Macroscopic coherence as an emergent property in molecular nanotubes

TL;DR

This paper demonstrates that natural molecular nanotubes can support macroscopic coherent excitonic states at room temperature, with coherence lengths over 1000 molecules, due to emergent geometric and cooperative effects.

Contribution

It reveals that macroscopic coherence in natural nanotubes arises from emergent properties of structure and cooperativity, not just local interactions or long-range couplings.

Findings

Natural nanotubes support large thermal coherence lengths (~1000 molecules).

Macroscopic coherence is due to emergent geometry and cooperativity, not just coupling magnitude.

Enhanced coupling (super-transfer) lowers the density of states near the ground state.

Abstract

Nanotubular molecular self-aggregates are characterized by a high degree of symmetry and they are fundamental systems for light-harvesting and energy transport. While coherent effects are thought to be at the basis of their high efficiency, the relationship between structure, coherence and functionality is still an open problem. We analyze natural nanotubes present in Green Sulfur Bacteria. We show that they have the ability to support macroscopic coherent states, i.e. delocalized excitonic states coherently spread over many molecules, even at room temperature. Specifically, assuming a canonical thermal state, in natural structures we find a large thermal coherence length, of the order of 1000 molecules. By comparing natural structure with other mathematical models, we show that this macroscopic coherence cannot be explained either by the magnitude of the nearest-neighbour coupling between the molecules, which would induce a thermal coherence length of the order of 10 molecules, or by the presence of long-range interactions between the molecules. Indeed we prove that the existence of macroscopic coherent states is an emergent property of such structures due to the interplay between geometry and cooperativity (superradiance and super-transfer). In order to prove this, we give evidence that the lowest part of the spectrum of natural systems is determined by a cooperatively enhanced coupling (super-transfer) between the eigenstates of modular sub-units of the whole structure. Due to this enhanced coupling strength, the density of states is lowered close to the ground state, thus boosting the thermal coherence length.