Critical currents and electric fields of two-dimensional systems

We calculate the critical current of a thin, superconducting film with periodically modulated thickness placed in a perpendicular magnetic field. This problem is shown to be closely connected with the problem of the commensurate-incommensurate phase transition in two dimensions. The main mechanism responsible for the vortex motion appears to be the creation of a soliton in a vortex lattice on the boundary of a sample. The critical current is zero in the incommensurate phase. An analogous phenomenon is the critical longitudinal field for the Wigner crystal of electrons on the surface of liquid helium. The shear modulus for an Abrikosov vortex lattice in a thin film is found for a wide range of parameters.     

Use of a Sigmoid Function to Describe Second Peak in Magnetization Loops

Order-disorder transitions of a vortex lattice transfer type-II superconductors from a low critical current state to a high one. The similar transition between different current states can be caused by electromagnetic granularity. A sigmoid curve is proposed to describe the corresponding peak in a field dependence of the macroscopic critical density. Using the extended critical state model, analytic expressions are obtained for the field dependencies of the local critical current density, the depth of equilibrium surface region, and the macroscopic critical current density. The expressions are well fit to published data.     

Vortex dimples at charged helium interfaces

We have investigated theoretically the profile of electrically charged helium interfaces in the presence of a superfluid vortex line perpendicular to the interface. The free surface of superfluid³He-B as well as the interface between phase-separated³He and⁴He mixtures have been considered. The superflow-induced dimple profile around the vortex line was calculated within the linear theory assuming a weak curvature of the interface. Our results show that the depth of the dimple increases strongly with an external electric field as the critical value of the electrohydrodynamic surface instability is approached. The chances of observing these charge-enhanced dimples are also discussed.     

Observation of the Curved and Entangled Vortex Tubes in 3D Mesoscopic Cubic Superconductors

The time-dependent Ginzburg-Landau equations have been solved numerically by a finite element analysis for the superconducting samples with a cubic shape in a uniform external magnetic field. We obtain the different vortex patterns as a function of the magnetic field perpendicular to its surface. The vortex tubes must reach the surface perpendicularly in order to avoid a supercurrent component pointing outwards the surface. At the same time, we observed the arrangement of spiral vortices in the cubic superconductor. These results show that our approach is an effective and useful to interpret experimental data on vortex states in the mesoscopic superconductors.     

Vortex Escape from Columnar Defect in a Current-Loaded Superconductor

The problem of Abrikosov vortices depinning from extended linear (columnar) defect in 3D-anisotropic superconductor film under non-uniformly distributed Lorentz force is studied for the case of low temperatures, disregarding thermal activation processes. We treat it as a problem of mechanical behavior of an elastic vortex string settled in a potential well of a linear defect and exerted to Lorentz force action within the screening layer about the London penetration depth near the specimen surface. The stability problem for the vortex pinning state is investigated by means of numerical modeling, and conditions for the instability threshold are obtained as well as the critical current density \(j_\mathrm{c}\) and its dependence on the film thickness and magnetic field orientation. The instability leading to vortex depinning from extended linear defect first emerges near the surface and then propagates inside the superconductor. This scenario of vortex depinning mechanism at low temperatures is strongly supported by some recent experiments on high-Tc superconductors and other novel superconducting materials, containing columnar defects of various nature.     

Vortex state in thin films of multicomponent ferroelectrics

The distribution of polarization in thin film of multicomponent weak anisotropic ferroelectric under application of an axially symmetric electric field induced by circular electrodes is studied. We demonstrate that above a certain critical field the instability towards abrupt formation of ferroelectric vortex state appears. The polarization distribution in the vortex state exhibits a doughnut shape in agreement with experimental data.     

A Quantum Mechanical Treatment of the Vortex–Vortex Interaction in Anisotropic Superconductors

Using a new Hamiltonian of interaction [Int. J. Mod. Phys. B16, 4809 (1002); B17, 4763 (2003)], we calculate the vortex–vortex interaction energy in anisotropic superconductors. We present here the analytical formulae. The interaction energy has a minimum at a certain vortex–vortex distance. This distance decreases with the increase of the angle θ between the vortex line and the crystalline axis. Also, we calculate the elastic force and the nonharmonicity coefficients of the vortex lattice. Both coefficients decrease with increasing angle. Generally, the interaction energy decreases with the angle increase. Finally, we present the self-energy of a vortex in a lattice, the energy of a single isolated vortex and the lower critical field. Also, the surface of the vortex in the lattice has a form of a rosette with six petals, which are all equals for θ = 0 and become nonequals for θ ≠ 0. The surface of the first isolated vortex has a form of an oval for θ ≠ 0, and becomes a circle for θ = 0. PACS numbers: PACS 74.25.Qt.     

