Abstract
We report a new effect regarding the collective flux pinning in sputtered amorphous Nb 3Ge films. Thick films ( d = 5−8 μm) show the two dimensional collective pinning (2DCP) at low fields and all temperatures. Below 0.7 T c, however, an extremely sharp and large peak develops in the macroscopic pinning force at fields around 0.75 B c2. It is argued that this peak is not the one usually observed in 2DCP, caused by a structural transition in the vortex lattice. Strong evidence is found that the onset of this peak is triggered by a dimensional cross-over in the collective pinning (2D to 3D) expected when the longitudinal correlation length L c becomes smaller than d/2. A discrepancy with the 3D expression of the Larkin-Ovchinnikov theory for L c nevertheless remains.
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