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 ${\bar{V}}_p$ and V̄_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 $I_{pc}$ and $I_{sc}$ the primary and secondary flux-flow resistances $R_p$ and R_s, and a coupling current I₀, 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.