Driven Vortex States and Relaxation in Single Crystal $YBa_2Cu_4O_8$
  The vortex response to various ac-drives has been studied in high quality $YBa_2Cu_4O_8$ single crystals.   Well within the vortex solid phase a re-entrant resistive state has been observed that is characterized by long relaxation times. Pulse current response and IV characteristics reveal that in this state the driven vortex system relaxes in a way to increase t he effective pinning force. A phenomenological model accounts for the observed features, in particular the clock-wise hysteresis of the IV-curves and the increase of the apparent critical current with applied current amplitude. 
V. Metlushko, U. Welp, I. Aranson, S. Scheidl, V. M. Vinokur, G. W.   Crabtree, K. Rogacki, B. Dabrowski