Vortex dynamics below the flux-lattice melting transition in YBa2Cu3O7- delta

Dynamics of two-dimensional colloids on a disordered substrate

Using Langevin simulations, we numerically study the dynamics of two-dimensional colloids on a disordered substrate. With a decreasing strength of the interaction between colloids, we find a crossover from elastic to plastic depinnings, where a substantial increase in the depinning force is observed. Furthermore, we find a dynamical phase transition from the moving liquid to the moving smectic at high driving forces by decreasing the temperature. Peak effect occurs in the dynamical critical driving force across the transition, accompanied by a clear crossing of velocity-force dependence curves.

Critical currents at the bragg glass to vortex glass transition

We present simulations of the transport properties of superconductors at the transition from the Bragg glass (BG) to the vortex glass (VG) phase. We study the frustrated anisotropic 3D XY model with point disorder, which has been shown to have a first-order transition as a function of the intensity of disorder. We add an external current to the model, and we obtain current-voltage curves as a function of disorder at a low temperature. We find that the in-plane critical current has a steep increase at the BG-VG transition, while the c-axis critical current has a discontinuous jump down, this later result in agreement with the first-order character of the transition.

Nonequilibrium phase transitions of vortex matter in three-dimensional layered superconductors

Large-scale simulations on the three-dimensional (3D) frustrated anisotropic XY model have been performed to study the nonequilibrium phase transitions of vortex matter in weak random pinning potential in layered superconductors. The first-order phase transition from the moving Bragg glass to the moving smectic is clarified, based on thermodynamic quantities. A washboard noise is observed in the moving Bragg glass in 3D simulations for the first time. It is found that the activation of the vortex loops plays the dominant role in the dynamical melting at high drive.

History effects and phase diagram near the lower critical point in YBa2Cu3O7 single crystals

Using a sensitive torque magnetometer we have studied magnetization curves for untwinned overdoped YBa2Cu3O7 single crystals in fields of up to 28 T. We demonstrate the existence of history effects below the lower critical point and provide a full demarcation of the Bragg-glass phase. A pronounced symmetry is observed in the behavior of the phase lines, irreversible magnetization, and value of the magnetization jump near both critical points.

Effects of point defects on the phase diagram of vortex states in high- T(c) superconductors in the B parallel to c axis

The phase diagram for the vortex states of high- T(c) superconductors with point defects in the B--> parallel to c axis is drawn by large-scale Monte Carlo simulations. The vortex slush (VS) phase is found between the vortex glass (VG) and vortex liquid (VL) phases. The first-order transition between this novel normal phase and the VL phase is characterized by a sharp jump of the density of dislocations. The first-order transition between the Bragg glass (BG) and VG or VS phases is also clarified. These two transitions are compared with the melting transition between the BG and VL phases.

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.

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 time scales 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.