Suppression of matching field effects by splay and pinning energy   dispersion in YBa_2Cu_3O_7 with columnar defects
  We report measurements of the irreversible magnetization M_i of a large number of YBa_2Cu_3O_7 single crystals with columnar defects (CD). Some of them exhibit a maximum in M_i when the density of vortices equals the density of tracks, at temperatures above 40K. We show that the observation of these matching field effects is constrained to those crystals where the orientational and pinning energy dispersion of the CD system lies below a certain threshold. The amount of such dispersion is determined by the mass and energy of the irradiation ions, and by the crystal thickness. Time relaxation measurements show that the matching effects are associated with a reduction of the creep rate, and occur deep into the collective pinning regime. 
D. Niebieskikwiat, A.V. Silhanek, L. Civale, G. Nieva, P. Levy and L.   Krusin-Elbaum