Polaronic optical absorption of the Holstein and Su-Schrieffer-Heeger models

TL;DR

This paper compares the optical absorption features of polarons in the Holstein and Su-Schrieffer-Heeger models, revealing a new absorption band unique to the SSH model due to bond-modulation effects.

Contribution

It provides a detailed analysis of polaronic optical absorption in two models, highlighting a novel absorption feature in the SSH model not seen in the Holstein model.

Findings

Both models show similar polaronic absorption bands.

The SSH model exhibits an additional strong absorption band.

The new band involves transitions from bonding to antibonding states on shortened bonds.

Abstract

The optical conductivity of a polaronic charge carrier in the intermediate and strong-coupling regimes is calculated for a tight-binding electron using exact diagonalization. Two different simple models of the electron-phonon coupling are considered: the Holstein model, with coupling of the lattice displacement to the local electron density, and the Su-Schrieffer-Heeger type of coupling arising from modulation of the electronic overlap integral. The two models are shown to exhibit a similar polaronic absorption band. However, the Su-Schrieefer-Heeger form of coupling gives rise to a second strong absorption band not present in the Holstein model. This additional feature corresponds to transitions from the ground state of bonding character on a shortened bond to excited states of anti-bonding character on the same shortened bond.