Critical dynamics associated with dynamic disordering near the depinning transition in different vortex phases

We study a depinning transition based on transient dynamics of vortices driven by a suddenly applied dc current, focusing on whether a difference in the equilibrium vortex phase that can lead to a different vortex flow will change the critical behavior. After preparing an ordered initial vortex configuration, we measure the time evolution of voltage associated with dynamic disordering in three magnetic fields, corresponding to the ordered phase (OP), disordered phase (DP), and coexistence phase. The critical behavior of the depinning transition is commonly observed in these phases, pointing to the universality of the transition. However, the critical behavior is most marked in the coexistence phase, while the suppression of the critical region and that of dynamic disordering are observed in OP and DP, respectively, whose origin is attributed to the different flow states among these phases.     

Excess resistance in the superconducting transition of a mesoscopic Al disk

We report resistance measurements on a mesoscopic Al disk whose size is comparable to the superconducting coherence length. As the magnetic field increases, resistance peaks successively appear and some of the peak resistances are larger than the normal state value R_N. These peaks are ascribed to the transitions between different vortex states in the superconducting Al disk. The experimental results suggest that some anomalous energy dissipation is caused by the dynamics of the vortices in the confined geometry.     

Long-range nonlocal flow of vortices in narrow superconducting channels

We report a new nonlocal effect in vortex matter, where an electric current confined to a small region of a long and sufficiently narrow superconducting wire causes vortex flow at distances hundreds of intervortex separations away. The observed remote traffic of vortices is attributed to a very efficient transfer of a local strain through the one-dimensional vortex lattice (VL), even in the presence of disorder. We also observe mesoscopic fluctuations in the nonlocal vortex flow, which arise due to "traffic jams" when vortex arrangements do not match a local geometry of a superconducting channel.     

Maximizing critical currents in superconductors by optimization of normal inclusion properties

The movement of vortices in superconductors due to an applied current can induce a loss of perfect conductivity. Experimental observations show that material impurities can effectively prevent vortices from moving. In this paper, we provide numerical studies to investigate vortex pinning and critical currents through the use of an optimal control approach applied to a variant of the time-dependent Ginzburg-Landau model that can account for normal inclusions. The effects that the size and boundary of the sample and the number, size, shape, orientation, and location of the inclusion sites have on the critical current and vortex lattices are studied. In particular, the optimal control approach is used to determine the optimal properties of the impurities so as to maximize the critical current, i.e., the largest current that can pass through a superconductor without resistance.     

Faster vortex core switching with lower current density using three-nanocontact spin-polarized currents in a confined structure

We perform micromagnetic simulations on the switching of magnetic vortex core by using spin-polarized currents through a three-nanocontact geometry. Our simulation results show that the current combination with an appropriate current flow direction destroys the symmetry of the total effective energy of the system so that the vortex core can be easier to excite,resulting in less critical current density and a faster switching process. Besides its fundamental significance, our findings provide an additional route to incorporating magnetic vortex phenomena into data storage devices.     

Directional dependence of vortex core resonance in a square-shaped ferromagnetic dot

The resonant property of the magnetic vortex confined in a square-shaped ferromagnetic dot has been investigated. We showed that the field dependence of the resonant frequency has a unique directional dependence originating from a four-fold rotational symmetry of the square. The resonant frequency is found to be strongly modulated by the magnetic field along the diagonal direction although the magnetic field applied along the side of the square hardly modified the resonant frequency. The modulation ratio of the resonant frequency defined by the ratio between minimum and maximum frequencies for the vortex resonance was found to be tuned by the lateral dimension of the square. These unique frequency tunabilities controlled by the magnitude and the direction of the magnetic field may provide additional functions in the application of the magnetic vortex systems.     

Point-vortex cluster formation in the plane and on the sphere: an energy bifurcation condition

The dynamics of a system of point vortices is considered in the plane and on the sphere. Particular attention is given to the formation of vortex clusters and to global vortex dynamics, especially in the spherical case. For integrable systems and systems with given symmetries, we show the existence of a critical energy above or below which (depending on the geometry of the surface) the system splits into clusters and vortex dynamics is confined to a particular region. The case of nonidentical vortices is of particular interest since we observe quite different global dynamics depending on the energy and the initial conditions. Furthermore we identify all the relative equilibria configurations as critical points of the reduced energy and we give an instability criterion to deduce instability for certain configurations.     

