Excitons in insulating cuprates

TL;DR

This paper investigates electronic excitations in insulating cuprates, revealing excitonic states with specific symmetries and their relation to optical and Raman spectra, based on a detailed multi-orbital model.

Contribution

It introduces a comprehensive six-band model to analyze excitons in insulating cuprates, identifying their symmetries and explaining experimental optical and Raman observations.

Findings

Identification of hybrid $d_{xy}$-$p_ au$ excitons with $A_{2g}$ symmetry

Observation of $B_{1g}$, $E_u$, and $A_{1g}$ excitons

Agreement with experimental optical absorption and Raman data

Abstract

We study the electronic excitations near the charge-transfer gap in insulating CuO$_2$ planes, starting from a six-band model which includes $% p_\pi $ and $d_{xy}$ orbitals and Cu-O nearest-neighbor repulsion $U_{pd}$. While the low lying electronic excitations in the doped system are well described by a modified $t-J$ model, the excitonic states of the insulator include hybrid $d_{xy}-$ $p_\pi $ states of $A_{2g}$ symmetry. We also obtain excitons of symmetries $B_{1g}$ and $E_u$, and eventually $A_{1g}$, which can be explained within a one-band model. The results agree with observed optical absorption and Raman excitations.