A nanoscale SQUID operating at high magnetic fields

A washer-free Nb nanoSQUID has been developed for measuring magnetization changes from nanoscale objects. The SQUID loop is etched into a 250 nm wide Au/Nb bilayer track and the diameter of the SQUID hole is ～70 nm. In the presence of a magnetic field perpendicular to the plane of the SQUID, vortex penetration into the 250 nm wide track can be observed via the critical current-applied field characteristic and the value at which vortex first penetrates is consistent with the theoretical prediction. Upon removing the applied field, the penetrated vortices escape the track and the critical current at zero field is restored.     

Current–Voltage Measurements and Vortex Dynamics in a Bi2Sr2CaCu2O x Whisker: Observation of the Peak Effect and Calculation of the Critical Current in the Flux Flow Regime with and Without Applied Field

In this article, we report current–voltage measurements carried out on a Bi₂Sr₂CaCu₂O _x whisker in a large temperature range below the critical temperature with and without applied magnetic field. We examine the critical current peak effect and the vortex dynamics at low field. The critical current peak effect consists of the initial increase of the critical current that subsequently decreases as the applied field is increased. For current–voltage measurements, this effect is associated with a change in the current–voltage curves that are typical of the flux flow regime at low fields and resemble flux creep characteristics for higher fields. As a general rule, our observations are consistent with the models that link the peak effect to vortex phase transitions. We calculate the critical current in self-field in the flux flow regime taking into account intervortex forces. We suppose that most vortices are pinned by defects while mobile vortices move through plastic channels between the strongly pinned vortex regions. When an external field is applied, we suggest that the increase in the critical current that is observed is linked to oscillations of the pinned vortices.     

Magnetic Flux Penetration and Motion in Antiferromagnetic Superconductors

The paper reviews a concept of induced spin-flop domain inside vortices in an antiferromagnetic superconductor. Such phenomenon may occur when an external magnetic field is strong enough to flip over magnetic moments in the core of the vortex from their ground state configuration. The formation of the domain structure inside vortices modifies the surface energy barrier of the superconductor. During this process the entrance of the flux is stopped and a newly created state exhibits perfect shielding. Such behavior should be visible as a plateau on the dependence of flux density as a function of the external magnetic field. The end of the plateau determines the critical field, which has been called the second critical field for flux penetration. Moreover, it is predicted and described how this phenomenon modifies flux creep in layered superconductors. The various scenarios of changing the creep regime from thermal to quantum and vice versa at constant temperature are discussed.     

Temperature Dependence of Vortex Nucleation in Gaseous Bose-Einstein Condensates

The formation of quantized vortices in trapped, gaseous Bose-Einstein condensates is considered. The thermodynamic stability of vortex states and the essential role of the surface excitations as a route for vortex penetration into the condensate are discussed. Special focus is on finite-temperature effects of the vortex nucleation process. It is concluded that the critical angular frequencies for exciting surface modes with the relevant multipolarities yield, also at finite temperatures, the appropriate thresholds for the nucleation of vortices in dilute Bose-Einstein condensates, in fair agreement with the recent experiments.     

螺旋桨梢涡卷起数值计算

通过解析公式计算得到面源和面偶的诱导速度，在面偶附近区域，用涡环代替面偶，计算得到螺旋桨的速度场，根据尾涡面必须和当地流体流速相切的原则，修正原来尾涡面的形状，逐步迭代直至尾涡形状收敛，用这种方法预报了三维水翼和螺旋桨的尾涡面的形状和梢涡卷起，与试验结果有较好的一致性。     

