χ″ _n) parts of even harmonic susceptibility (n⩽6) are measured as a function of external DC field (H _dc) in the field increasing (H↑) and decreasing (H↑) cycle. Hysteresis is observed betweenH↑ andH↓ cycles. In theH↓ cycle, at a field,H _comp=4·2G, bothχ′ andχ″ of all the even harmonics vanish indicating a true cancellation of fields in the intergrain region (H _eff≈0) caused by the balance betweenH _dc and the remanent magnetization of the grains. The position of the extrema and the zero of the various harmonics undergo a shift proportional to the remanent magnetization of the grains at that particular field.     

AC and DC magnetic studies of critical currents in ceramics of the Ho1−xPrxBa2Cu3O7−δ system

AC magnetic susceptibility measurements were performed for ceramics of the Ho_1−xPr_xBa₂Cu₃O_7−δ system as a function of temperature, frequency, field amplitude and static magnetic field. From the measurements, temperature dependences of intergranular critical currents and corresponding pinning energies at grain boundaries were determined for different Pr concentrations. These critical currents strongly decrease with Pr doping. They are limited by superconductor–insulator–superconductor or superconductor–normal metal–superconductor junctions and for higher Pr concentrations additionally suppressed by flux creep. Also intragranular critical current at 4.3 K was determined as a function of Pr concentration from magnetic hysteresis loops. It is a monotonically decreasing function of Pr doping.     

Comparison between self-field AC losses of a single-phase Bi-2223 cable measured by electric and calorimetric methods

High-T_c power cables based on Bi-2223 tapes are under development worldwide. An important parameter for design and cost optimization is the AC loss expected during 50/60 Hz operation. Its measurement can be affected by several experimental problems that may alter the results. For this purpose, the 50 Hz losses of a 1.5 m long conductor, made by helically winding four layers of Bi-2223 tapes, were measured simultaneously by electrical and calorimetric methods in a wide range of currents up to 2 kA in order to investigate the reasons for possible discrepancies. The present, specifically designed, calorimetric system is not based on temperature distribution analysis but on the measurement of the gas boil-off rate of the liquid nitrogen bath.     

Superconducting transition temperature for semimetals like bismuth

The superconducting transition temperatureT _c for semimetals like bismuth has been calculated as a function of the densityn _c of the electron and hole charge carriers. A simplified two-band model for describing the longitudinal dielectric function for such a system has been used in our model calculation. We find that the attractive interaction responsible for the instability of the normal ground state comes not only from the exchange of lattice phonons, but also from the electronhole sound mode, provided the ratio of the averaged hole to electron mass,m _h /m _e ≠ 1. We have compared our theoretical values ofT _c (n _c ) with experimental results for bismuth under hydrostatic pressure, and find reasonable agreement ifm _h /m _e is assumed to have a value which is only slightly larger than that at atmospheric pressures. A linear variation of the negative band gap as a function of pressure has been assumed for the sake of this comparison.     

Comparison of transport and magnetic AC losses in Bi-2223/Ag tapes — the role of superconducting core geometry

AC losses were measured by 4-probe transport method and by external magnetization method in three samples of Bi-2223/Ag tape: a multifilamentary tape with separated filaments, another multifilamentary tape with ‘bridges' between filaments, and a two-shell tape. The transport losses agreed with those calculated using I_c from DC experiment. Magnetization experiments gave indications about the various paths of induced currents. For the tape with well separated filaments the main part of screening current closes inside individual filaments. Additional screening of the whole filamentary zone involves the normal metal matrix, leading to frequency dependent losses. In the case of tape with ‘bridged' filaments, supercurrents interconnect the filaments into bundles whose screening (and loss) is frequency-independent. Matching the experimental data indicates that a typical bundle was composed of 8 filaments. Magnetic losses of the two-shell tape were explained by a model for magnetization of superconducting wire with elliptical cross-section.