MAGNETIC-FIELD DEPENDENCE OF THE THERMAL-ACTIVATION ENERGY IN EPITAXIAL YBA2CU3O7-DELTA THIN-FILMS

Thermally activated flux motion has been studied in c-axis oriented ep itaxial YBa2Cu3O7-delta thin films with a critical current density J(c ) of 4 x 10(6) A/cm(2) at 77 K in the absence of an external magnetic field. The magnetoresistivity and the critical current density were me asured in applied magnetic fields H, up to 20 T, for H perpendicular a cid parallel to the c-axis of the films. Critical current densities at 77 K are still as high as 7 X 10(4) A/cm(2) in 20 T field for H perpe ndicular to c-axis and 2 x 10(3) A/cm(2) in 10 T field for H parallel to c-axis. It was found that the magnetic-field dependence of the acti vation energy follows a power law, U-0(H) similar to H--alpha. Crossov ers of the a values were observed from alpha(1) = 0.22 to alpha(2) = 0 .5 at 0.3 T for H perpendicular to c-axis and from alpha(1) = 0.33 to alpha(2) = 1 at 6 T for H parallel to c-axis. These crossovers were in terpreted as transitions from a low-field single-vortex pinning to a c ollective-vortex pinning in high field as predicted by Yeshurun and Ma lozemoff [Phys. Rev. Lett. 60 (1988) 2202]. In the collective-pinning regime, the observed anisotropic dependences of U-0(H) are assumed to be due to the intrinsic planar pinning for H perpendicular to c-axis a nd extrinsic point pinning for H parallel to c-axis.