Evidence for mean free path fluctuation induced pinning in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ films

The critical current ${\mathit{j}}_{\mathit{c}}$ and the pinning energy ${\mathit{U}}_{\mathit{c}}$ have been determined for three types of yttrium-based superconducting films from current ${\mathit{j}}_{\mathit{s}}$ and dynamic relaxation Q=d ln${\mathit{j}}_{\mathit{s}}$/d ln(dB/dt) data by means of the generalized inversion scheme. For B<2 T and T<80 K the temperature dependence of ${\mathit{j}}_{\mathit{c}}$ and ${\mathit{U}}_{\mathit{c}}$ for all films is found to be in excellent agreement with a model of single vortices pinned by randomly distributed weak pinning centers via spatial fluctuations of the charge carrier mean free path. Pinning due to spatial fluctuations of ${\mathit{T}}_{\mathit{c}}$ is not observed.