Dynamical melting and mode locking of vortex matter in 2H-NbSe2 pure single crystals

We have investigated flow properties of vortex matter around the peak effect region in 2H-NbSe2 pure single crystals by mode locking (ML) techniques. ML is a dynamic synchronization phenomenon between ac drive superimposed on dc drive and collective lattice modes excited over the driven vortex lattice. Around the peak effect regime, we observe the vanishing of the ML resonance, which is velocity dependent: At small velocity, ML vanishes just below the peak field Hp for the peak effect, while at large velocity, it vanishes above Hp. The velocity dependence of the vanishing field agrees qualitatively with a dynamical melting picture proposed by Koshelev and Vinokur.