New University of Chicago study discovers similarities between photosynthesis, supercooling of atoms
CHICAGO (CBS) -- We learn about photosynthesis in elementary school – if only as a basic explanation of how plants generate their own energy, and why we need to water them and keep them near a light source for them to do so.
We might learn a little bit later about absolute zero – the lowest temperature possible where there is no motion or heat – and the state of matter that forms when atoms are cooled down almost to that point.
But unless we go into science as an academic discipline, we probably won't be writing sentences like these: "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. Here, we establish a link between the two phenomena by investigating the potential for exciton-condensate-like amplification of energy transport in room-temperature light harvesting."
Those sentences come from the abstract of a new University of Chicago study, "Exciton-Condensate-Like Amplification of Energy Transport in Harvesting," published this past Friday in the journal "PRX Energy." What does it all mean?
It means, as the study found, that the process of photosynthesis in plants and the process of cooling atoms down to near absolute zero – that would be what is involved with "Bose-Einstein condensation of excitons" – are a lot more alike as physical processes than we may think.
As explained by UChicago, the study found links at the atomic level between photosynthesis and exciton condensates – a puzzling state of physics in which energy can flow through a material frictionlessly.
"As far as we know, these areas have never been connected before, so we found this very compelling and exciting," study co-author Professor David Mazziotti of the Department of Chemistry said an a UChicago News article.
Along with study co-authors Anna Schouten and LeeAnn Sager-Smith, Mazziotti observed what happens at a molecular level during photosynthesis.
What they found was that when a photon hits a leaf, it knocks loose an electron in a specially-designed molecule, UChicago said. Both the electron – and the hole left behind when it was knocked loose – can then travel around the leaf, the study explained.
This carries the energy of the sun to another area, where it sets forth a chemical reaction that generates sugars for the plant to consume, the study said.
The electron-and-hole pair together are called an exciton. When the research team broadened their view, they noticed that when multiple excitons were moving around during photosynthesis, it looked a lot like Bose-Einstein condensate – or the fifth state of matter, UChicago said.
"In this material, excitons can link up into the same quantum state—kind of like a set of bells all ringing perfectly in tune," wrote Louise Lerner of the UChicago News Office. "This allows energy to move around the material with zero friction."
Models created by the research team determined the excitons in a leaf during photosynthesis can link up in ways similar to excitons in a Bose-Einstein condensate where atoms have been cooled to near absolute zero, UChicago said. Until now, exciton condensates had only ever been seen when materials were cooled significantly below room temperature, UChicago said.
"It'd be kind of like seeing ice cubes forming in a cup of hot coffee," Lerner wrote.
There was more than one way the findings were surprising. As noted in the abstract, behavior seen in exciton condensates is usually only observed in highly-ordered materials such as a graphene double layer, which is two layers of carbon atoms arranged in a hexagonal lattice structure. The structures involved in a leaf are not rigidly organized in such a fashion at all.
"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," Schouten was quoted in the UChicago News article.
While the effect is not total in photosynthesis and results in what are more like "islands" of exciton condensates, there is still enough going on to augment energy transfer in the system – with the scientists' models showing it can as much as double the system's efficiency, according to UChicago.
The study opens up new possibilities for generating synthetic materials for technological purposes, according to Mazziotti.
"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," Mazziotti was quoted by UChicago.