Midinfrared spectrum of undoped cuprates: d-dtransitions studied byab initiomethods

Abstract

We present results of ab initio calculations for d-d transitions, which arise in the midinfrared spectrum of undoped cuprate compounds. It has been suggested that these transitions arise at energies as low as 0.4 eV in ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ and ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}.$ We study the differences in d-d transition energies in a series of cuprates that contains compounds in which the Cu ions are sixfold, fivefold, or fourfold coordinated. Furthermore, we analyze the dependence of the ${3d}_{{x}^{2}\ensuremath{-}{y}^{2}}\ensuremath{\rightarrow}{3d}_{{z}^{2}}$ excitation energy on the ratio of the in-plane and apex copper---ligand distances in the model system CuO. Our cluster calculations do not support the assignment of the 0.4--1-eV band to phonon and magnon sidebands of a d-d transition. On the other hand, we confirm the interpretation of the peak around 1.7 eV observed in ${\mathrm{CuGeO}}_{3}$ as arising from d-d transitions.