Angular dependence of the activation energy in YBCO/PBCO multilayers

The activation energy in several (YBCO)(m)/(PBCO)(n) superconducting multil ayers was deduced from resistivity measurements in magnetic fields up to 16 T, as a function of the angle between the magnetic field and the ab planes . The activation energy U-0(B,theta) scales with a reduced field B-red=B(si n(2) theta+cos(2) theta/Gamma(2))(1/2), with an angle dependent anisotropy parameter Gamma. A crossover from U(0)proportional to log B-red at low B-re d, to U(0)proportional to(B-red)(alpha) at high B-red, is also observed The implications of these results on vortex dynamics and on the coupling betwe en the superconducting layers will be discussed.