Driven motion of vortices in superconductors (original) (raw)
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Dynamics of superconducting vortices driven by oscillatory forces in the plastic-flow regime
Physical Review B, 2011
We study experimentally and theoretically, the reorganization of superconducting vortices driven by oscillatory forces near the plastic depinning transition. We show that the system can be taken to configurations that are tagged by the shaking parameters but keep no trace of the initial conditions. In experiments performed in N bSe2 crystals, the periodic drive is induced by ac magnetic shaking fields and the overall order of the resulting configuration is determined by non invasive ac susceptibility measurements. With a model of interacting particles driven over random landscapes, we perform molecular dynamics simulations that reveal the nature of the shaking dynamics as fluctuating states similar to those predicted for other interacting particle systems.
Nanomechanics of Individual, Isolated Vortices in a Cuprate Superconductor
Nature Physics, 2009
Superconductors often contain quantized microscopic whirlpools of electrons, called vortices, that can be modelled as one-dimensional elastic objects 1 . Vortices are a diverse area of study for condensed matter because of the interplay between thermal fluctuations, vortex-vortex interactions and the interaction of the vortex core with the three-dimensional disorder landscape 2-5 . Although vortex matter has been studied extensively 1,6,7 , the static and dynamic properties of an individual vortex have not. Here, we use magnetic force microscopy (MFM) to image and manipulate individual vortices in a detwinned YBa 2 Cu 3 O 6.991 single crystal, directly measuring the interaction of a moving vortex with the local disorder potential. We find an unexpected and marked enhancement of the response of a vortex to pulling when we wiggle it transversely. In addition, we find enhanced vortex pinning anisotropy that suggests clustering of oxygen vacancies in our sample and demonstrates the power of MFM to probe vortex structure and microscopic defects that cause pinning.
Structure and orientation of the moving vortex lattice in clean type-II superconductors
Physical Review B, 2004
The dynamics of moving vortex lattice is considered in the framework of the time dependent Ginzburg - Landau equation neglecting effects of pinning. At high flux velocities the pinning dominated dynamics is expected to cross over into the interactions dominated dynamics for very clean materials recently studied experimentally. The stationary lattice structure and orientation depend on the flux flow velocity. The vortex lattice will have a different orientation for V>V_c. The two orientations can be desribed as motion "in channels" and motion of "lines of vortices perpendicular to the direction of motion. Although we start from the lowest Landau level approximation, corrections to conductivity and the vortex lattice energy dissipation from higher Landau levels are systematically calculated and compared to a recent experiment.
1997
Using large scale simulations on parallel processors, we analyze in detail the dynamical behavior of superconducting vortices undergoing avalanches. In particular, we quantify the effect of the pinning landscape on the macroscopic properties of vortex avalanches and vortex plastic flow. These dynamical instabilities are triggered when the external magnetic field is increased slightly, and are thus driven by a flux gradient rather than by thermal effects. The flux profiles, composed of rigid flux lines that interact with 100 or more vortices, are maintained in the Bean critical state and do not decay away from it. By directly determining vortex positions during avalanches in the plastically moving lattice, we find that experimentally observable voltage bursts correspond to the pulsing movement of vortices along branched channels or winding chains in a manner reminiscent of lightning strikes. This kind of motion cannot be described by elastic theories. We relate the velocity field and cumulative patterns of vortex flow channels with statistical quantities, such as distributions of avalanche sizes. Samples with a high density of strong pinning sites produce very broad avalanche distributions. Easy-flow vortex channels appear in samples with a low pinning density, and typical avalanche sizes emerge in an otherwise broad distribution of sizes. We observe a crossover from interstitial motion in narrow channels to pin-to-pin motion in broad channels as pin density is increased.
Surface instabilities and vortex transport in current-carrying superconductors
Physical Review B, 1998
We investigate the stability of the vortex configuration in thin superconducting films and layered Josephsoncoupled superconductors under an applied current analytically and by numerical simulations of the timedependent Ginzburg-Landau equation. We show that the stationary vortex lattice becomes unstable with respect to long-wavelength perturbations above some critical current I c. We find that at currents slightly exceeding I c the vortex phase develops plastic flow, where large coherent pieces of the lattice are separated by lines of defects and slide with respect to each other. At elevated currents a transition to elastic flow is observed. We obtained the effective one-dimensional Ginzburg-Landau equation for a description of the vortex penetration from the edges. We discuss this transition in terms of a one-dimensional phase-slip phenomenon in superconducting wires with a periodically modulated temperature. We found several distinct dynamic vortex phases in the layered current-carrying superconductors. We show that for some intermediate range of the current, depending on the coupling between the layers, the coherent motion of the pancake vortices in different layers becomes unstable leading to dynamic decoupling. ͓S0163-1829͑98͒07205-1͔
Dynamic vortex phases and pinning in superconductors with twin boundaries
Phys Rev B, 2000
We investigate the pinning and driven dynamics of vortices interacting with twin boundaries using large scale molecular-dynamics simulations on samples with near one million pinning sites. For low applied driving forces, the vortex lattice orients itself parallel to the twin boundary and we observe the creation of a flux gradient and vortex-free region near the edges of the twin boundary. For increasing drive, we find evidence for several distinct dynamical flow phases which we characterize by the density of defects in the vortex lattice, the microscopic vortex flow patterns, and orientation of the vortex lattice. We show that these different dynamical phases can be directly related to microscopically measurable voltage-current V(I) curves and voltage noise. By conducting a series of simulations for various twin boundary parameters we derive several vortex dynamic phase diagrams.
