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-

The 2D quantum system of electrons at a GaAs/GaAlAs heterojunction in high magnetic field at low temperature is shown to exhibit conduction typical of pinned charge-density waves. Crossover from Ohmic conduction occurs on the same boundary at which radio-frequency resonances signal the onset of transverse elasticity. A further small non-Ohmic region is isolated from the main area by a v-j quantum-Hall-effect phase. The relationship found between the threshold conduction field and the resonance frequency is well accounted for by a model of a pinned electron crystal.