Rectifying effect in a Josephson junction ratchet array

We have studied numerically a rectifying effect in an underdamped Josephson junction ratchet array driven by dc and ac current. The array consists of both alternating potential barriers and alternating inter-capacitances along the direction of vortex flow. The guide banks of high critical currents are assigned for all the longitudinal junctions to prevent the percolative pattern of vortex motion. In some junction parameters, we see a rectifying effect which indicates a finite value of the time-averaged voltage at zero dc bias. The directional dependence of the vortex motion becomes fairly large when the junction parameters lie in an optimal range which gives rise to a Shapiro step at zero dc bias. Such a rectifying effect survives for small thermal fluctuation, but eventually disappear beyond a certain critical temperature.     

轴流式涡流管内三维流场的大涡模拟

根据涡流管内可压缩气体的强旋转运动是导致涡流管能量分离的根本原因,提出了在涡流管内加入一个"X"型导流片迫使气流产生强旋转运动,使其进气方式变为轴流式。利用计算流体动力学(CFD)的方法,建立了轴流式和切流式涡流管内部气体流动的三维大涡数值模型,对其内部气流流场进行了数值模拟。数值模拟结果表明:"X"型导流片可以改变轴向进入气体的运动方向,使气体产生高速旋转运动,得出"X"型导流片的叶片夹角为θ=120°时,气体的旋转效果最好;大涡模拟可以较好地模拟涡流管内气流的三维流场。     

Vortex motion in high-Tc films and a micropattern-induced phase transition

A micropattern-induced transition in the mechanism of vortex motion and vortex mobility is observed in high-T_c thin films. The competition between the anomalous Hall effect and the guidance of vortices by rows of microholes (antidots) lead to a sudden change in the direction of vortex motion that is accompanied by a change in the critical current density and microwave losses. The latter effect demonstrates the difference in vortex mobility in different phases of vortex motion in between and within the rows of antidots.     

偶极-偶极相互作用下玻色-爱因斯坦凝聚体中涡旋的非线性动力学研究

基于平均场理论和分裂算符谱算法, 研究了偶极-偶极相互作用下玻色爱因斯坦凝聚体中涡旋的非线性动力学. 研究发现外势运动速度小于临界值时,偶极-偶极相互作用对系统涡旋的非线性动力学影响较小,而外势运动速度超过临界速度时,偶极-偶极相互作用对涡旋的非线性动力学影响很大,可使系统产生涡旋对、涡旋偶极子和简单涡旋,并使它们形成涡街.     

偶极-偶极相互作用下玻色-爱因斯坦凝聚体中涡旋的非线性动力学研究

基于平均场理论和分裂算符谱算法,研究了偶极-偶极相互作用下玻色爱因斯坦凝聚体中涡旋的非线性动力学.研究发现外势运动速度小于临界值时,偶极-偶极相互作用对系统涡旋的非线性动力学影响较小,而外势运动速度超过临界速度时,偶极-偶极相互作用对涡旋的非线性动力学影响很大,可使系统产生涡旋对、涡旋偶极子和简单涡旋,并使它们形成涡街.     

Critical current densities and flux creep rate in Co-doped BaFe2As2 with columnar defects introduced by heavy-Ion irradiation

We report the formation of columnar defects in Co-doped BaFe₂As₂ single crystals with different heavy-ion irradiations. The formation of columnar defects by 200 MeV Au ion irradiation is confirmed by transmission electron microscopy and their density is about 40% of the irradiation dose. Magneto-optical imaging and bulk magnetization measurements reveal that the critical current density J_c is enhanced in the 200 MeV Au and 800 MeV Xe ion irradiated samples while J_c is unchanged in the 200 MeV Ni ion irradiated sample. We also find that vortex creep rates are strongly suppressed by the columnar defects. We compare the effect of heavy-ion irradiation into Co-doped BaFe₂As₂ and cuprate superconductors.     

