The Mechanism of Doping and the Features of Phase Diagrams of HTSC Cuprates and Ferropnictides

We propose a generalized model of electronic structure modification in HTSC cuprates and ferropnictides under doping. In this model, the role of doping consists in only a local change in the electronic structures of the parent phases of cuprates and ferropnictides due to the formation of trion complexes comprising a doped carrier localized in unit cell and charge transfer (CT) excitons around it. These CT excitons emerge in CuO₄ or AsFe₄ plaquettes in the CuO₂ or FeAs basal planes (CT plaquettes) under the influence of the doped carrier, restricting its itinerancy. As the dopant concentration is increased, CT plaquettes combine into clusters of the so-called CT phase. It is this CT phase that is related in the model to the HTSC phase. In support of this assumption, we determined the ranges of dopant concentrations conforming to the existence of percolation clusters of the CT phase; these ranges were shown to coincide with the positions of the superconducting domes on the phase diagrams of these compounds. The model also perfectly describes subtle features of the phase diagrams of various cuprates and ferropnictides including the "1/8" anomaly, narrow peaks in the dependences of the London penetration depth on the concentration of the dopant, and other specific features. The mechanism of the generation of free carriers in the CT phase, provided by intrinsic self-doping, was considered. The mechanism is not directly related to external doping, but is due to the interaction of band electrons with so-called Heitler–London (HL) centres inherently existing in the percolation cluster of the CT phase and representing pairs of adjacent CuO₄ or AsFe₄ CT plaquettes in the CuO₂ or FeAs basal planes. Material in the CT phase was shown to represent a medium, in which the mechanism of excitonic superconductivity, specified by the interaction of band electrons with HL centres, can be realized.