Preferential diagonal penetration of vortices into square superconducting networks

We have observed vortex penetration into Nb thin films with square arrays of square holes with variable sizes and lattice constants. We find that when the lattice spacing is large and the width of superconducting line is narrow, vortices penetrate diagonally rather than parallel to the nearest neighbor direction. This phenomenon is also confirmed in NbTiN samples with the same geometry. We also confirm that the direction of edge relative to that of hole array is not relevant. Possible origin of such a preferential penetration is proposed.     

Pinning effects on the vortex critical velocity in type-II superconducting thin films

We study the influence of artificial pinning centers on the vortex critical velocity in Al thin films deposited on top of a periodic array of Permalloy (FeNi) square rings. We demonstrate that the field dependence of the flux flow velocity strongly depends on the particular magnetic state of the rings. In particular, we find that, even when the rings are in a flux closure state, i.e. with little stray field, the vortex critical velocity shows a non-monotonic magnetic field dependence. This behaviour is in sharp contrast with the results obtained in a reference plain film, with no rings underneath. A comparison with the intrinsic strong pinning Nb films previously studied, suggests an interpretation in terms of a channel-like motion of vortices, here induced by the artificial pinning structure.     

Dynamic vortex Mott transition in triangular superconducting arrays

The proximity-coupled superconducting island arrays on a metallic film provide an ideal platform to study the phase transition of vortex states under mutual interactions between the vortex and potential landscape. We have developed a top-down microfabrication process for Nb island arrays on Au film by employing an Al hard mask. A current-induced dynamic vortex Mott transition has been observed under the perpendicular magnetic fields of $f$ magnetic flux quantum per unit cell, which is characterized by a dip-to-peak reversal in differential resistance d$V$/d$I$ vs. $f$ curve with the increasing current. The d$V$/d$I$ vs. $I$ characteristics show a scaling behavior near the magnetic fields of $f= {1}/{2}$ and $f=1$, with the critical exponents $\varepsilon$ of 0.45 and 0.3, respectively, suggesting different universality classes at these two fields.     

Development of surface magneto-optical imaging method

We have observed single vortices in a NbTiN thin film by using magneto-optical (MO) imaging method, in which the light reflected from the surface of the sample produces MO contrasts. The gap between the sample and the indicator film is estimated by the Newton ring and curve fitting to the flux density profile of single vortices.     

Magneto-optical imaging of iron-oxypnictide SmFeAsO1−xFx and SmFeAsO1−y

We have prepared iron-oxypnictide SmFeAsO_1−xF_x by ambient-pressure technique and SmFeAsO_1−y by high-pressure technique, and characterized their bulk and local magnetic properties by using SQUID magnetometer and magneto-optical imaging. While the high-pressure samples have densities close to the theoretical value, the ambient-pressure samples have several small voids. Despite these structural differences between the two kinds of samples, they both have superconducting transition temperature above 50 K. In addition, magneto-optical images for both samples show similar kinds of inhomogeneities with large current concentrated in several grains and with small intergranular current. The estimated intragranular currents for both samples are over 10⁵ A/cm² at low temperatures and low fields.     

Direct visualization of the vortex distributions in a superconducting film with a Penrose array of magnetic pinning centers: Symmetry induced giant vortex state

Using scanning Hall probe microscopy a direct visualization of the flux distribution in a Pb film covering a fivefold Penrose array of Co dots is obtained. We demonstrate that stable vortex configurations can be found for fields H ∼ 0.8H₁, H₁ and 1.6H₁, where H₁ corresponds to one flux quantum per pinning site. The vortex pattern at 0.8H₁ corresponds to one vacancy in one of the vertices of the thin tiles whereas at 1.6H₁ the vortex structure can be associated with one interstitial vortex inside each thick tile. Strikingly, for H = 1.6H₁ interstitial and pinned vortices arrange themselves in ring-like structures (“vortex corrals”) which favor the formation of a giant vortex state at their center.     

Effect of surface defects on vortex penetration in small superconducting squares

We report the dependence of the vortex penetration on the number and positions of surface defects in mesoscopic superconducting squares. The vortex penetration is detected by the change in the transport properties of small tunnel junctions attached to the sample (small-tunnel-junction method). We find that for vorticities L = 0 and 1, a defect at the center of a side facilitate the vortex penetration, while a defect at a corner does not affect the vortex penetration field. For higher vorticities, the influence of defects becomes complicated, indicating that not only the existence of surface defects but also the vortex arrangement influences the vortex penetration. Simple explanation for this mesoscopic vortex penetration effect is given.     

Vortex structures in nano-scaled superconducting networks

Using the de Gennes–Alexander equation, we have investigated the stable vortex structures in finite superconducting networks (10 × 10 holes) with disordered wires under an external magnetic field. Vortex structures change gradually with increasing magnetic field. For the network with a disordered wire at the edge, vortices are not pinned disordered hole, but enter into the network at the holes with the disorder. But for the network with two disordered wires, the vortex enters at the hole between two disordered wires. This behavior can be considered as the result of the nonlocality of superconductivity.     

