Bose-Einstein condensation of excitons — in which excitons condense into a single coherent quantum state, known as an exciton condensate — enables frictionless energy transfer, but typically occurs under extreme conditions in highly ordered materials, such as graphene double layers. In contrast, photosynthetic light-harvesting complexes demonstrate extremely efficient transfer of energy in disordered systems under ambient conditions. Now, physicists have established a link between the two phenomena by investigating the potential for exciton-condensate-like amplification of energy transport in room-temperature light harvesting.
Schouten et al. find links at the atomic level between photosynthesis and exciton condensates, a strange state of physics that allows energy to flow frictionlessly through a material. Image credit: Evgeni Tcherkasski.
“As far as we know, these areas have never been connected before, so we found this very compelling and exciting,” said Professor David Mazziotti and his colleagues at the University of Chicago.
The team specializes in modeling the complicated interactions of atoms and molecules as they display interesting properties.
There’s no way to see these interactions with the naked eye, so computer modeling can give scientists a window into why the behavior happens — and can also provide a foundation for designing future technology.
In particular, the researchers have been modeling what happens at the molecular level when photosynthesis occurs.
When a photon from the Sun strikes a leaf, it sparks a change in a specially designed molecule. The energy knocks loose an electron.
The electron, and the ‘hole’ where it once was, can now travel around the leaf, carrying the energy of the Sun to another area where it triggers a chemical reaction to make sugars for the plant.
Together, that traveling electron-and-hole-pair is referred to as an exciton.
When the team took a birds-eye view and modeled how multiple excitons move around, they noticed something odd. They saw patterns in the paths of the excitons that looked remarkably familiar.
In fact, it looked very much like the behavior in a material that is known as a Bose-Einstein condensate, sometimes known as the fifth state of matter.
In this material, excitons can link up into the same quantum state — kind of like a set of bells all ringing perfectly in tune. This allows energy to move around the material with zero friction.
According to the team’s models, the excitons in a leaf can sometimes link up in ways similar to exciton condensate behavior.
This was a huge surprise. Exciton condensates have only been seen when the material is cooled down significantly below room temperature. It’d be kind of like seeing ice cubes forming in a cup of hot coffee.
“Photosynthetic light harvesting is taking place in a system that is at room temperature and what’s more, its structure is disordered — very unlike the pristine crystallized materials and cold temperatures that you use to make exciton condensates,” said University of Chicago researcher Anna Schouten.
This effect isn’t total — it’s more akin to ‘islands’ of condensates forming.
“But that’s still enough to enhance energy transfer in the system,” said University of Chicago researcher LeeAnn Sager-Smith.
In fact, the team’s models suggest it can as much as double the efficiency.
This opens up some new possibilities for generating synthetic materials for future technology.
“A perfect ideal exciton condensate is sensitive and requires a lot of special conditions, but for realistic applications, it’s exciting to see something that boosts efficiency but can happen in ambient conditions,” Professor Mazziotti said.
The finding also plays into a broader approach his team has been exploring for a decade.
The interactions between atoms and molecules in processes like photosynthesis are incredibly complex — difficult even for a supercomputer to handle — so scientists have traditionally had to simplify their models in order to get a handle on them.
“We think local correlation of electrons are essential to capturing how nature actually works,” Professor Mazziotti said.
The study appears in the journal PRX Energy.
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Anna O. Schouten et al. 2023. Exciton-Condensate-Like Amplification of Energy Transport in Light Harvesting. PRX Energy 2 (2): 023002; doi: 10.1103/PRXEnergy.2.023002
