Model pinning centers and spatial distribution of the Lorentz force in a type I superconductor

Flux tubes which are pinned at well-defined pinning sites in a type I superconductor can be used as a microprobe for the local force at the location of the pinning center. The method requires a measurement of the driving force which just cancels the well-known pinning force of a particular pinning site. Such information can easily be obtained using the high-resolution magnetooptical technique for magnetic flux detection. We report on measurements performed at 4.2 K with superconducting lead films of 6 Μm thickness. Well-defined pinning sites could be realized in the form of small circular indentations (diameter 3 μm; depth 0.2 μm) etched into the surface of the Pb films. By a suitable geometric arrangement of the pinning sites the dependence of the driving force upon the sample coordinate can be studied. As an example, we have investigated the spatial dependence of the Lorentz force in a current-carrying strip. Our results agree reasonably with the existing models for the various forces acting upon a flux tube.