STM Imaging of Flux Line Arrangements in the Peak Effect Regime
  We present the results of a study of vortex arrangements in the peak-effect regime of 2H-NbSe_2 by scanning tunneling microscopy. By slowly increasing the temperature in a constant magnetic field, we observed a sharp transition from collective vortex motion to positional fluctuations of individual vortices at the temperature which coincides with the onset of the peak effect in ac-susceptibility. We conclude that the peak effect is a disorder driven transition, with the pinning energy winning from the elastic energy. 
A. M. Troyanovski, M. van Hecke, N. Saha, J. Aarts and P. H. Kes