Thermally activated flux motion in a-axis ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$/${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ superlattices

We have investigated resistive properties of a-axis ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$/a,b-axis ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ superlattices in magnetic fields to study thermally activated flux-flow resistance. We find that in magnetic fields perpendicular to the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$/${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ layering the activation energies of these superlattices deduced from the tail of the resistive transitions have a logarithmic magnetic-field dependence for small ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ thicknesses (<300 Å), in agreement with recent theories of thermally activated resistance in strongly pinned two-dimensional superconductors.