Plastic vortex-creep in YBa$_2$Cu$_3$O$_{7-x}$ crystals
  Local magnetic relaxation measurements in YBa$_2$Cu$_3$O$_{7-x}$ crystals show evidence for plastic vortex-creep associated with the motion of dislocations in the vortex lattice. This creep mechanism governs the vortex dynamics in a wide range of temperatures and fields below the melting line and above the field corresponding to the peak in the ''fishtail'' magnetization. In this range the activation energy $U_{pl}$, which decreases with field, drops below the elastic (collective) creep activation energy, $U_{el}$, which increases with field. A crossover in flux dynamics from elastic to plastic creep is shown to be the origin of the fishtail in YBa$_2$Cu$_3$O$_{7-x}$. 
Y. Abulafia, A. Shaulov, Y. Wolfus, R. Prozorov, L. Burlachkov, and Y.   Yeshurun, D. Majer E. Zeldov, H. W\"uhl, V.B. Geshk...