Transport of magnetic vortices by surface acoustic waves

In a thin film of superconducting Y Ba₂Cu₃O₇ the impact of surface acoustic waves (SAWs) traveling on the piezoelectric substrate is investigated. A pronounced interaction between the ultrasonic waves and the vortex system in the type II superconductor is observed. The occurrence of a SAW-induced dc voltage perpendicular to the sound path is interpreted as dragging of vortices by the piezoacoustic SAW, which acts as a conveyor for the flux quanta. The antisymmetry of this voltage with respect to the magnetic field directly evidences the induced, directed flux motion. This dynamic manipulation of vortices can be seen as an important step towards flux-based electronic devices.     

Current-driven dynamics in Josephson junction networks with an asymmetric potential

Current-driven dynamics of Josephson junction networks (JJNs) is studied using numerical simulations. We consider a JJN with an asymmetric and periodic potential of vortices, which is realized by saw-tooth modulation of junction critical currents. When external ac currents are applied to the JJN in a magnetic field, there appears a ratchet effect, and then directed motion of vortices is induced in certain system parameter regimes. A ratchet behavior is observed even for JJNs with weak structural disorder. We clarify the vortex pinning and dynamics in the JJN as a ratchet system.     

Directed motion of vortices in a current-driven Josephson junction network

Dynamics of directed motion of vortices in a Josephson junction network (JJN) with a ladder structure is studied using a numerical simulation. By applying spatial and temporal modulation of external bias currents, directed motion of vortices occurs in the absence of a ratchet-type asymmetric potential. In the present system, the asymmetry of the directed motion emerges as a dynamical effect due to the modulated bias current. Some dynamical effects such as mode-locking and vortex–antivortex excitation are relevant to the directed dynamics. We clarify the details of the directed motion of vortices in the JJN.     

Dynamic and static phases of vortices under an applied drive in a superconducting stripe with an array of weak links

Static and dynamic properties of superconducting vortices in a superconducting stripe with a periodic array of weakly-superconducting (or normal metal) regions are studied in the presence of external magnetic and electric fields. The time-dependent Ginzburg-Landau theory is used to describe the electronic transport, where the anisotropy is included through the spatially-dependent critical temperature T _c . Superconducting vortices penetrating into the weak-superconducting region with smaller T _c are more mobile than the ones in the strong superconducting regions. We observe periodic entrance and exit of vortices which reside in the weak link for some short interval. The mobility of the weakly-pinned vortices can be reduced by increasing the uniform applied magnetic field leading to distinct features in the voltage vs. magnetic field response of the system.     

Dynamic phases and the peak effect in dirty type II superconductors

We study numerically and experimentally the dynamics of driven vortex matter. Our London-Langevin simulations find that the critical current exhibits a peak both across the Bragg glass to vortex glass transition and across the melting line. The peak is accompanied by a clear crossing of the I-V curves. We report transport measurements on untwinned YBCO crystals, in complete accordance with these findings. At higher drives disorder is averaged to reduced values, and in three dimensions the vortices reorder into a "moving solid." The effect of the disorder can be well represented with a "shaking temperature" which is inversely proportional to the velocity.     

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.     

Directed transport in quantum Hall bilayers

The pseudo-spin model for a double layer quantum Hall system with the total landau level filling factor ν=1 is discussed. In contrast to the “traditional” model where the interlayer voltage enters as a static magnetic field along pseudo-spin hard axis, taking into account the realistic experimental situation, in our model we interpret the influence of applied voltage as a source of additional relaxation process in the double layer system. We show that the Landau-Lifshitz equation for the considered pseudo-magnetic system well describes existing experimental data and reduces to the dc driven and damped sine-Gordon equation. As a result, the mentioned model predicts novel directed intra-layer transport phenomenon in the system. In particular, unidirectional intra-layer energy transport can be realized due to the motion of topological kinks induced by applied voltage. Experimentally this should be manifested as counter-propagating intra-layer inhomogeneous charge currents proportional to the interlayer voltage and total topological charge of the pseudo-spin system.     

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.     

Disorder induced control of discrete soliton ratchets

We demonstrate that directed transport of topological solitons in damped, biharmonically driven Frenkel-Kontorova chains can be strongly enhanced by introducing suitable phase disorder into the asymmetric periodic driving. From a collective coordinate formalism, we theoretically deduce an effective deterministic equation of motion governing the dynamics of the soliton center-of-mass for which we predict the dependence of maximal soliton drift on disorder strength according to recently proposed general scaling laws concerning directed transport induced by symmetry breaking of temporal forces. We find that these results are in excellent agreement with those of computer simulations of the original Frenkel-Kontorova chains.     

Accumulation and Release of Self-Field Generated Vortices in Percolative YBa2Cu3O7-δ-Films

Pulsed laser heating of current biased percolative YBa₂Cu₃O_7--films can be used to generate and release self-field induced vortices. A laser pulse yields a voltage signal due to penetration of bias current induced magnetic flux in absence of an external magnetic field. Upon retooling, strongly pinned vortices remain in the film. These remanent vortices have been detected after disconnecting the bias source in the currentless film. Applying a subsequent second laser pulse, again yields a signal voltage but of inverse sign due to magnetic flux redistribution.     

