Search

It is shown that many anomalies observed in underdoped cuprates, including anomalous spectral weight transfer and a large pseudogap, appear to have a common nature due to both the cluster structure of the underdoped phase and the specific mechanism of superconducting pairing. The combined action of these factors leads to the fact that at a temperature T lying in a certain temperature range Tc

We have previously shown that most of the anomalies in the superconducting characteristics of cuprates and ferropnictides observed at dopant concentrations within the superconducting dome, as well as the position of the domes in the phase diagrams, do not require knowledge of the details of their electronic structure for explanation, but can be understood and calculated with high accuracy within the framework of a simple model describing the cluster structure of the superconducting phase. This fact suggests a change in the paradigm that forms our understanding of HTSC. In this paper, we propose a unified view on the transformation of the electronic structure of cuprates and ferropnictides upon heterovalent and isovalent doping, based on the assumption of self-localization of doped carriers. In this representation, in undoped cuprates and ferropnictides, which initially have different electronic structures (Mott insulator and semimetal), local doping forms percolation clusters with the same electronic structure of a self-doped excitonic insulator where a specific mechanism of superconducting pairing is implemented, which is genetically inherent in such a system. The proposed model includes a mechanism for generating additional free carriers under heterovalent and isovalent doping and makes it possible to predict their sign, which, in the general case, does not coincide with the sign of doped carriers., Comment: 21 pages, 5 figures

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 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 CT phase and representing pairs of adjacent CuO4 or AsFe4 CT plaquettes in the CuO2 or FeAs basal planes. Material in 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., Comment: 22 pages, 7 figures

Here we suggest a simple physical model of HTSC cuprates that provides qualitative explanation of the main features of these compounds, including the nature of Fermi arcs, pseudogap anomalies and the origin of free carriers. According to this model, the unusual properties of HTSC cuprates result from their unique electronic structure favourable for the formation of diatomic negative-U centers (NUCs) and realization of an unusual mechanism of electron-electron interaction., Comment: 13 pages, 5 figures