Electronic State and Optical Response in a Hydrogen-Bonded Molecular Conductor

TL;DR

This paper presents a theoretical study of hydrogen-bonded molecular conductors, revealing competing electronic and protonic phases and their effects on optical properties, with implications for recent experimental findings.

Contribution

It introduces a model Hamiltonian capturing proton-electron interactions and analyzes phase diagrams and optical spectra using mean-field and exact diagonalization methods.

Findings

Identification of charge density wave, polar charge-ordered, and antiferromagnetic phases.

Electron-proton coupling reduces effective electron-electron repulsion, softening inter-dimer excitations.

Proton-electron coupling hardens intra-dimer charge excitations.

Abstract

Motivated by recent experimental studies of hydrogen-bonded molecular conductors $\kappa$-$X_3$(Cat-EDT-TTF)$_2$ [$X$=H, D], interplays of protons and correlated electrons, and their effects on magnetic, dielectric, and optical properties, are studied theoretically. We introduce a model Hamiltonian for $\kappa$-$X_3$(Cat-EDT-TTF)$_2$, in which molecular dimers are connected by hydrogen bonds. Ground-state phase diagram and optical conductivity spectra are examined by using the mean-field approximation and the exact diagonalization method in finite-size cluster. Three types of the competing electronic and protonic phases, charge density wave phase, polar charge-ordered phase, and antiferromagnetic dimer-Mott insulating phase are found. Observed softening of the inter-dimer excitation due to the electron-proton coupling implies reduction of the effective electron-electron repulsion, i.e. "Hubbard $U$", due to the quantum proton motion. Contrastingly, the intra-dimer charge excitation is harden due to the proton-electron coupling. Implications of the theoretical calculations to the recent experimental results in $\kappa$-$X_3$(Cat-EDT-TTF)$_2$ are discussed.