The surface impedance of 2H-NbSe2 in the
mixed state was measured over the frequency range 10–3000 MHz
in fields up to 2 T. A crossover between pinned and viscous
flux-line dynamics is observed at a pinning frequency, wp.
The measured wp is compared
to that predicted by a single-particle "washboard potential" model
of pinning. When the flux lattice is in an ordered state, wp
is in good agreement with the model. However, when it is in
a metastable disordered state, wp
is nearly two orders of magnitude larger than expected, indicating
the appearance of internal degrees of freedom and glassy dynamics.