High-TC superconductivity in ultrathin crystals: implications for microscopic theory

High-transition temperature (high-T-C) superconductivity is associated with layered crystal structures. This work considers superconductivity in ultrathin crystals (of thickness equal to the transverse structural periodicity distance d for one formula unit) of 32 cuprate, ruthenate, rutheno-cuprate, iron pnictide, organic and transuranic compounds, wherein intrinsic optimal (highest) transition temperatures T-C0 (10-150 K) are assumed. Sheet transition temperatures T-CS = alpha T-C0, where alpha < 1, are determined from Kosterlitz-Thouless (KT) theory of phase transitions in two-dimensional superconductors. Calculation of a involves superconducting sheet carrier densities N-S derived theoretically from crystal structure, ionic valences, and known doping, a two-fluid model for the temperature dependence of the superconducting magnetic penetration depth, and experimental data on KT transitions; a is on average 0.83 (varying with standard deviation 0.11). Experiments on several thin crystal structures of thickness d(F) approaching d are shown to be consistent with the calculations of T-C0 from microscopic superconductivity theory and with T-CS from KT theory, where the presence of disorder is also taken into account; careful analyses of these thin film studies indicate a minimum thickness d(F) approximate to d for superconductivity.