Excitonic diffusion length in complex quantum systems: The effects of disorder and environmental fluctuations on symmetry-enhanced supertransfer

TL;DR

This paper investigates how symmetry-induced supertransfer effects can significantly enhance exciton diffusion lengths in complex quantum systems, considering both ideal and disordered environments, and analyzes the transition from quantum to classical transport regimes.

Contribution

It provides an analytical expression for exciton diffusion length incorporating supertransfer and explores how disorder and dephasing influence this enhancement.

Findings

Supertransfer can increase diffusion length by a factor of n in symmetric structures.

Disorder reduces but does not eliminate supertransfer effects.

Transition from coherent to incoherent transport coincides with the loss of supertransfer scaling.

Abstract

Symmetric couplings among aggregates of $n$ chromophores increase the transfer rate of excitons by a factor $n^2$, a quantum mechanical phenomenon called "supertransfer." In this work we demonstrate how supertransfer effects induced by geometrical symmetries can enhance the exciton diffusion length by a factor $n$ along cylindrically symmetric structures, consisting of arrays of rings of chromophores, and along spiral arrays. We analyse both closed system dynamics and open quantum dynamics, modelled by combining a random bosonic bath with static disorder. In the closed system case, we use the symmetries of the system within a short-time approximation to obtain a closed analytical expression for the diffusion length that explicitly reveals the supertransfer contribution. When subject to disorder, we show that supertransfer can enhance excitonic diffusion lengths for small disorders and characterize the crossover from coherent to incoherent motion. Owing to the quasi-1D nature of the model, disorder ultimately localizes the excitons, diminishing but not destroying the effects of supertransfer. When dephasing effects are included, we study the scaling of diffusion with both time and number of chromophores and observe that the transition from a coherent, ballistic regime to an incoherent, random-walk regime occurs at the same point as the change from supertransfer to classical scaling.