Unsteady effects on flame extinction limits during gaseous and two-phase flame/vortex interactions

In highly fluctuating flows, it happens that high values of the strain-rate do not induce extinction of the flame front. Unsteady effects minimize the flame response to rapidly varying strain fields. In the present study, the effects of time-dependent flows on non-premixed flames are investigated during flame/vortex interactions. Gaseous flames and spray flames in the external sheath combustion regime are considered. To analyse the flame/vortex interaction process, the velocity field and the flame geometry are simultaneously determined using particle imaging velocimetry and laser-induced fluorescence of the CH radical. The influence of vortex flows on the extinction limits for different vortex parameters and for different gaseous and two-phase flames is examined. If the external perturbation is applied over an extended period of time, the extinction strain-rate is that corresponding to the steady-state flame, and this critical value mainly depends on the fuel and oxidizer compositions and the injection temperature. If the external perturbation is applied during a short period of time, extinction occurs at strain-rates above the steady-state extinction strain-rate. This deviation appears for flow fluctuation timescales below steady flame diffusion timescales. This behaviour is induced by diffusive processes, limiting the ability of the flame to respond to highly fluctuating flows. With respect to unsteady effects, the spray flames investigated in this article behave essentially like gaseous flames, because evaporation takes place in a thin layer before the flame front. Extinction limits are only slightly modified by the spray, controlling process being the competition between aerodynamic and diffusive timescales.     

Magnetic vortex core dynamics in cylindrical ferromagnetic dots

We report direct imaging by means of x-ray photoemission electron microscopy of the dynamics of magnetic vortices confined in micron-sized circular permalloy dots that are 30 nm thick. The vortex core positions oscillate on a 10 ns time scale in a self-induced magnetostatic potential well after the in-plane magnetic field is turned off. The measured oscillation frequencies as a function of the aspect ratio of the dots are in agreement with theoretical calculations presented for the same geometry.     

Magneto-optical imaging of vortex arrangements in Pb finite superconducting networks

We have fabricated finite-sized Pb superconducting networks with 10 × 10 square (each 6 × 6 μm²) holes by using the electron beam lithography and vortex arrangements are visualized by using magneto-optical imaging. We find that the vortex penetration at low temperature is controlled by defects in the network. We also find nearly regular arrangements of vortices with defects close to 1/2 and1/3 of the matching field.     

Magnetoresistance anisotropy in a hexagonal lattice of Co antidots obtained by thermal evaporation

Patterned soft magnetic materials are eligible for use in magnetic random access memories. A hexagonal-lattice pattern of circular antidots was produced by optical lithography in a Co film. In order to test the effect of geometry on the local magnetisation configuration of such a structure, we performed room-temperature angle-resolved magnetisation measurements aimed to check the pinning of domain walls by the pattern's lattice. Magnetoresistance (MR) room-temperature measurements were performed at various angles between the magnetic field direction and the macroscopic electrical current vector, to clarify whether and how the local current density configuration affects the MR response. We found that the magnetoresistance is of anisotropic type (AMR) and has a local origin. Furthermore, the largely unsaturating behaviour of MR at high fields may be explained only by considering that tiny portions of the pattern constitute highly frustrated regions and align their magnetisation at rather high fields. A simplified model based on a local anisotropy term is shown to account for the experimental results for both M and MR.     

Patterns at the onset of electroconvection in freely suspended smectic films

We report the results of experiments on electrically driven convection that occurs in a thin, freely suspended film of smectic A liquid crystal when an electric field is applied in the plane of the film. Convection in a vortex pattern is found above a well-defined critical voltage. The film behaves as a two-dimensional isotropic liquid: neither its thickness nor the director field are modified by the flow. We present measurements of the critical voltage at the onset of convection in two experimental configurations—one which allows the injection of charges into the film from the electrodes, and one which does not. When injection is present, the critical voltage for the onset of flow increases monotonically with increasing frequency of applied field. With no injection, there is no instability at DC and the critical voltage diverges there. The nature of the flow pattern observed at onset changes with frequency. Below a certain frequency the film flows in vortices that extend over the width of the film; above this frequency the flow is confined to two lines of smaller vortices localized along the electrodes. We present a simple discussion of the mechanisms which drive the convection.     

Simulation of Dynamics in Two-Dimensional Vortex Systems in Random Media

Dynamics in two-dimensional vortex systems with random pinning centres is investigated using molecular dynamical simulations. The driving force and temperature dependences of vortex velocity are investigated. Below the critical depinning force Fc, a creep motion of vortex is found at low temperature. At forces slightly above Fc, a part of vortices flow in winding channels at zero temperature. In the vortex channel flow region, we observe the abnormal behaviour of vortex dynamics： the velocity is roughly independent of temperature or even decreases with temperature at low temperatures. A phase diagram that describes different dynamics of vortices is presented.