Boundary conditions in the macroscopic theory of superconductivity

This note is devoted to the role played by the boundary conditions in the macroscopic theory of superconductivity. For “usual” superconductors one uses the boundary condition which does not contain any kind of a constant characterizing the superconductor. But in the general case the boundary condition involves a certain length $\Lambda$ (the extrapolation length). Taking into account of a more general boundary condition may turn out to be significant for high-temperature superconductors in which the coherence length is small.