Angular dependence of flux pinning in high-T_c superconducting films

We have investigated the angular dependence of the dissipative characteristics of YBa₂>Cu₃>O_7-y(>YBCO) and >Bi₂>Sr₂>CaCu₂>O_8-y(>BSCCO) films by measuring the resistivity in a field. The dissipation can be described by the thermal activated flux creep model, >R=R_O exp(-U(H,T,θ)/k_BT). U depends on the magnetic field as well as the angle, θ, the magnetic field makes with the c-axis. We found that the activation energy of YBCO films decreases by a factor of 2 while that of BSCCO films decreases by a factor of about 10 in >H = 5.5>T and at >T/T_c = 0.93 as one varies θ from 90° to 0°, where θ is the angle between the c axis and the direction of the field.