Electron-doped organics: Charge-disproportionate insulators and Hubbard-Fr\"ohlich metals

TL;DR

This study uses ab-initio calculations to explore the structure and electronic properties of electron-doped PAH crystals, revealing that the lowest energy state is insulating with charge disproportionation, challenging assumptions about metallicity in these materials.

Contribution

It demonstrates that three-electron doping does not necessarily lead to metallic behavior and introduces a Hubbard-Frohlich model for understanding superconductivity in PAHs.

Findings

Lowest energy state is a band insulator with charge disproportionation.

Metastable metallic structure retains higher symmetry and equivalent PA ions.

Dimerizing distortion can open a gap, affecting superconductivity potential.

Abstract

Several examples of metallic electron doped polycyclic aromatic hydrocarbons (PAHs) molecular crystals have recently been experimentally proposed. Some of them have superconducting components, but most other details are still unknown beginning with structure and the nature of metallicity. We carried out ab-initio density functional calculations for La-Phenanthrene (La-PA), here meant to represent a generic case of three-electron doping, to investigate structure and properties of a conceptually simple case. To our surprise we found first of all that the lowest energy state is not metallic but band insulating, with a disproportionation of two inequivalent PA molecular ions and a low P1 symmetry, questioning the common assumption that three electrons will automatically metallize a PAH crystal. Our best metallic structure is metastable and slightly higher in energy, and retains equivalent PA ions and a higher P21 symmetry -- the same generally claimed for metallic PAHs. We show that a "dimerizing" periodic distortion opens very effectively a gap in place of a symmetry related degeneracy of all P21 structures near the Fermi level, foreshadowing a possible role of that special intermolecular phonon in superconductivity of metallic PAHs. A Hubbard-Frohlich model describing that situation is formulated for future studies.