闭口单箱颤振的流场机理

应用粒子图像测试技术(PIV)测试了闭口单箱颤振过程中尾部风嘴附近的旋涡变化过程,采用相位平均的方法研究模型周期性振动与旋涡规律性演化之间的关系。当风速低于颤振临界风速时,模型尾部风嘴附近下侧的旋涡控制结构运动状态,结构振动幅度较小;当风速接近颤振临界风速时,尾部风嘴上侧的旋涡尺度明显增大并达到与下侧旋涡相匹配的程度时,结构振幅明显增加,尾部风嘴处上下侧旋涡的交替作用主导了结构的振动。结合计算流体动力学(CFD)的数值计算方法获得颤振时刻模型表面的压力场。通过对模型表面进行合理分区,并利用分块分析的思想研究了颤振过程中气流能量输入特点。分析结果表明振动中的模型通过迎风端风嘴从气流中吸收了大量的能量。在颤振临界风速下,一个完整的振动周期内气流输入到振动系统的能量不断增加,造成单箱的颤振多为结构稳定性的突然性丧失。     

Influence of grain properties on the Abrikosov vortices interaction with Josephson junction

The problem of the displacement of an Abrikosov vortex is investigated when the magnetic field decreases to H _cl ^G in a grain of a type II superconductor. It is shown that near grain boundaries the vortex line generates an intergranular current which depends on the normalized grain size τ and the anisotropy ratio ν. These parameters strongly influence the conditions of the Josephson vortex generation as a result of the interaction of the Abrikosov vortex with the Josephson junction. We support our theory with calculations of the intergrain critical current taking into account two types of the vortex configuration, triangular and square, for different grain characteristics. The results are of interest for the charge transport in granular superconductors as well as the relaxation process in devices that contain Josephson junctions in micro- and nanoelectronics in magnetic field.     

p-Wave Superconductors in Sr2RuO4 and Bechgaard Salts

We review our recent works on the vortex state in p-wave superconductors. First, in a magnetic field parallel to the c-axis, the square vortex lattice is most stable, except in the immediate vicinity of T = T _c0. Second, the effect of impurities on H _c2 is studied, which exhibits characteristics of unconventional superconductors. Finally, the a–b anisotropy in the upper critical field in a magnetic field is considered. This anisotropy provides important information about the fourfold term arising from the Fermi surface effect.     

涡旋光束通过非高斯随机粗糙面的场分布特性

基于角谱衍射理论,利用Johnson传递系统数值模拟非高斯粗糙面,研究了拉盖尔-高斯涡旋光束通过随机非高斯粗糙表面的场分布特性。在分析了非高斯粗糙面方向自相关长度、峰度、偏斜以及均方根粗糙度对涡旋光束场分布影响的基础上,研究了涡旋光束通过随机粗糙表面后光束光强分布变化时的均方根粗糙度取值范围,并通过实验,将实验数据与仿真结果进行了对比分析。结果表明:当非高斯粗糙面方向相关长度为20 mm,偏斜为0.001,峰度为6,均方根粗糙度大于0.12 mm时,拉盖尔-高斯光束透过随机表面的光强分布不再保持空心分布,对应的相位奇点消失。     

Interplay Between Surface Effects and the Magnetic Properties in Polycrystalline Superconductors

The effects of the grain boundaries on the magnetic properties are studied in the polycrystalline superconductors. The interaction of the Abrikosov vortex with both the surface and Josephson junction as well as with other vortices and applied magnetic field are taken into account in this study by the London theory approach. It is shown that the magnetic behavior is strongly dependent of the anisotropy ratio as well as the normalized grain size and the coupling parameter between the grains. The first flux entry field H _p , the lower critical field H _{c 1} and the Gibbs free energy are computed. The vortex–vortex interaction and magnetization M(H) are also investigated.     

The paramagnetic Meissner effect resulting from the persistence of the giant vortex state

The paramagnetic Meissner effect (PME), recently observed in high-T_c materials and also in Nb, can be succesfully explained by the persistence of the giant vortex state with the fixed orbital quantum number L. This state is formed in superconductors in the field cooled regime at the third critical field. The self-consistent numerical solution of the Ginzburg-Landau equations clearly shows that the compression of the flux trapped inside the giant vortex state results in the PME. The PME is suppressed, and the normal diamagnetic response is recovered, by increasing the applied field.     

Topological Electronic Transitions in Vortex Cores in Type-II Superconductors

On the basis of microscopic theory, we study topological transitions in quasiparticle spectra of vortex systems, governed by an external magnetic field and transport current. We analyze two generic examples of such transitions: (i) opening of Fermi surface segments corresponding to the creation of a vortex-antivortex pairs; (ii) merging of different Fermi surface segments via the Landau–Zener tunneling. The basic properties of vortex matter such as pinning and transport characteristics, heat transport in the vortex state and peculiarities of the local density of states are strongly affected by these changes in the Fermi surface topology.