Modeling of Kinetic-Inductance Coplanar Striplin with NbN Thin Films

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Abstract
In order to determine transmission line parameters of superconducting coplanar striplines with thickness smaller than the magnetic penetration depth of the striplines, in which the influences of enlarged kinetic inductance and strongly nonuniform current distribution make it useless to apply the conventional design formula developed for normal conductors, we have conducted theoretical and experimental studies on the geometrical effects of the kinetic inductance and the microwave loss of the thin coplanar striplines. The analytical expression for the kinetic inductance of the coplanar stripline is given using the conformal mapping technique, and it is shown with the London equation and the two fluid-model that the microwave loss is directly proportional to the kinetic inductance. The comparison has been made between theory and experiment concerning the kinetic inductance and the microwave quality factor of NbN coplanar striplines with various thicknesses and widths. It is shown that the observed characteristics are well modeled by the analytical expression.