**High- T_{C} Superconductivity in Ultra-thin Crystals: Implications for microscopic theory**, D. R. Harshman and A. T. Fiory [arXiv]

High transition temperature (high-

*T*

_{C}) superconductivity is associated with layered crystal structures. This work considers superconductivity in ultra-thin crystals (of thickness equal to the transverse structural periodicity distance

*d*for one formula unit) of thirty-two 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}= α

*T*

_{C0}, where α < 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 thin crystal structures approaching thickness d are shown to be consistent with 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}≈

*d*for superconductivity.

D. R. Harshman and A. T. Fiory, Emerging Materials Research **1**, 4 (2012).