Exciton doublet in the Mott-Hubbard LiCuVO$_4$ insulator identified by spectral ellipsometry

TL;DR

This study uses spectral ellipsometry to identify an exciton doublet in LiCuVO$_4$, revealing how Coulomb interactions influence electronic excitations in this Mott-Hubbard insulator.

Contribution

The paper reports the first spectroscopic identification of an exciton doublet in LiCuVO$_4$, linking it to Coulomb interactions in the Mott-Hubbard gap.

Findings

Identification of a two-peak exciton doublet at 2.15 and 2.95 eV.

Spectral weight of the exciton doublet increases near magnetic ordering temperature.

Methodology applicable to other copper-oxide chain and plane compounds.

Abstract

Spectroscopic ellipsometry was used to study the dielectric function of LiCuVO$_{4}$, a compound comprised of chains of edge-sharing CuO$_4$ plaquettes, in the spectral range (0.75 - 6.5) eV at temperatures (7-300) K. For photon polarization along the chains, the data reveal a weak but well-resolved two-peak structure centered at 2.15 and 2.95 eV whose spectral weight is strongly enhanced upon cooling near the magnetic ordering temperature. We identify these features as an exciton doublet in the Mott-Hubbard gap that emerges as a consequence of the Coulomb interaction between electrons on nearest and next-nearest neighbor sites along the chains. Our results and methodology can be used to address the role of the long-range Coulomb repulsion for compounds with doped copper-oxide chains and planes.