Within the Ginzburg-Landau approach a theoretical study is performed of 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 {\em mesoscopic} of linear size $L$ the surface induces superconductivity in the interior for $T < T_{c}(L)$, with $T_c(L) > T_{cs}$. In analogy with adsorbed fluids, superconductivity in thin films of type-I materials is akin to {\em 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 \equiv \lambda/\xi $, goes to zero, using analytic calculations. While standard finite-size scaling holds for the transition in non-zero 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 $T_c(R)-T_c(\infty)$ inversely proportional to the radius $R$.Comment: 37 pages, 5 figures, accepted for PR
The superconducting state of an infinitely long superconducting cylinder surrounded by a medium which enhances its superconductivity near the boundary is studied within the nonlinear Ginzburg-Landau theory. This enhancement can be due to the proximity of another superconductor or due to surface treatment. Quantities like the free energy, the magnetization and the Cooper-pair density are calculated. Phase diagrams are obtained to investigate how the critical field and the critical temperature depend on this surface enhancement for different values of the Ginzburg-Landau parameter κ. Increasing the superconductivity near the surface leads to higher critical fields and critical temperatures. For small cylinder diameters only giant vortex states nucleate, while for larger cylinders multivortices can nucleate. The stability of these multivortex states also depends on the surface enhancement. For type-I superconductors we found the remarkable result that for a range of values of the surface extrapolation length the superconductor can transit from the Meissner state into superconducting states with vorticity L > 1. Such a behaviour is not found for the case of large κ, i.e. type-II superconductivity.74.60. De, 74.20.De, 74.25.Dw
Using Ginzburg-Landau theory it is predicted that the critical temperature Tc of a mesoscopic superconductor with surface enhancement can be increased significantly with respect to the bulk Tc, by introducing surface curvature. While confinement and reduction of the dimensionality lead to a critical-point shift Tc(L) − Tc(∞) which decays exponentially with the ratio L/ξ of sample size to coherence length, surface curvature gives rise to a more important algebraic shift Tc(R) − Tc(∞) proportional to the curvature ξ/R. As an example, order-of-magnitude estimates for the increase of Tc are given for cold-worked mesoscopic InBi cylindrical wires and spherical grains.
4He wetting layers on Cs exhibit an unusually long-lived metastable state upon undercooling to temperatures well below the wetting temperature T(w) approximately 1.9 K. The decay of this state by homogeneous thermal nucleation of holes is disfavored by the incipient divergence of the free energy barrier separating the metastable thick film from the stable thin-film state. We propose that interface deformations ("dimples") created by electrons bound at the 4He liquid-vapor interface can be used as nuclei for the heterogeneous nucleation of holes. The size and excess free energy of the dimple can be tuned by an applied electric field E which allows the lifetime of the metastable film to be controlled.
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