# On the vibron nature in the system of two parallel macromolecular chains: the influence of interchain coupling

## TL;DR

This paper investigates how interchain coupling and temperature influence vibron behavior and polaron formation in two parallel macromolecular chains, revealing a transition from free vibron to small polaron with varying system parameters.

## Contribution

It introduces a partial dressing method to analyze the continuous and discontinuous transitions of vibron dressing in a coupled chain system across different regimes.

## Key findings

- In non-adiabatic regime, dressing increases smoothly with coupling.
- In adiabatic regime, a sharp jump in dressing occurs.
- Interchain coupling effectively increases the system's adiabaticity.

## Abstract

We studied the properties of the intramolecular vibrational excitation (vibron) at finite temperature in a system which consists of two parallel macromolecular chains. It was assumed that vibron interacts exclusively with dispersionless optical phonons and the whole system is considered to be in thermal equilibrium. Particular attention has been paid to the examination of the impact of the temperature and strength of the interchain coupling on the \emph{small polaron} crossover. For that purpose we employed partial dressing method which enables the study of the degree of the phonon dressing of the vibron excitations in a wide area of system parameter space. We found that in the non--adiabatic regime the degree of dressing as a function of coupling constant continuously increases reflecting the smooth transition of the slightly dressed, practically free vibron, to a heavily dressed one: small polaron. As "adiabaticity" rises this transition becomes increasingly steeper, and finally, in the adiabatic limit, a discontinuous "jump" of the degree of dressing is observed. The interchain coupling manifests itself through the increase of the effective adiabatic parameter of the system.