Theory of Excitons in Insulating Cu-Oxide Plane

TL;DR

This paper develops a local model to analyze low-energy charge transfer excitations in insulating Cu-O planes, revealing exciton properties and their agreement with experimental spectra.

Contribution

It introduces a novel local model for excitons in Cu-O planes, detailing their structure, movement, and symmetry, with predictions matching experimental data.

Findings

Identifies a bound spin singlet exciton as the elementary excitation.

Shows excitons can move freely without disturbing antiferromagnetic order.

Model predictions align with electron energy loss spectra in Sr₂CuO₂Cl₂.

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$_2$.