Dynamics of two-dimensional vortex system in a strong square pinning array at the second matching field

We study the dynamics of a two-dimensional vortex system in a strong square pinning array at the second matching field. Two kinds of depinning behaviors, a continuous depinning transition at weak pinning and a discontinuous one at strong pinning, are found. We show that the two different kinds of vortex depinning transitions can be identified in transport as a function of the pinning strength and temperature. Moreover, interstitial vortex state can be probed from the transport properties of vortices.     

Electronic properties of graphene with a topological defect

Various types of topological defects in graphene are considered in the framework of the continuum model for long-wavelength electronic excitations, which is based on the Dirac–Weyl equation. The condition for the electronic wave function is specified, and we show that a topological defect can be presented as a pseudomagnetic vortex at the apex of a graphitic nanocone; the flux of the vortex is related to the deficit angle of the cone. The cases of all possible types of pentagonal defects, as well as several types of heptagonal defects (with the numbers of heptagons up to three, and six) are analyzed. The density of states and the ground state charge are determined.     

Non-axial nature of an optical vortex and Wigner function

Before we use an optical vortex for any application, it is important to know if the vortex is axial or non-axial, since properties of one type of the vortex are quite different from another. We show that the Wigner distribution can provide a solution, being different for axial and non-axial vortex. We use transport properties of the Wigner function to study the propagation of an axial as well as non-axial vortex and find them different.     

Irregular mixing due to a vortex pair interacting with a fixed vortex

The paper examines scalar advection caused by a point–vortex pair encountering a fixed point vortex in a uniform flow. The interaction produces two types of vortex motion. First is unbounded as the pair moves unrestrictedly after encountering the fixed vortex. The scalar exchanging between the pair's bubble and fixed vortex's neighbourhood is numerically estimated. Second is bounded as the pair's vortices periodically oscillate about the fixed vortex. The pair's periodic motion perturbs scalar motion causing a portion of scalar trajectories to manifest chaotic behaviour. We analyse scalar transport using Poincaré sections, which reveal regular and chaotic transport regions.     

Abrikosov vortex escape from a columnar defect as a topological electronic transition in a vortex core

The microscopic scenario of vortex escape from a columnar defect under the influence of a transport current has been studied. For defect radii smaller than the superconducting coherence length the depinning process is shown to be a consequence of two subsequent topological electronic transitions in a trapped vortex core. The first transition at a critical current j _L is associated with the opening of Fermi surface segments corresponding to the creation of a vortex-antivortex pair bound to the defect. The second transition at a certain current j _d > j _L is caused by merging of different Fermi surface segments, which accompanies the formation of a freely moving vortex.     

Vortex system dynamics and energy losses in a current-carrying 2D superconducting wafer

The dynamic processes occurring in the vortex system of a 2D superconducting wafer carrying transport current are investigated using the model of the vortex system of high-temperature superconductors. Calculations are performed by the Monte Carlo method. For the first time, the dynamics of magnetic field penetration in a current-carrying HTSC wafer is demonstrated and the energy losses associated with a change in transport current are calculated. It is shown that changes in the transport current amplitude and in the number of defects lead to a change in the energy liberation mechanism: hysteresis energy losses are replaced by the losses in the saturated layer.     

Equilibration and dynamic phase transitions of a driven vortex lattice

We report on the observation of two types of current driven transitions in metastable vortex lattices. The metastable states, which are missed in usual slow transport measurements, are detected with a fast transport technique in the vortex lattice of undoped 2H-NbSe2. The transitions are seen by following the evolution of these states when driven by a current. At low currents we observe an equilibration transition from a metastable to a stable state, followed by a dynamic crystallization transition at high currents.     

A single Abrikosov vortex trapped in a mesoscopic superconducting cylindrical surface

We investigate the behaviour of a single Abrikosov vortex trapped in a mesoscopic superconducting cylindrical surface with a magnetic field applied transverse to its axis. In the framework of the time-dependent Ginzburg-Landau formalism we show that, provided the transport current and the magnetic field are not large, the vortex behaves as an overdamped quasi-particle in a tilted washboard potential. The cylindrical thin strip with the trapped vortex exhibits E(J) curves and time-dependent electric fields very similar to the ones exhibited by a resistively shunted Josephson weak link.     

