Varieties of dynamics in a disordered flux-line lattice

Dynamics of a flux-line lattice (FLL) has been studied in the anisotropic superconductor 2H-NbSe2, where pinning is extremely weak (critical current/depairing current similar to 10(-6)), the lattice is well-formed and a robust ''peak effect'' occurs slightly below H-c2. The strong H dependence of the rigidity of the FLL is used to explore the crossover between interaction-dominated and disorder-dominated dynamics. Three distinct types of dynamics, describing elastic flow, plastic flow and fluid flow, respectively, are observed as the lattice softens with the approach to the superconducting-to-normal phase boundary. A power-law behavior, V similar to (I-I-c)(beta), describes the I-V curves for both elastic and fluid flow (with different beta) reasonably well, but not for plastic flow. The latter dominates the intermediate regime, where the peak effect occurs, and is accompanied by qualitative changes in the I-V curves related to tearing of the FLL. In this regime, a ''fingerprint phenomenon'' is observed in the current-dependent differential resistance describing a specific sequence of depinning of ''chunks'' or filaments in a given sample. At large driving forces, the disorder due to pinning is less significant, the dynamically generated defects heal and a nearly defect-free coherent motion is recovered. In some samples, the I-V curves at the onset of motion in this regime become discontinuous and hysteretic as in first-order transitions. A ''non-equilibrium phase diagram'' describing various regimes of dynamics is constructed. A time-averaged velocity correlation length characterizes the spatial inhomogeneity of a moving FLL and thus distinguishes between the various dynamical regimes. These dynamical phenomena are compared and contrasted with phenomena attributed to thermodynamic phase transitions in the cuprate superconductors.