Theory of Excitons in Insulating Cu-Oxide Plane

Abstract

We use a local model to study the formation and the structure of the low energy charge transfer excitations in the insulating Cu-O${}_2$ plane. The elementary excitation is a bound exciton of spin singlet, consisting of a Cu${}^{+}$ and a neighboring spin singlet of Cu-O holes. The exciton can move through the lattice freely without disturbing the antiferromagnetic spin background, in contrast to the single hole motion. There are four eigen-modes of excitons with different symmetry. The p-wave-like exciton has a large dispersion width. The s-wave-like exciton mixes with p-state at finite momentum, and its dipole transition intensity is strongly anisotropic. The model is in excellent agreement with the electron energy loss spectra in the insulating Sr${}_2$CuO${}_2$Cl${}_2$.

5 pages, 5 figures and 1 table. Submitted to Phys. Rev. B