Statics and Dynamics of Vortex Liquid Crystals

Using numerical simulations we examine the static and dynamic properties of the recently proposed vortex liquid crystal state. We confirm the existence of a smectic-A phase in the absence of pinning. Quenched disorder can induce a smectic state even at T=0. When an external drive is applied, a variety of anisotropic dynamical flow states with distinct voltage signatures occur, including elastic depinning in the hard direction and plastic depinning in the easy direction. We discuss the implications of the anisotropic transport for other systems which exhibit depinning phenomena, such as stripes and electron liquid crystals.Comment: 4 pages, 4 postscript figure

Static and Dynamic Phases for Vortex Matter with Attractive Interactions

Exotic vortex states with long range attraction and short range repulsion have recently been proposed to arise in superconducting hybrid structures and multi-band superconductors. Using large scale simulations we examine the static and dynamic properties of such vortex states interacting with random and periodic pinning. In the absence of pinning this system does not form patterns but instead completely phase separates. When pinning is present there is a transition from inhomogeneous to homogeneous vortex configurations similar to a wetting phenomenon. Under an applied drive, a dynamical dewetting process can occur from a strongly pinned homogeneous state into pattern forming states. We show that a signature of the exotic vortex interactions under transport measurements is a robust double peak feature in the differential conductivity curves.Comment: 5 pages, 4 postscript figure

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.Comment: Phys. Rev. Lett. 83, in press (1999); 4 pages, 5 .gif figures; figures also available at http://www-personal.engin.umich.edu/~nori/ratche

Deformation and Depinning of Superconducting Vortices from Artificial Defects: A Ginzburg-Landau Study

Using Ginzburg-Landau theory, we have performed detailed studies of vortices in the presence of artificial defect arrays, for a thin film geometry. We show that when a vortex approaches the vicinity of a defect, an abrupt transition occurs in which the vortex core develops a ``string'' extending to the defect boundary, while simultaneously the supercurrents and associated magnetic flux spread out and engulf the defect. Current induced depinning of vortices is shown to be dominated by the core string distortion in typical experimental situations. Experimental consequences of this unusual depinning behavior are discussed.Comment: 10 pages,9 figure

Dynamical Phases of Driven Vortices Interacting with Periodic Pinning

The finite temperature dynamical phases of vortices in films driven by a uniform force and interacting with the periodic pinning potential of a square lattice of columnar defects are investigated by Langevin dynamics simulations of a London model. Vortices driven along the [0,1] direction and at densities for which there are more vortices than columnar defects ($B>B_{\phi}$) are considered. At low temperatures, two new dynamical phases, elastic flow and plastic flow, and a sharp transition between them are identified and characterized according to the behavior of the vortex spatial order, velocity distribution and frequency-dependent velocity correlationComment: 4 pages with 4 figures. To be published in Phys. Rev. B Rapid Communication

Colloidal Dynamics on Disordered Substrates

Using Langevin simulations we examine driven colloids interacting with quenched disorder. For weak substrates the colloids form an ordered state and depin elastically. For increasing substrate strength we find a sharp crossover to inhomogeneous depinning and a substantial increase in the depinning force, analogous to the peak effect in superconductors. The velocity versus driving force curve shows criticality at depinning, with a change in scaling exponent occuring at the order to disorder crossover. Upon application of a sudden pulse of driving force, pronounced transients appear in the disordered regime which are due to the formation of long-lived colloidal flow channels.Comment: 4 pages, 4 postscript figure

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.Comment: accepted to Physical Review Letter

Orientational pinning and transverse voltage: Simulations and experiments in square Josephson junction arrays

We study the dependence of the transport properties of square Josephson Junctions arrays with the direction of the applied dc current, both experimentally and numerically. We present computational simulations of current-voltage curves at finite temperatures for a single vortex in the array ($Ha^2/\Phi_0=f=1/L^2$), and experimental measurements in $100\times1000$ arrays under a low magnetic field corresponding to $f\approx0.02$. We find that the transverse voltage vanishes only in the directions of maximum symmetry of the square lattice: the [10] and [01] direction (parallel bias) and the [11] direction (diagonal bias). For orientations different than the symmetry directions, we find a finite transverse voltage which depends strongly on the angle $\phi$ of the current. We find that vortex motion is pinned in the [10] direction ($\phi=0$), meaning that the voltage response is insensitive to small changes in the orientation of the current near $\phi=0$. We call this phenomenon orientational pinning. This leads to a finite transverse critical current for a bias at $\phi=0$ and to a transverse voltage for a bias at $\phi\not=0$. On the other hand, for diagonal bias in the [11] direction the behavior is highly unstable against small variations of $\phi$, leading to a rapid change from zero transverse voltage to a large transverse voltage within a few degrees. This last behavior is in good agreement with our measurements in arrays with a quasi-diagonal current drive.Comment: 9 pages, 9 figure

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.Comment: 5 figures, to appear in Phys. Rev.