Gyroscopic dynamics of antiferromagnetic vortices in domain boundaries of yttrium orthoferrite

It is established experimentally that the magnetic field directed along the b axis has little effect on the velocities of antiferromagnetic vortices in the domain boundary (DB) of yttrium orthoferrite and fails to explain the presence of an appreciable gyroscopic force acting on these vortices. This force is induced by the dynamic canting of magnetic sublattices proportional to the DB velocity. Due to the canting, the velocities of antiferromagnetic vortices depend initially quadratically on the DB velocity, as was experimentally found in this work. The dynamics of antiferromagnetic vortices in the yttrium orthoferrite DBs is gyroscopic and quasi-relativistic, with the limiting velocity of 20 km/s equal to the velocity of spin waves at the linear portion of their dispersion curve.     

Rectified vortex motion in an Nb film with a spacing-graded array of holes

Superconducting Nb thin films with a spacing-graded array of holes were prepared by electron beam lithography. Two films with different hole gradients were fabricated. The ac-driven vortices were investigated in Nb superconductors with a spacing-graded array of holes. The measurements revealed pronounced rectified voltage when the vortex lattice is driven by an ac injected current. The rectified voltage is mainly caused by the strength of the vortex–vortex interaction. The rectified motion of a vortex is affected by the pinning potential of the spacing-graded array and the applied magnetic field. The vortex–vortex interaction strength changes the effective pinning landscape of the vortices and an asymmetric potential is formed. Vortices depin easily from high concentration to low concentration of pinning sites. In both samples, the ac-driven vortices exhibit a variety of dynamical responses and the rectified voltage is tunable with the applied magnetic field.     

Pinning-induced formation of vortex clusters and giant vortices in mesoscopic superconducting disks

Merged, or giant, multiquanta vortices (GVs) are known to appear in very small superconductors near the superconducting transition due to strong confinement of magnetic flux. Here we present evidence for a new, pinning-related, mechanism for vortex merger. Using Bitter decoration to visualize vortices in small Nb disks with varying degrees of disorder, we show that confinement in combination with strong disorder causes individual vortices to merge into clusters or even GVs well below Tc and Hc2, in contrast to well-defined shells of individual vortices found in the absence of pinning.     

Dynamics of kinematic vortices in a mesoscopic superconducting loop

Using the time-dependent Ginzburg–Landau formalism, we study the dynamic properties of a submicron superconducting loop in applied current and in presence of a perpendicular magnetic field. The resistive state of the sample is caused by the motion of kinematic vortex–antivortex pairs. Vortices and antivortices move in opposite directions to each other, perpendicularly to the applied drive, and the periodic creation and annihilation of such pairs results in periodic oscillations of the voltage across the sample. The dynamics of these kinematic pairs is strongly influenced by the applied magnetic field, which for high fields leads to the flow of just vortices. Kinematic vortices can be temporarily pinned inside the loop with observable trace in the voltage vs. time characteristics.     

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.     

扰动对有机磁体器件自旋极化输运特性的影响

基于紧束缚模型和格林函数方法,研究了有机磁体晶格扰动和侧基自旋取向扰动对金属/有机磁体/金属三明治结构有机自旋器件自旋极化输运特性的影响.计算结果表明:晶格扰动的存在降低了器件的起始偏压,减小了导通电流,并使得电流-电压曲线的量子台阶效应不再显著,扰动不太强时电流仍呈现较高的自旋极化率;而侧基自旋取向扰动减小了体系的自旋劈裂,增加了器件的起始偏压,低偏压下随着扰动的增强器件电流及其自旋极化率明显降低.进一步模拟了温度对器件自旋极化输运的影响. 关键词： 有机自旋电子学 有机磁体 自旋极化输运 自旋过滤     

Unusual vortex dynamics in the quantum-liquid phase of a-MoxSi1−x films

We find the unusual vortex dynamics in the low-temperature liquid phase of amorphous Mo_xSi_1−x films by measuring the fluctuating component of the flux-flow voltage δV(t) about the average voltage. For the thick film, in which the quantum vortex liquid (QVL) phase has been well-determined in the field-temperature plane, δV(t) originating from the vortex motion is clearly visible in the QVL phase, where the distribution of δV(t) is anomalously asymmetric, implying large velocity and/or number fluctuations of driven vortices. For the thin film, in which the QVL phase has not been determined from the static transport measurements, similar unusual vortex motion is observed in nearly the same reduced-temperature regime. We suggest that vortex dynamics in the low-temperature liquid phase of thick and thin films is dominated by common physical mechanisms related to quantum-fluctuation effects.     

Vortex entanglement in disordered superconductors

Vortex entanglement and pinning in multiply connected disordered superconductors are studied. It is shown that the winding of vortices around repulsive obstacles is greatly enhanced by quenched columnar disorder and suppressed by point disorder, compared to the clean case. This leads to an additional contribution to the effective pinning force acting on vortices, which, unlike the conventional mechanisms of pinning, grows with temperature.     

Carrier confinement in disordered GaAs/AlAs superlattices probed by capacitance–voltage experiments

The effects of the intentional disorder in (GaAs)_m/(AlAs)₆ superlattices were studied using transport techniques. Evidences of a strong electron localization in the superlattices even in the presence of extend states were found. We interpret this result taking into account the disorder which causes the local breakdown of the coherence of the miniband transport and, therefore, give rise to the electron localization. In order to support our experiments, we numerically calculate the capacitance–voltage characteristics of the superlattices and the results were found in good agreement with the measured ones.     

Self-assembly and vortices formed by microparticles in weak electrolytes

We carried out experimental studies of the self-assembly of metallic micron-size particles in poorly conducting liquid subject to a constant electric field. Depending on the experimental conditions, the particles self-assemble into long chains directed along the electric field lines and form vortices and other structures. The vortices perform Brownian-type random motion due to self-induced chaotic hydrodynamic flows. We measured the diffusivity constant of the vortices and the conductivity and mechanical stiffness of the chains.