Published for the American Physical Society by the American Institute of Physics
Physical Review B. Condensed matter vol:62 issue:21 pages:14359-14372
Within the Ginzburg-Landau approach a theoretical study is performed on the effects of confinement on the transition to superconductivity for type-I and type-II materials with surface enhancement. The superconducting order parameter is characterized by a negative surface extrapolation length b. This leads to an increase of the critical field H-c3 and to a surface critical temperature in zero field T-cs, which exceeds the bulk T-c. When the sample is mesoscopic of linear size L the surface induces superconductivity in the interior for T < T,(L), with T-c(L) > T-cs. In analogy with adsorbed fluids, superconductivity in thin films of type-I materials is akin to capillary condensation and competes with the interface delocalization or ''wetting" transition. The finite-size scaling properties of capillary condensation in superconductors are scrutinized in the limit that the ratio of magnetic penetration depth to superconducting coherence length kappa equivalent to lambda/xi goes to zero, using analytic calculations. While standard finite-size scaling holds for the transition in nonzero magnetic field H, an anomalous critical-point shift is found for H = 0. The increase of T-c for H = 0 is calculated for mesoscopic films, cylindrical wires, and spherical grains of type-I and type-II materials. Surface curvature is shown to induce a significant increase of T-c, characterized by a shift T-c(R) - T-c(infinity) inversely proportional to the radius R.