Superconducting phase diagrams of cuprates and pnictides as a key to understanding the HTSC mechanism
Lebedev Physical Institute, Russian Academy of Sciences, Leninsky prosp. 53, Moscow, 119991, Russian Federation
This paper reviews the experimental phase diagrams of cuprates and pnictides to demonstrate that specific features of the superconducting phase diagrams in both HTSC families can be understood within the framework of the proposed approach which assumes the formation, at heterovalent doping, of localized trion complexes consisting of a doped carrier and charge transfer (CT) excitons. The geometry of such cells containing CT excitons (CT plaquettes) in the basal plane of the crystal is determined by its crystal structure and the type of the dopant so that the dopant concentration range corresponding to the existence of a percolation cluster of CT plaquettes can be readily determined for each particular compound. These dopant concentration ranges coincide to good accuracy with the experimental ranges of superconducting domes on the phase diagrams of the HTSC compounds considered. The emergence of free carriers and the mechanism of superconducting pairing is in this pattern related to biexciton complexes (Heitler—London centres) emerging on neighboring CT plaquettes.