Collective vortex motion ina-MoGe superconducting thin films

We report linear and nonlinear electrical transport in perpendicular magnetic fields of very thin amorphous ${\mathrm{Mo}}_{79}$ ${\mathrm{Ge}}_{21}$ superconducting films. At low fields, we demonstrate the existence of a barrier to vortex motion away from the sample edges. In regimes where the vortex motion is not substantially affected by the sample edges, we observe manifestly collective activated behavior. We relate the field dependence of the observed activation energies to possible mobile configurations of vortices.