Model for the penetration of magnetic flux in Nb superconducting films with lithographically defined indentations and defects due to inhomogeneities

We consider the penetration of magnetic flux perpendicularly to a Nb superconducting film containing edge indentations and/or defects. The induced currents undergo an abrupt change of direction around the indentations and the defects, giving rise to discontinuity lines (d-lines). It has recently been shown, by means of magneto-optical imaging and numerical models, that the detailed structure of the d-lines generated with lithographically-defined micro-indentations carry information about their size and shape, and vary with temperature [1]. In this talk, we describe the models which can be used to describe the d-lines and discuss the effects of temperature, demagnetization, and flux creep, as well as the influence of a field-dependent critical current density on the distribution of the magnetic field. We extend the model to discontinuities produced by defects arising from inhomogeneous properties of the film and discuss the information contained in the resulting d-line structure. Last, in contrast to what has been repeatedly predicted in the literature, artificial indentations were not observed to act as preferred nucleation spots for flux avalanches [1]. To investigate this result, we estimate the levels of electric field strengthening arising near indentations and defects, and compare with the levels necessary for triggering thermomagnetic instabilities.