Optical absorption spectra of A6C60 and A6C70: Reduction of effective Coulomb interactions in Frenkel excitons

TL;DR

This paper theoretically investigates how heavy doping in alkali metal doped fullerides reduces effective Coulomb interactions, affecting their optical absorption spectra and related dielectric properties.

Contribution

It demonstrates that doping significantly lowers Coulomb interaction parameters, explaining experimental dielectric constant increases in heavily doped fullerides.

Findings

Coulomb interactions are nearly halved in heavily doped systems.

Optical spectra are explained by reduced Coulomb parameters.

Dielectric constants approximately double with doping.

Abstract

We theoretically investigate optical absorption spectra of $\soc^{6-}$ and $\rug^{6-}$, and discuss relations with the optical properties of alkali metal doped fullerides $A_6\soc$ and $A_6\rug$. This is a valid approach for systems where Frenkel exciton effects are dominant. We use a tight binding model with long ranged Coulomb interactions and bond disorder. Optical spectra are obtained by the Hartree-Fock approximation and the configuration interaction method. We find that the Coulomb interaction parameters, which are relevant to the optical spectra of $A_6\soc$ ($A_6\rug$) in order to explain the excitation energies and relative oscillator strengths of absorption peaks, are almost the half of those of the neutral $\soc$ ($\rug$). The reduction of the effective Coulomb interactions is concluded for the heavily doped case of $\soc$ and $\rug$. This finding is closely related with the experimental fact that dielectric constants of fullerides which are maximumly doped with alkali metals become about twice as large as those of the neutral systems.