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-O2 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 Sr2CuO2Cl2.
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