Correlating exciton coherence length, localization, and its optical lineshape. I. a finite temperature solution of the Davydov soliton model

TL;DR

This paper develops a finite-temperature model linking exciton coherence, localization, and optical lineshape, providing insights into the stability of soliton states in biological and molecular systems.

Contribution

It introduces a numerically exact, self-consistent finite-temperature approach to the Davydov soliton model, connecting spectral lineshape with thermal lattice vibrations.

Findings

Identifies a critical temperature for exciton self-trapping stability.

Shows good agreement with experimental data on molecular J-aggregates.

Resolves issues regarding soliton stability at finite temperatures in biological systems.

Abstract

The lineshape of spectroscopic transitions offer windows into the local environment of a system. Here, we present a novel approach for connecting the lineshape of a molecular exciton to finite-temperature lattice vibrations within the context of the Davydov soliton model (A. S. Davydov and N. I. Kislukha, Phys. Stat. Sol. {\bf 59},465(1973)). Our results are based upon a numerically exact, self-consistent treatment of the model in which thermal effects are introduced as fluctuations about the zero-temperature localized soliton state. We find that both the energy fluctuations and the localization can be described in terms of a parameter-free, reduced description by introducing a critical temperature below which exciton self-trapping is expected to be stable. Above this temperature, the self-consistent ansatz relating the lattice distortion to the exciton wavefunction breaks down. Our theoretical model coorelates well with both experimental observations on molecular J-aggregate and resolves one of the critical issues concerning the finite temperture stability of soliton states in alpha-helices and protein peptide chains.