WETTING, PREWETTING AND SURFACE TRANSITIONS IN TYPE-I SUPERCONDUCTORS

Within the Ginzburg-Landau theory, which is quantitatively correct for classical superconductors, it is shown that a type-I superconductor c an display an interface delocalization or ''wetting'' transition, in w hich a macroscopically thick superconducting layer intrudes from the s urface into the bulk normal phase. The condition for this transition t o occur is that the superconducting order parameter \psi\(2) is enhanc ed at the surface. This corresponds to a negative surface extrapolatio n length b. The wetting transition takes place at built two-phase coex istence of normal and superconducting phases, at a temperature T-D bel ow the critical temperature T-c, and at magnetic field H-D = H-c(TD). The field is applied parallel to the surface. Surprisingly, the order of the wetting transition is controlled by a bulk material constant, t he Ginzburg-Landau parameter kappa. This is very unusual, since in oth er systems (fluids, Ising magnets,...) the order of the wetting transi tion depends on surface parameters that are difficult to determine or control. For superconductors, first-order wetting is predicted for 0 l ess than or equal to kappa<0.374, and critical wetting for 0.374<kappa <1/root 2. In the case of first-order wetting, the prewetting extensio n is also found. Unlike in standard wetting problems, the prewetting l ine does not terminate at a critical point but changes from first to s econd order at a tricritical point. Twinning-plane superconductivity ( TPS) is reinterpreted as a prewetting phenomenon. The possibility of c ritical wetting in superconductors is especially interesting because t his phenomenon has largely eluded experimental verification in any sys tem until now. Furthermore, superconductors provide a realization of w etting in systems with short-range (exponentially decaying) interactio ns. This is very different from the usual long-range (algebraically de caying) interactions, such as van der Waals forces, and has important consequences for the wetting characteristics.