Angular Dependence of the Activation Energy in YBCO/PBCO Multilayers

The activation energy in several (YBCO)_m/(PBCO)_nsuperconducting multilayers 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 U₀(B,) scales with a reduced field B_red=B(sin²+cos²/²)^1/2, with an angle dependent anisotropy parameter . A crossover from U₀log B_red, at low B_red, to U₀(B_red)at high B_red, is also observed. The implications of these results on vortex dynamics and on the coupling between the superconducting layers will be discussed.