Field-Induced Broadening of the Resistive Transition and Two-Dimensional Nature of Flux Pinning in ${\mathrm{Y}}_2$${\mathrm{Ba}}_4$${\mathrm{Cu}}_8$${\mathrm{O}}_{16}$ Films

We report a unique dual nature of the magnetic-field-dependent resistive losses in epitaxial films of superconducting ${\mathrm{Y}}_2$ ${\mathrm{Ba}}_4$ ${\mathrm{Cu}}_8$ ${\mathrm{O}}_{16}$. For excitation currents coplanar with the magnetic field, the losses are identical in the Ohmic regime for both longitudinal and transverse field orientations. In the nonlinear current-voltage regime, the transverse field results in the Lorentz-force-induced instability in the fluxline lattice and a higher dissipation. Comparison of the flux-pinning potential in ${\mathrm{Y}}_2$ ${\mathrm{Ba}}_4$ ${\mathrm{Cu}}_8$ ${\mathrm{O}}_{16}$ deduced from the thermally activated nonlinear voltage with data on Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ suggests an important role of the increased separation between Cu-O superconducting planes in ${\mathrm{Y}}_2$ ${\mathrm{Ba}}_4$ ${\mathrm{Cu}}_8$ ${\mathrm{O}}_{16}$ in flux pinning.