Collective pinning of vortices in very thin amorphous Mo79Ge21 films.

Abstract

The collective pinning of vortices in very thin amorphous superconducting films is experimentally investigated by a systematic study of the nonlinear electrical response of ${\mathrm{Mo}}_{\mathit{x}}$${\mathrm{Ge}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ (x=79%) alloy films of various thickness measured in perpendicular magnetic field. In the thickest of these films, the standard collective pinning analysis yields an unusual field dependence of the pinning force, suggesting that more than one type of pinning center may play a substantial role. We then examine the field dependence of the pinning forces in progressively thinner samples. In the thinnest films the influence of the additional pinning centers in this system appears to be less pronounced than in thicker films, although the interplay between changes in the microscopic pinning and the breakdown of the standard collective pinning picture remains to be clarified.