arXiv subjects

C. Reichhardt

Publications and source records attributed to C. Reichhardt.

305 records · Page 17Linked to original sources

arXiv1999

Melting of Moving Vortex Lattices With Periodic Pinning

We study numerically the effects of temperature on moving vortex lattices interacting with periodic pinning arrays. For low temperatures the vortex lattice flows in channels, forming a hexatic structure with long range transverse and longitudinal ordering. At higher temperatures, a transition to a smectic state occurs where vortices wander between channels and longitudinal order is lost while transverse order remains. At the highest temperatures the vortex lattice melts into an isotropic liquid.

cond-mat.supr-con↗

arXiv1999

Superconducting Fluxon Pumps and Lenses

We study stochastic transport of fluxons in superconductors by alternating current (AC) rectification. Our simulated system provides a fluxon pump, "lens", or fluxon "rectifier" because the applied electrical AC is transformed into a net DC motion of fluxons. Thermal fluctuations and the asymmetry of the ratchet channel walls induce this "diode" effect, which can have important applications in devices, like SQUID magnetometers, and for fluxon optics, including convex and concave fluxon lenses. Certain features are unique to this novel two-dimensional (2D) geometric pump, and different from the previously studied 1D ratchets.

cond-mat.supr-con↗

arXiv1999

Vortices Freeze like Window Glass: the Vortex Molasses Scenario

We overview several recent experimental and numerical observations, which are at odds with the Vortex Glass theory of the freezing of disordered vortex matter. To reinvestigate the issue, we performed numerical simulations of the overdamped London - Langevin model, and use finite size scaling to analyze the data. Upon approaching the transition the initial Vortex Glass type criticality is arrested at some crossover temperature. Below this temperature the timescales continue growing very quickly, consistent with the Vogel-Fulcher form, while the spatial correlation length xi stops exhibiting any observable divergence. We call this mode of freezing the Vortex Molasses scenario.

cond-mat.supr-con↗

arXiv1999

Dynamic Vortex Phases and Pinning in Superconductors with Twin Boundaries

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.

cond-mat.supr-con↗

arXiv1999

Transverse depinning in strongly driven vortex lattices with disorder

Using numerical simulations we investigate the transverse depinning of moving vortex lattices interacting with random disorder. We observe a finite transverse depinning barrier for vortex lattices that are driven with high longitudinal drives, when the vortex lattice is defect free and moving in correlated 1D channels. The transverse barrier is reduced as the longitudinal drive is decreased and defects appear in the vortex lattice, and the barrier disappears in the plastic flow regime. At the transverse depinning transition, the vortex lattice moves in a staircase pattern with a clear transverse narrow-band voltage noise signature.

cond-mat.supr-con↗

arXiv1999

Topological Invariants in Microscopic Transport on Rough Landscapes: Morphology, Hierarchical Structure, and Horton Analysis of Riverlike Networks of Vortices

River basins as diverse as the Nile, the Amazon, and the Mississippi satisfy certain topological invariants known as Horton's laws. Do these macroscopic (up to 10^3 km) laws extend to the micron scale? Through realistic simulations, we analyze the morphology and statistical properties of networks of vortex flow in flux-gradient-driven superconductors. We derive a phase diagram of the different network morphologies, including one in which Horton's laws of length and stream number are obeyed -- even though these networks are about 10^9 to 10^15 times smaller than geophysical river basins.

cond-mat.supr-con↗

arXiv1998

Phase Locking, Devil's Staircases, Farey Trees, and Arnold Tongues in Driven Vortex Lattices with Periodic Pinning

Using numerical simulations, we observe phase locking, Arnold tongues, and Devil's staircases for vortex lattices driven at varying angles with respect to an underlying superconducting periodic pinning array. This rich structure should be observalble in transport measurments. The transverse $V(I)$ curves have a Devil's staircase structure, with plateaus occurring near the driving angles along symmetry directions of the pinning array. Each of the plateaus corresponds to a different dyanmical phase with a distinctive vortex structure and flow pattern.

cond-mat.supr-con↗

arXiv1998

Nonequilibrium Dynamic Phase Diagram for Vortex Lattices

The new dynamic phase diagram for driven vortices with varying lattice softness we present here indicates that, at high driving currents, at least two distinct dynamic phases of flux flow appear depending on the vortex-vortex interaction strength. When the flux lattice is soft, the vortices flow in independently moving channels with smectic structure. For stiff flux lattices, adjacent channels become locked together, producing crystalline-like order in a coupled channel phase. At the crossover lattice softness between these phases, the system produces a maximum amount of voltage noise. Our results relate spatial order with transport and are in agreement with experiments.

cond-mat.supr-con↗

arXiv1998

Fractal Networks, Braiding Channels, and Voltage Noise in Intermittently Flowing Rivers of Quantized Magnetic Flux

We analyze the microscopic dynamics of vortex motion through channels that form river-like fractal networks in a variety of superconducting samples, and relate it to macroscopic measurable quantities such as the power spectrum. As a function of pinning strength, we calculate the fractal dimension, tortuosity, and the corresponding voltage noise spectrum. Above a certain pinning strength, a remarkable universal drop in both tortuosity and noise power occurs when the vortex motion changes from shifting braiding channels to unbraided channels. We compare our results with experiments.

cond-mat.supr-con↗

arXiv1998

Commensurate and Incommensurate Vortex States in Superconductors with Periodic Pinning Arrays