Phase transitions in a two-dimensional vortex system with defects: Monte Carlo simulation

AbstractThe phase states and phase transitions in a system consisting of a two-dimensional vortex lattice with defects are studied by the Monte Carlo method. It is shown that a “rotating lattice” phase, which is an intermediate phase between the vortex crystal and vortex liquid phases, is present. The dependence of the temperature of the transition from the rotating lattice phase into a vortex liquid on the strength of the defect potential is determined. The current-voltage characteristics of the system are calculated at various temperatures for point, square, and linear defects. It is shown that the phase state of the system strongly affects its transport properties.     

Characteristic parameters of drift vortices coupled to Alfvén waves in an inhomogeneous space plasma

We present detailed measurements of ion scale vortices of drift type coupled to Alfvén waves in an inhomogeneous and collisionless space magnetoplasma. The two free parameters of a dipolar vortex, intensity and spatial radius, are measured. The vortices are driven by a strong density gradient on a boundary layer with scale size of the same order as the vortex diameter. Observations of vortices off the gradient show that symmetry-breaking conditions in a real inhomogeneous plasma can lead not only to cross-field but also to cross-boundary anomalous transport of particles and energy.     

Cherenkov radiation emitted by a vortex into an anisotropic dielectric

The fields produced by a Josephson vortex moving in a sandwich placed into an anisotropic dielectric are investigated. When the vortex velocity exceeds the velocity of light in the direction of the normal to the sandwich surface, Cherenkov emission of electromagnetic waves propagating from the sandwich to the bulk of the dielectric takes place. The Poynting vector of outgoing waves is determined. It is shown that the radiation directivity considerably depends on the degree of anisotropy in the permittivity. The radiation loss power of the vortex is determined, and the relation between the transport current and the vortex velocity is established.     

Vortex motion rectification in Josephson junction arrays with a ratchet potential

By means of electrical transport measurements we have studied the rectified motion of vortices in ratchet potentials engineered on overdamped Josephson junction arrays. The rectified voltage as a function of the vortex density shows a maximum efficiency close a matching condition to the period of the ratchet potential indicating a collective vortex motion. Vortex current reversals were detected varying the driving force and vortex density revealing the influence of vortex-vortex interaction in the ratchet effect.     

Finite superconducting square wire-network based on two-dimensional crystalline Mo2C

Superconducting wire-networks are paradigms to study Cooper pairing issues, vortex dynamics and arrangements. Recently, emergent low-dimensional crystalline superconductors were reported in the minimal-disorder limit, providing novel platforms to reveal vortices-related physics. Study on superconducting loops with high-crystallinity is thus currently demanded. Here, we report fabrication and transport measurement of finite square-network based on two-dimensional crystalline superconductor Mo₂C. We observe oscillations in the resistance as a function of the magnetic flux through the loops. Resistance dips at both matching field and fractional fillings are revealed. Temperature and current evolutions are carried out in magnetoresistance to study vortex dynamics. The amplitude of oscillation is enhanced due to the interaction between thermally activated vortices and the currents induced in the loops. The driving current reduces the effective activation energy for vortex, giving rise to stronger vortex interaction. Moreover, by the thermally activated vortex creep model, we derive the effective potential barrier for vortex dissipation, which shows well-defined correspondence with structures in magnetoresistance. Our work shows that low-dimensional crystalline superconducting network based on Mo₂C possesses pronounced potential in studying the modulation of vortex arrangements and dynamics, paving the way for further investigations on crystalline superconducting network with various configurations.     

Wave-induced bed flows by a Lagrangian vortex scheme

Nominally 2-dimensional viscous flow induced by gravity waves over a spatially periodic bed is simulated by a Lagrangian vortex scheme. A vortex sheet is introduced on the surface at each time step to satisfy the zero velocity conditions. The sheet is discretised; the vortex-in-cell method is used to convect vorticity and random walks are added to effect viscous diffusion. Good agreement with analytical theory is obtained for velocity profiles in uniform sinusoidal flow and for mass transport due to linear waves. Mass transport for finite amplitude waves is also obtained. For separated flow over rippled beds, which is still liminar, a vortex decay factor is required to produce agreement with experiment and is thought to compensate for large scale 3-dimensional effects.     

