Optical Spectroscopy and Superconductivity of Cuprates (Review)

Abstract

The optical properties of low-dimensional dielectric cuprates, including parent systems for high-temperature superconductors, such as La₂CuO₄, Sr₂CuO₂Cl₂, and YBa₂Cu₃O₆ 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 Cu¹⁺–Cu³⁺-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.