Probing dynamics and pinning of single vortices in superconductors at nanometer scales
The dynamics of quantized magnetic vortices and their pinning by materials defects determine electromagnetic properties of superconductors, particularly their ability to carry non-dissipative currents. Despite recent advances in the understanding of the complex physics of vortex matter, the behavior of vortices driven by current through a multi-scale potential of the actual materials defects is still not well understood, mostly due to the scarcity of appropriate experimental tools capable of tracing vortex trajectories on nanometer scales. Using a novel scanning superconducting quantum interference microscope we report here an investigation of controlled dynamics of vortices in lead films with sub-Angstrom spatial resolution and unprecedented sensitivity. We measured, for the first time, the fundamental dependence of the elementary pinning force of multiple defects on the vortex displacement, revealing a far more complex behavior than has previously been recognized, including striking spring softening and broken-spring depinning, as well as spontaneous hysteretic switching between cellular vortex trajectories. Our results indicate the importance of thermal fluctuations even at 4.2 K and of the vital role of ripples in the pinning potential, giving new insights into the mechanisms of magnetic relaxation and electromagnetic response of superconductors. T he ability to carry non-dissipative electric currents in strong magnetic fields is one of the fundamental features of type-II superconductors crucial for many applications 1–10. The current, however, exerts a transverse Lorentz force on vortices, leading to their dissipative motion and to finite resistance unless materials defects immobilize (pin) vortices at current densities below some critical value J c. Pinning potential wells U(r) produced by defects are the key building blocks that determine collective pinning phenomena, in which a flexible vortex line is pinned by multiple defects. The global electromagnetic response of the vortex matter is thus governed by a complex interplay of individual pinning centers, interaction between vortices, and thermal fluctuations 11. Recent advances in materials science have enabled several groups to controllably produce nano-structures of nonsuperconducting precipitates to optimize the pinning of vortices in superconductors and to achieve J c up to 10–30% of the fundamental depairing current density J d at which the current breaks Cooper pairs 2–10. In artificially-engineered pinning structures, the shape of U(r) can also be made asymmetric to produce the intriguing vortex ratchet and rectification phenomena 12–17. Local studies of individual vortices using scanning probe techniques 18–20 based on STM, MFM, Hall probes and SQUIDs have allowed controllable manipulation of single vortices 21 and have revealed phase transitions 22–24 , vortex dynamics and pinning at grain boundaries 25,26 , collective creep of a vortex lattice 27,28 , and collective motion 29,30 and dissipative hopping of individual vortices in response to an ac magnetic field 31. Even so, the intrinsic structure of a single pinning potential well U(r), which determines the fundamental interaction of vortices with pinning centers, has not yet been measured directly. Difficulties with the measurement of U(r) arise from the necessity of deconvoluting the effect of multiple pinning defects along the elastic vortex line, and from the lack of experimental techniques for extracting U(r) on the scale of the superconducting coherence length j (ranging from 2 to 100 nm for different materials) that quantifies the size of the Cooper pair. To avoid the complexity of the situation in which the vortex line behaves like an elastic string pinned by multiple defects 21 , the vortex length should to be of the order of j. This situation can be achieved in thin superconducting films of thickness d%j in a perpendicular magnetic field. Moreover, in order to simplify the interpretation of experimental data, it is desirable to choose a film in which the three basic length
On the origin of high smectic fluidity of the vortex lattice in a 2D superconductor
Europhysics Letters (EPL), 2006
We study the response of the vortex lattice in a strongly type-II 2D superconductor to quasi-static sweeping magnetic induction using the Ginzburg-Landau approach in the lowest Landau level approximation. It is found that, due to small size and relative softness of the vortex core as compared to the atomic core of a typical crystal potential, the vortices tend to flow in independently moving channels with smectic structure. A model for calculating the friction coefficient of moving vortices in the presence of a few pinning centers under conditions of minimal power dissipation is presented.
Physical Review B, 2006
In this work we investigate the dynamics of vortices in a square mesoscopic superconductor. As time evolves we show how the vortices are nucleated into the sample to form a multivortex, single vortex, and giant vortex states. We illustrate how the vortices move around at the transition fields before they accommodate into an equilibrium configuration. We also calculate the magnetization and the free energy as functions of the applied magnetic field for several values of temperature. In addition, we evaluate the upper critical field.