As a function of applied field, we find a rich variety of ordered and partially-ordered vortex lattice configurations in systems with square or triangular arrays of pinning sites. We present formulas that predict the matching fields at which commensurate vortex configurations occur and the vortex lattice orientation with respect to the pinning lattice. Our results are in excellent agreement with recent imaging experiments on square pinning arrays [K. Harada et al., Science 274, 1167 (1996)].

cond-mat.supr-con↗

arXiv1997

Superconducting vortex avalanches, voltage bursts, and vortex plastic flow: Effect of the microscopic pinning landscape on the macroscopic properties

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. Videos available at http://www-personal.engin.umich.edu/~nori

cond-mat.supr-con↗

arXiv1997

Dynamic Phases of Vortices in Superconductors with Periodic Pinning

We present results from extensive simulations of driven vortex lattices interacting with periodic arrays of pinning sites. Changing an applied driving force produces a rich variety of novel dynamical plastic flow phases which are very distinct from those observed in systems with random pinning arrays. Signatures of the transition between these different dynamical phases include sudden jumps in the current-voltage curves as well as marked changes in the vortex trajectories and the vortex lattice order. Several dynamical phase diagrams are obtained as a function of commensurability, pinning strength, and spatial order of the pinning sites.

cond-mat.supr-con↗

arXiv1996

Plastic Flow, Voltage Bursts, and Vortex Avalanches in Superconductors

We use large-scale parallel simulations to compute the motion of superconducting magnetic vortices during avalanches triggered by small field increases. We find that experimentally observable voltage bursts correspond to pulsing vortex movement along branched channels or winding chains, and relate vortex flow images to features of statistical distributions. As pin density is increased, a crossover occurs from interstitial motion in narrow easy-flow winding channels with typical avalanche sizes, to pin-to-pin motion in broad channels, characterized by a very broad distribution of sizes. Our results are consistent with recent experiments.

cond-mat.supr-con↗

arXiv1996

Vortex Plastic Motion in Twinned Superconductors

We present simulations, without electrodynamical assumptions, of $B(x,y,H(t)), M(H(t))$, and $J_c(H(t))$, in hard superconductors, for a variety of twin-boundary pinning potential parameters, and for a range of values of the density and strength of the pinning sites. We numerically solve the overdamped equations of motion of up to 10^4 flux-gradient-driven vortices which can be temporarily trapped at $\sim 10^6$ pinning centers. These simulations relate macroscopic measurements (e.g., M(H), ``flame'' shaped $B(x,y)$ profiles) with the underlying microscopic pinning landscape and the plastic dynamics of individual vortices.

cond-mat.mtrl-sci↗

arXiv1996

Spatio-temporal dynamics and plastic flow of vortices in superconductors with periodic arrays of pinning sites

We present simulations of flux-gradient-driven superconducting rigid vortices interacting with square and triangular arrays of columnar pinning sites in an increasing external magnetic field. These simulations allow us to quantitatively relate spatio-temporal microscopic information of the vortex lattice with typically measured macroscopic quantities, such as the magnetization $M(H)$. The flux lattice does not become completely commensurate with the pinning sites throughout the sample at the magnetization matching peaks, but forms a commensurate lattice in a region close to the edge of the sample. Matching fields related to unstable vortex configurations do not produce peaks in $M(H)$. We observe a variety of evolving complex flux profiles, including flat terraces or plateaus separated by winding current-carrying strings and, near the peaks in $M(H)$, plateaus only in certain regions, which move through the sample as the field increases.

supr-con↗

arXiv1996

Vortex Plastic Flow, $B(x,y,H(t)), M(H(t)), J_c(B(t))$, Deep in the Bose Glass and Mott-Insulator Regimes

We present simulations of flux-gradient-driven superconducting vortices interacting with strong columnar pinning defects as an external field $H(t)$ is quasi-statically swept from zero through a matching field $B_ϕ$. We analyze several measurable quantities, including the local flux density $ B(x,y,H(t))$, magnetization $M(H(t))$, critical current $J_{c}(B(t))$, and the individual vortex flow paths. We find a significant change in the behavior of these quantities as the local flux density crosses $B_ϕ$, and quantify it for many microscopic pinning parameters. Further, we find that for a given pin density $J_c(B)$ can be enhanced by maximizing the distance between the pins for $ B < B_ϕ $.

supr-con↗

arXiv1995

Microscopic Derivation of Magnetic Flux Density Profiles, Magnetization Hysteresis Loops, and Critical Currents in Strongly Pinned Superconductors

We present a microscopic derivation, without electrodynamical assumptions, of $B(x,y,H(t))$, $M(H(t))$, and $J_c(H(t))$, in agreement with experiments on strongly pinned superconductors, for a range of values of the density and strength of the pinning sites. We numerically solve the overdamped equations of motion % dynamics of these flux-gradient-driven vortices which can be temporarily trapped at pinning centers. The field is increased (decreased) by the addition (removal) of flux lines at the sample boundary, and complete hysteresis loops can be achieved by using flux lines with opposite orientation. The pinning force per unit volume we obtain for strongly-pinned vortices, $J_c B \sim n_p f_p^{1.6}$, interpolates between the following two extreme situations: very strongly-pinned independent vortices, where $J_c B \sim n_p f_p$, and the 2D Larkin-Ovchinikov collective-pinning theory for weakly-pinned straight vortices, where $J_c B \sim n_p f_p^{2}$. Here, $n_p$ and $f_p$ are the density and maximum force of the pinning sites.

cond-mat.mtrl-sci↗