AC Dynamics and Metastability of a Flux-Line Lattice

We have measured the complex surface impedance of 2H-NbSe_2 in the mixed state over a wide range of magnetic field(0-2T) and frequency(10-3000MHz). A crossover between pinned and viscous dynamics of flux-lines is observed at a pinning frequency, $\omega_p$. The measured $\omega_p$ is compared to that predicted by a single-particle "washboard potential" model for the pinning interaction. When the flux lattice is prepared in an ordered state by zero-field cooling or by driving it with a large current, $\omega_p$ is in good agreement with the predicted value. However, when it is prepared in a metastable disordered state by field cooling, $\omega_p$ is nearly two orders of magnitude higher than expected indicating a complete break-down of the single-particle model.