A solid may have a superconducting surface layer because of changes from the bulk in structure, atomic spacing, and composition. If fluxoids do not come into thermal equilibrium by penetration of the surface barrier, magnetic screening by the layer may simulate in part the diamagnetic susceptibility of a bulk superconductor if the layer thickness exceeds $\frac{{2\lambda }^2}{R}$, where $\lambda$ is the penetration depth and $R$ the radius of the cylindrical insulator. Thus the critical thickness for screening may be less than $\lambda$. Continuous coverage is not necessary for screening to obtain. It is essential to carry out a true Meissner cooling before identifying a material as a superconductor.

• Received 14 May 1987

DOI:https://doi.org/10.1103/PhysRevB.37.642