Symmetric behavior of vortex states of superconducting networks in a magnetic field

We present magnetic field dependence of phase transition temperature and vortex configuration of superconducting networks based on theoretical study. The applied magnetic field is called “filling field” that is defined by applied magnetic flux (in unit of the flux quantum) per unit loop of the superconducting network. If a superconducting network is composed of very thin wires whose thicknesses are less than coherence length, the de Gennes–Alexander (dGA) theory is applicable. We have already shown that field dependences of transition temperature curves have symmetric behavior about the filling field of 1/2 by solving the dGA equation numerically in square lattices, honeycomb lattices, cubic lattices and those with randomly lack of wires networks. Many experimental studies also show the symmetric behavior. In this paper, we make an explicit theoretical explanation of symmetric behaviors of superconducting network respect to the applied field.     

Experimental distinction between giant vortex and multivortex states in mesoscopic superconducting squares

We study the distinction between giant vortex states and multivortex states in a thin mesoscopic superconducting square by using the temperature dependence of the vortex expulsion fields. We find that the results agree well with those obtained from the multiple-small-tunnel-junction method, indicating that the distinction by the temperature dependence of the vortex expulsion fields is applicable to superconducting squares.     

Dynamic behavior of diffusion flame interacting with a large-scale vortex by laser imaging techniques

Instantaneous and simultaneous measurements of two-dimensional temperature and OH-LIF profiles by combining Rayleigh scattering with laser induced fluorescence (LIF) were demonstrated in a nitrogen-diluted hydrogen (H₂ 30% + N₂ 70%) laminar normal diffusion flame interacting with a large-scale vortex by oscillating central fuel flow or in an inverse diffusion flame by oscillating central airflow. The dynamic behavior of the diffusion flame extinction and reignition during the flame–vortex interaction processes was investigated. The results obtained are described as follows. (1) The width of the reaction zone decreases remarkably, and a decrease in flame temperature and OH-LIF is seen with increasing central airflow in an inverse diffusion flame. OH-LIF increases, and temperature does not change with increasing central fuel flow in a normal diffusion flame. The computations predict the experimental results well, and it is revealed that flame temperature characteristics result from the preferential diffusion of heat and species, which induces excess enthalpy or on enthalpy deficit, and an increase or decrease in H₂ mole fraction in the flame. (2) When a large velocity fluctuation is given to the central flow, the temperature and the OH-LIF at the reaction zone become thin at the convex and circumferential part of the vortex where a high temperature layer exists, and the temperature at the reaction zone is lowered in the inverse flame and the normal flame. (3) The width and temperature of the reaction zone interacting with the vortex recover quickly to that of the laminar steady flame after the vortex passing in the normal flame, but the recovery to that of the steady flame after the vortex passing is delayed in the inverse flame. (4) When a remarkably large velocity fluctuation is given to the central airflow in the inverse flame, thinning of temperature and reaction zone starts at the convex and circumferential part of the vortex, resulting in a and flame extinction completely occurs at the tail part of the vortex and makes the pair of edge flames. The outside edge flame reignites and connects with the upstream reaction zone. The inside edge flame finally extinguishes as the supply of fuel is interrupted by the outside edge flame.     

Effect of supercurrent injection on vortex penetration and expulsion fields in mesoscopic superconducting squares

We report an experimental study on the effect of supercurrent injection on vortex states in mesoscopic superconductors. The sample consists of an Al square with side of 1.1 μm, which is directly connected to Al leads for current injection. The vortex penetration/expulsion is detected by the voltage change across a small tunnel junction attached to the square. We find that the vortex penetration/expulsion fields are significantly influenced by the external current of the order of 10 μA. The shift of the vortex penetration/expulsion fields is interpreted in terms of the forces exerted by the external current.     

Staircase magnetization anomalies in superconducting Pb/Au bilayer films patterned with antidot lattices

Superconducting Pb(x)/Au(25 nm) bilayers (x = 50, 100 nm) patterned with antidot lattices exhibit various matching field anomalies depending on experimental conditions. Magnetization peaks at applied fields H = n[20 Oe] (n = integer) resemble superconducting wire network data; cusps are also observed, consistent with predictions of “giant” vortices in low-kappa films. Sharp “staircase” anomalies spaced by 1–3 Oe are observed in AC magnetization, possibly a result of depinning of intermediate state domains, or macroscopic quantum tunneling between reproducible states of different quantized flux.     

