Franck-Condon Factors as Spectral Probes of Polaron Structure

TL;DR

This paper uses advanced variational methods to analyze polaron properties in molecular crystals, revealing spectral signatures and constructing a comprehensive phase diagram that identifies different polaron regimes and transitional structures.

Contribution

It introduces a combined variational approach to map the polaron phase diagram across all parameters, identifying new classes of polarons and transitional regimes.

Findings

Identification of spectral signatures for polaron self-trapping

Construction of a comprehensive polaron phase diagram

Discovery of a new class of 'compact' polarons

Abstract

We apply the Merrifield variational method to the Holstein molecular crystal model in D dimensions to compute non-adiabatic polaron band energies and Franck-Condon factors at general crystal momenta. We analyze these observable properties to extract characteristic features related to polaron self-trapping and potential experimental signatures. These results are combined with others obtained by the Global-Local variational method in 1D to construct a polaron phase diagram encompassing all degrees of adiabaticity and all electron-phonon coupling strengths. The polaron phase diagram so constructed includes disjoint regimes occupied by "small" polarons, "large" polarons, and a newly-defined class of "compact" polarons, all mutually separated by an intermediate regime occupied by transitional structures.