“Wetting” phase transitions in type-I superconductors

We predict on the basis of the Ginzburg-Landau theory that a type-I superconductor can exhibit an interface delocalization or “wetting” transition, in which a macroscopically thick superconducting layer intrudes from the surface into the bulk normal phase. The condition for this transition to occur is that the superconducting order parameter |Ψ|2 is enhanced at the surface. Surprisingly, the order of the wetting transition is controlled by a bulk material constant, the Ginzburg-Landau parameter κ. First-order wetting is predicted for 0 ≤ κ < 0.374, and critical wetting for 0.374 < κ < 1/√2. The possibility of critical wetting is especially interesting because this phenomenon has largely eluded experimental verification in any system until now.