On the nature of the optical conductivity of polaron

TL;DR

This paper investigates how the optical spectra of polarons evolve with increasing electron-phonon coupling, revealing a crossover from light to heavy phonon clouds and clarifying the nature of strong coupling regimes.

Contribution

It provides a detailed theoretical analysis comparing exact Monte Carlo and diagonalization methods to understand polaron optical spectra and the crossover from weak to strong coupling regimes.

Findings

Optical spectra are explained by superposition of light and heavy phonon cloud states.

The strong coupling Franck-Condon regime is not achievable at realistic coupling values.

The crossover from weak to strong coupling involves a transition from large to small polarons.

Abstract

We study theoretically a fundamental issue in solids: the evolution of the optical spectra of polaron as the electron-phonon coupling increases. By comparing the exact results obtained by diagrammatic Monte Carlo method and the data got through exact diagonalization within an appropriate subspace of the phononic wavefunctions, the physical nature of the crossover from weak to strong coupling is revealed. The optical spectra are well understood by the quantum mechanical superposition of states with light and heavy phonon clouds corresponding to large and small polarons, respectively. It is also found that the strong coupling Franck-Condon regime is not accessible at realistic values of the coupling constant.