Dominant role of the Cu---O charge-transfer energy, electronic polarizability and associated factors in the superconductivity of cuprates

Abstract

Although it is believed that there is strong hybridization between the Cu(3d) and O(2p) orbitals in the layered cuprates and that the parent compounds such as La₂CuO₄ are charge-transfer gap insulators, very few models consider the Cu---O charge-transfer energy, Δ, or the hybridization strength, t_pd, to be the important factors responsible for the superconductivity of these materials. Based on the crucial experimental observation that the relative intensity of the features in Cu(2p) photoemission of several families of cuprates varies systematically with the hole concentration, n_h, we have been able to show that both these properties vary smoothly with Δ/t_pd. More importantly, we show that the electronic polarizability of the CuO₂ sheets, α, is sufficiently large to favour hole pairing and that the value α also depends on Δ/t_pd. Both n_h and α increase smoothly with decreasing Δ/t_pd. Considering that the maximum T_c in the various cuprate families containing the same number of CuO₂ sheets occurs around the same n_h value (e.g., n_h≈0.2 in cuprates with two CuO₂ sheets). The present study demonstrates how Δ/t_pd, α and such chemical bonding characteristics have an important bearing on the superconducting properties of the cuprates.