1.5 T关节磁共振成像超导磁体的设计、制作与测试

磁共振成像(magnetic resonance imaging,MRI)是当今世界上最先进的医学影像技术之一,现阶段MRI技术正朝着成像质量更清晰、功能更强大、效率更高、个体化更强的趋势发展.与全身MRI设备相比,专科型MRI设备具有体积小、重量轻、成本低、病人舒适度高、成像质量高、功能更强等优点.但是关节专用超导MRI系统需要长度方向上被严格限制的超导磁体在160 mm直径球域(diameter sphere volume,DSV)内产生高均匀度的磁场.本文综合考虑了超导线用量、中心磁感应强度和成像区磁场不均匀度等因素,使用0-1规划和遗传算法相结合的方法设计了一种非屏蔽型1.5 T关节MRI超导磁体,该磁体的室温孔径为280 mm,总长度为520 mm,液氦量为30 L,载流区最大磁场为5.48 T,5高斯线范围为径向3.2 m、轴向2.6 m,160 mm DSV的磁场不均匀度设计值为22 ppm,考虑加工误差及冷缩因素,磁体加工完成并经过被动匀场后的预估值为60 ppm.经过绕制、固化、组装、焊接等工序,该磁体已制作完成.经过3次锻炼后成功励磁到1.5 T,经过被动匀场后160 mm DSV的磁场不均匀度达到50 ppm,各项指标均达到设计目标.     

Magneto-optical and micromagnetic simulation study of the current-driven domain wall motion in ferromagnetic (Ga,Mn)As

We have studied current-driven domain wall motion in modified Ga_0.95Mn_0.05As Hall bar structures with perpendicular anisotropy by using spatially resolved polar magneto-optical Kerr effect microscopy and micromagnetic simulation. Regardless of the initial magnetic configuration, the domain wall propagates in the opposite direction to the current with critical current of 1-2×10⁵ A/cm². Considering the spin-transfer torque term as well as various effective magnetic field terms, the micromagnetic simulation results are consistent with the experimental results. Our simulated and experimental results suggest that the spin-torque rather than Oersted field is the reason for current-driven domain wall motion in this material.     

中国超导电子学研究及应用进展

超导体的发现距今已有近110年了,高温超导体的发现也已经有30多年了.超导材料的电子学应用在最近一二十年取得了突破性进展.高温超导微波器件显示了比传统微波器件更优越的性能,已经在移动通信、雷达和一些特殊通信系统中取得了规模化应用.超导量子干涉器件以其磁场和电流测量的超高灵敏度,成为地质勘探、磁共振成像和生物磁成像等领域不可替代的手段.包括超导隧道结混频器、超导热电子混频器、超导转变沿探测器及超导单光子探测器等在内的超导传感器/探测器可以探测全波段的电磁波及各种宇宙辐射,具有接近量子极限的超高灵敏度,在地球物理、天体物理、量子信息技术、材料科学及生物医学等众多前沿领域发挥越来越重要的作用.超导参量放大器已经成为实现超导量子计算的关键器件.超导集成电路技术已被列入国际器件与系统技术路线图,成为后摩尔时代微电子领域的前沿阵地之一.在计量科学中,超导约瑟夫森效应及约瑟夫森结阵器件被广泛应用于量子电压基准和国际单位制基本单位的重新定义中.在当前的量子信息技术热潮中,超导电子学扮演重要角色,同时量子热潮也大力推动了超导电子学的发展.本文主要对近几年我国超导电子学研究和应用的现状与进展进行概括总结.     

射频超导腔加速性能的改进

为提高超导加速腔的加速梯度和Q值,改进了薄膜型超导腔的加速性能。研究证明,对于铜铌溅射腔,在无氧铜衬底和铌膜之间加入NbN 层可以提高铌膜的超导转变温度,改善晶格结构;对纯铌超导腔提出了改进方法,在传统的纯铌超导腔表面制备多层的超导-绝缘-超导复合膜可以屏蔽Nb腔表面的界面场,提高超导腔的临界磁场,从而提高了铌腔的加速梯度。     

Phase boundary of the hexagonal—prism superconducting network in a magnetic field

In this paper, we systematically study the phase boundary T_c(\vec H) of a hexagonal-prism superconducting network in an external magnetic field \vec H of arbitrary magnitude and direction. The result indicates that the phase boundary of the hexagonal-prism superconducting circuit varies more sharply than that of the cubic circuit. The potential applications of the hexagonal-prism superconducting circuit are also discussed.     

超导技术在未来电网中的应用

大力发展可再生能源并实现清洁能源变革,是当今能源领域的大趋势。随着可再生能源越来越多地接入电网,将对直流输电和大规模储能技术提出愈加迫切的需求。在此背景下,超导直流输电技术、超导直流限流器以及基于超导电性的电力储能技术等具有潜在的应用前景。文章较为系统地介绍上述直流超导电力装置的原理、优势以及近些年国内外的进展等。     

Construction of curved global vortex

The complex scalar field models which possess, in the spectra of solutions, the vortices with cylindrical symmetry are considered. The rigorous construction of the curved, constant width, vortices is presented. In the framework of the used formalism an example of the curved vortex of constant width is discussed in detail.