Optical spectroscopy and superconductivity of cuprates (review)

The optical properties of low-dimensional dielectric cuprates, including parent systems for high-temperature superconductors, such as La$_{2}$CuO$_{4}$, Sr$_{2}$CuO$_{2}$Cl$_{2}$, and YBa$_{2}$Cu$_{3}$O$_{6}$ are briefly reviewed. The main focus is on the $d$–$d$ and $p$–$d$ charge transfer transitions, which determine the fundamental absorption band. The analysis of optical properties shows the instability of parent cuprates against the $d$–$d$ charge transfer with the formation of metastable electron–hole (EH) dimers of the Cu$^{1+}$–Cu$^{3+}$-type pairs coupled by the two-particle transfer and characterized by the giant electric polarizability. The formation of a system of stable EH dimers upon nonisovalent substitution determines the unconventional properties of the pseudogap and superconducting phases of doped cuprates.