Measurement of vortex motion in a type-II superconductor: A novel use of the neutron spin-echo technique

We have used the neutron spin-echo technique to measure the small energy change of neutrons which are diffracted by a moving vortex lattice in a low-pinning Nb-Ta superconducting sample. A transport current was passed in the mixed state to cause flux line movement. In the case of uniform motion, the flux velocity v(L) was given as expected by the values of electric and magnetic fields, via E = -v(L)wedgeB. We show that with a nonuniformly moving vortex lattice, one can measure the dispersion of the velocities, opening up new possibilities for investigating moving vortex lines.     

非定常瞬态流动过程中的Lagrangian拟序结构与物质输运作用

从Lagrangian角度数值分析了圆柱瞬时起动过程中的非定常瞬态流动现象,如分离泡产生、破裂和涡脱落等及其产生的非定常效应,揭示了所列现象诱导的物质输运和迁移效应,首先采用双时间步长的特征线算子分裂算法数值模拟了圆柱起动过程中的瞬时流场,然后采用数值方法从流场中提取出Lagrangian拟序结构(LCSs),并根据非线性动力学理论研究了流动分离和旋涡演化过程中的物质输运作用,结果表明,圆柱瞬时起动后所产生的非定常阻力与相应瞬态现象中的物质输运有密切的关系:对称分离泡产生及其在流向方向的生长,能够使分离泡内压力升高且分布均匀,从而减小阻力;对称分离泡的失稳增强了分离泡与主流之间的物质输运作用,最终导致涡的脱落,并有利于推迟流动分离和减小分离区域,非定常流动中LCSs所描述的物质输运和迁移作用对流动控制的机理研究具有一定指导意义。     

Vortex nanoliquid in high-temperature superconductors

Using a differential magneto-optical technique to visualize the flow of transport currents, we reveal a new delocalization line within the reversible vortex liquid region in the presence of a low density of columnar defects. This line separates a homogeneous vortex liquid, in which all the vortices are delocalized, from a heterogeneous "nanoliquid" phase, in which interconnected nanodroplets of vortex liquid are caged in the pores of a solid skeleton formed by vortices pinned on columnar defects. The nanoliquid phase displays high correlation along the columnar defects but no transverse critical current.     

Mixed state of a dirty two-band superconductor: application to MgB2

We investigate the vortex state in a two-band superconductor with strong intraband and weak interband electronic scattering rates. Coupled Usadel equations are solved numerically, and the distributions of the pair potentials and local densities of states are calculated for two bands at different values of magnetic fields. The existence of two distinct length scales corresponding to different bands is demonstrated. The results provide qualitative interpretation of recent scanning tunneling microscopy experiments on vortex structure imaging in MgB2.     

Vortex-rectification effects in films with periodic asymmetric pinning

We study the transport of vortices excited by an ac current in an Al film with an array of nanoengineered asymmetric antidots. The vortex response to the ac current is investigated by detailed measurements of the voltage output as a function of ac current amplitude, magnetic field, and temperature. The measurements revealed pronounced voltage rectification effects which are mainly characterized by the two critical depinning forces of the asymmetric potential. The shape of the net dc voltage as a function of the excitation amplitude indicates that our vortex ratchet behaves in a way very different from standard overdamped models. Rather, the repinning force, necessary to stop vortex motion, is considerably smaller than the depinning force, resembling the behavior of the so-called inertia ratchets. Calculations based on an underdamped ratchet model provide a very good fit to the experimental data.     

Strong vortex liquid correlation from multiterminal measurements on untwinned YBa2Cu3O(7-delta) single crystals

Multiterminal transport measurements indicating a strongly correlated vortex liquid above the first order melting transition in untwinned YBa2Cu3O(7--delta) single crystals are presented. The correlation is found to be surprisingly stable for different weak variations of point disorder. We investigate the differences between intervortex and intravortex correlations in different contact geometries. The results are in agreement with the local conductivity model, with zero resistivity along straight vortex line segments.