Geometric Effects in the Superconducting Transition of Thin Films
by M. D. Reeber
A study is made of the effect of geometric factors and field orientation on the superconducting transition of bulk material whose dimensional ratios are comparable to those of thin evaporated films. An expression is derived for an effective demagnetizing coefficient in the direction of the applied field for an elliptic cylindrical superconductor, and it is shown that this deviates from 1 by quantities of the order of 10-2 or smaller for experimentally realizable conditions with typical films. These small coefficients give rise to surface fields sufficiently large for transition to the intermediate state. The shape of the transition for various field orientations has been obtained experimentally, and these curves are analyzed qualitatively on the basis of Landau's and Andrew's theory of the intermediate state. For temperatures below the transition point, and in the absence of fields other than the earth's magnetic field, the possibility that these films are in the intermediate state is explored.