Nature of optical excitations in the frustrated kagome compound Herbertsmithite

TL;DR

This study combines optical measurements and density functional theory to reveal that Herbertsmithite and related kagome compounds are charge-transfer insulators with specific electronic transition features, influenced by Coulomb interactions.

Contribution

It identifies the insulating nature and electronic transition mechanisms of Herbertsmithite and similar kagome compounds, highlighting their charge-transfer character and the role of Coulomb repulsion.

Findings

Band edges at 3.3 and 3.6 eV confirm insulating behavior.

Dipole-forbidden local d-d transitions occur between 1-2 eV.

Electrodynamic response is linked to Coulomb interactions and charge-transfer processes.

Abstract

Optical conductivity measurements are combined with density functional theory calculations in order to understand the electrodynamic response of the frustrated Mott insulators Herbertsmithite $\mathrm{ZnCu_{3}(OH)_{6}Cl_{2}}$ and the closely-related kagome-lattice compound $\mathrm{Y_{3}Cu_{9}(OH)_{19}Cl_{8}}$. We identify these materials as charge-transfer rather than Mott-Hubbard insulators, similar to the high-$T_c$ cuprate parent compounds. The band edge is at 3.3 and 3.6 eV, respectively, establishing the insulating nature of these compounds. Inside the gap, we observe dipole-forbidden local electronic transitions between the Cu $3d$ orbitals in the range 1--2 eV. With the help of \textit{ab initio} calculations we demonstrate that the electrodynamic response in these systems is directly related to the role of on-site Coulomb repulsion: while charge-transfer processes have their origin on transitions between the ligand band and the Cu $3d$ upper Hubbard band, \textit{local} $d$-$d$ excitations remain rather unaffected by correlations.