We present a mode locking (ML) phenomenon of vortex matter observed around
the peak effect regime of 2H-NbSe2 pure single crystals. The ML features
allow us not only to trace how the shear rigidity of driven vortices persists
on approaching the second critical field, but also to demonstrate a dynamic
melting transition of driven vortices at a given velocity. We observe the
velocity dependent melting signatures in the peak effect regime, which reveal a
crossover between the disorder-induced transition at small velocity and the
thermally induced transition at large velocity. This uncovers the relationship
between the peak effect and the thermal melting.Comment: To appear in Physical Review Lette