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

Abstract

High-transition temperature (high-TC) 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 TC0 (10−150 K) are assumed. Sheet transition temperatures TCS = αTC0, where α < 1, are determined from Kosterlitz–Thouless (KT) theory of phase transitions in two-dimensional superconductors. Calculation of α involves superconducting sheet carrier densities NS 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; α is on average 0·83 (varying with standard deviation 0·11). Experiments on several thin crystal structures of thickness dF approaching d are shown to be consistent with the calculations of TC0 from microscopic superconductivity theory and with TCS from KT theory, where the presence of disorder is also taken into account; careful analyses of these thin film studies indicate a minimum thickness dF ≈ d for superconductivity.