Charge transfer excitons in HTSC cuprates and nickelates

An analysis of the optical properties of compounds based on $3d$-elements provides valuable information about the electronic structure of the ground state and low-energy excitations. Thus, we show that the analysis of charge-transfer $d$–$d$ excitons in the dielectric antiferromagnetic phase of cuprates and metastable low-energy electron-hole EH dimers being a result of their evolution after electron-lattice relaxation, turns out to be very fruitful not only for describing linear and nonlinear optical properties and photoinduced effects, but also to develop a promising model of charge triplets to describe the low-energy electronic structure and $T$–$x$ phase diagrams of active CuO$_2$ planes in cuprates of the $T$-La$_2$CuO$_4$ or $T'$-Nd$_2$CuO$_4$ type, as well as NiO$_2$-planes in nickelates of the RNiO$_2$ type, and their evolution with changes in the main energy parameters.