Polarons with a twist

TL;DR

This paper introduces a polaron model incorporating molecular twist degrees of freedom, revealing unique nonlinear effects on electron transport relevant for complex molecules like DNA and molecular electronics.

Contribution

It presents a novel polaron model with twist-dependent hopping, demonstrating distinct nonlinear properties and energy level behaviors in the adiabatic regime.

Findings

Polaron properties are significantly affected by twist-dependent nonlinearities.

Energy level splitting increases with coupling strength in the adiabatic regime.

Bandwidth suppression follows a power-law with coupling, not exponential.

Abstract

We consider a polaron model where molecular \emph{rotations} are important. Here, the usual hopping between neighboring sites is affected directly by the electron-phonon interaction via a {\em twist-dependent} hopping amplitude. This model may be of relevance for electronic transport in complex molecules and polymers with torsional degrees of freedom, such as DNA, as well as in molecular electronics experiments where molecular twist motion is significant. We use a tight-binding representation and find that very different polaronic properties are already exhibited by a two-site model -- these are due to the nonlinearity of the restoring force of the twist excitations, and of the electron-phonon interaction in the model. In the adiabatic regime, where electrons move in a {\em low}-frequency field of twisting-phonons, the effective splitting of the energy levels increases with coupling strength. The bandwidth in a long chain shows a power-law suppression with coupling, unlike the typical exponential dependence due to linear phonons.