Proximity effect through higher derivatives in superconducting filaments

We show that extremely thin superconducting filaments placed in the midst o f insulating material do maintain their superconducting state, but only if higher derivative terms are included in the free energy. It is these higher derivatives that are mainly responsible for the proximity effect that coup les such thin filaments. The experimental signature of these terms is discu ssed. [S0163-1829(99)00114-9].