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Flow behavior of a flux line lattice in the layered superconductor 2//-NbSe2 is studied with a magnetic field parallel to the layers in the vicinity of a pronounced peak effect. A striking crossover in the current-voltage characteristics is observed as the system enters the peak regime. The results yield a nonequilibrium phase diagram where conventional depinning of an elastic medium occurs for a rigid lattice. A defective (plastic) flow instability occurs as the lattice softens, but heals at large drives. This defective flow dominates the dynamics for a very soft lattice.PACS numbers: 74.60. Ge, 7l.45.Lr, 62.20.Fe, 74.60Jg Dynamics of an Abrikosov flux line lattice (FLL), pinned by quenched random disorder, is representative of a generic problem of collective dynamics of a disordered elastic medium with many degrees of freedom. The competition between interaction, i.e., the elasticity of the medium and disorder in the form of pinning centers leads to a threshold behavior. Above the threshold for depinning, the system moves collectively. In a pioneering theoretical work, Fisher [1] has suggested that the depinning transition represents a "dynamical critical phenomenon." The subject is of wide interest since similar behavior is observed in a broad class of systems, such as the incommensurate charge density wave (CDW) [2], two-fluid interface in a random medium [3] or a rough substrate [4], and Wigner solids in semiconductor heterostructures [5]. Despite the easy tunability of the interaction through the magnetic field, little is known about the dynamics of the moving FLL in the two different regimes of behavior, where interaction and disorder, respectively, dominate the dynamics.In this paper we present an experimental determination of a "nonequilibrium phase diagram" describing the dynamics of the FLL in both regimes. We find a dramatic change in the l-V curve as the interaction among the flux lines is varied by varying the magnetic field in a low temperature superconductor. We attribute this to a crossover between a coherent motion of an elastic medium for a relatively stiff FLL, to a primarily defective ("plastic") flow of a soft FLL. Importantly, the plastic deformations heal at large drives and the elastic regime recovers. This recovery occurs at progressively larger drives as the FLL softens, such that the plastic regime dominates the dynamics for a very soft lattice.Measurements were performed on single crystal samples of the layered superconductor 2//-NbSe2. The sample (of dimension 1 mm x 1 mm x 30 pm) has a T c of 7.2 K (width -20 mK) and a residual resistance ratio /? -20. Both the magnetic field and the current are in the a-b plane and orthogonal to each other so as to maximize the driving Lorentz force F^-JxB. Pinning is weak in as-grown single crystals, i.e., the FLL is well formed; critical current density is in the lO'-lO 2 A/cm 2 range. The system is well described by the anisotropic Ginzburg-Landau (GL) model [6]. The GL parameter for Hlla,b is K\\ -30; the anisotropy factor in H C 2 between H...

We demonstrate that the changes in the elastic properties of the FeAs systems, as seen in our resonant ultrasound spectroscopy data, can be naturally understood in terms of fluctuations of emerging nematic degrees of freedom. Both the softening of the lattice in the normal, tetragonal phase as well as its hardening in the superconducting phase are consistently described by our model. Our results confirm the view that structural order is induced by magnetic fluctuations.

Strong metastability and history dependence are observed in DC and pulsed transport studies of flux-line lattices in 2H-N bSe2, leading to the identification of two distinct states of the lattice with different spatial ordering. The metastability is most pronounced upon crossing a transition line marked by a large jump in the critical current (the peak effect). Current induced annealing of the metastable state towards the stable state is observed with a strongly current dependent annealing time, which diverges as a threshold current is approached from above.PACS numbers: 74.60.Ge 74.60.Jg 74.60Ec In the absence of disorder the physics of a magnetic flux-line lattice (FLL) is governed by the interplay between thermal fluctuations, which favor melting, and interactions, that lead to ordering. The resulting phase diagram consists of a liquid and a solid phase with relatively simple dynamics. Quenched disorder causes the system to develop additional phases and complex dynamic effects such as pinning and irreversibility in the magnetic and electric responses. The role played by disorder and pinning in the physics of FLL in equilibrium has recently become an area of intense study [1,2]. A related but distinct topic of current interest concerns the role of motion on the spatial ordering of the FLL, the resulting dynamical transitions or crossovers that may occur, and the relation they bear to the disorder free situation [3][4][5][6][7].In this Letter we report on the existence of two distinct states of the FLL, one disordered, the other much less disordered (hereafter referred to as the ordered state) , with strikingly large differences between their transport properties. As a result the system displays a wide range of phenomena such as history-dependence, metastability, current-induced annealing and glassy relaxation. Each of these states is stable in its own sharply defined region of the (H,T) plane and metastable elsewhere and each can be accessed with a simple reproducible procedure. Our experiments show directly that the metastable state can be annealed into the "equilibrium" state by applying a current that depins the FLL. The annealing kinetics is found to be strongly current dependent, with the annealing time diverging as the depinning current is approached from above. The variation of these phenomena with field, temperature, and driving current provides direct access to the interplay between static and dynamic transitions and can elucidate the role of disorder in different parts of the FLL phase diagram. Our results can also be used to interpret the rich and complex history dependence studied earlier in low T c superconducting films [8,9].The history dependence is closely associated with the phenomenon of "peak effect". This effect, which is observed in many weak-pinning superconductors, is charac-