[en] We observe a zero-field current step (ZFS) in several square Josephson tunnel junctions of intermediate size (i.e., size congruent λ_j). This step clearly shows soliton-type motion along the two diagonals of the junction as predicted by Turlot et al. [Phys. Rev. B 42, 8418 (1990)], called isoperimetric pattern dynamics (IPD). In addition to the information given by the current-voltage characteristic, the dynamical behavior is investigated using low-temperature scanning electron microscopy (LTSEM). In parallel, extensive numerical simulations are performed to simulate the effect of the LTSEM ''hot spot'' on the IPD dynamics governed by a two-dimensional perturbed sine-Gordon equation. The comparison between the experimental data obtained by the LTSEM technique and numerical simulations clearly shows the validity of the IPD model in order to explain the observed ZFS