Theory of magnetically coupled type-II superconducting films

The current-voltage characteristics of a magnetically coupled pair of infinite type-II superconducting films of finite thickness, separated by an insulating layer, are calculated. The main features of the time-averaged primary and secondary flux-flow voltages ${\overline{V}}_p$ and ${\overline{V}}_s$ versus the primary and secondary currents $I_p$ and $I_s$ are found to depend upon only five parameters: the primary and secondary critical depinning currents $I_{\mathrm{pc}}$ and $I_{\mathrm{sc}}$ the primary and secondary flux-flow resistances $R_p$ and $R_s$, and a coupling current $I_0$, which is proportional to the maximum coupling force exerted on a secondary fluxoid by the displaced primary-fluxoid lattice. Model calculations of the current-voltage characteristics are performed to illustrate predicted behavior under varying strengths of current, pinning, and coupling.