“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 Ψ2 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.