Vortex dynamics in columnar-defected YBCO-films

By high-resolution SQUID-magnetometry, we study the influence of columnar d efects of field-equivalent density B-phi =1T on the flux creep in thin YBa2 Cu3O7-delta films. At temperatures T*(B,B-phi), the relaxation rates reveal a sharp crossover from a glassy dynamics to a faster algebraic flux creep which disappears at the transition lines T-c(B) to equilibrium superconduct ivity. As key results we report the effect of defect filling f = B/B-phi on the exponent mu of the glassy vortex dynamics at T < T*. For vanishing f w e find mu = 1/3 indicating a Mott-like variable range hopping of free vorti ces while mu(f = 0.1) = 0.7 agrees with a result by recent numerical work f or interacting flux lines in the Bose glass (BG) model. For larger filling, however, we observe mu(f --> 1) --> 1, which is inconsistent with these BG simulations. (C) 1999 Published by Elsevier Science B.V. All rights reserv ed.