After nearly a century of researching the nature of tiny, ephemeral objects called excitons, researchers finally managed to image the structure, hinting at the true location of an electron. The findings could ultimately help physicists create new states of matter or new quantum technologies.
Excitons are found in semiconductors and other materials such as insulators. When a semiconductor absorbs photons or light particles, it causes electrons to jump to higher energy levels, leaving positively charged holes in place. The electrons and the holes orbit each other to form an exciton – essentially the entire regime of an electron and the hole. Since the election has a negative charge and the hole a positive charge, the exciton itself is neutral. The excitons are temporary, because the electrons almost always click back into their holes. When the electrons fall back in, they emit a photon.
“Scientists first discovered excitons about 90 years ago,” said study co-author Keshav Dani, the head of the Okinawa Institute of Science and Technology’s Femtosecond Spectroscopy unit at a university. press release. “Until recently, however, people generally only had access to the optical signatures of excitons – for example, the light emitted by an exciton when it is extinguished. Other aspects of their nature, such as their momentum and how the electron and hole orbit each other, can only be described theoretically. “
Since electrons act as both particles and waves, their location and momentum cannot be determined at the same time. The “probability cloud” of an exciton – the sphere of influence it forms – is the best indicator of where the electron can lie around the hole.
The researchers tried to map the wave functions of the excitons, which would directly determine the shape and size of the structure. The work comes on the heels of recent Research by the same team, who described a method for detecting exciton momentum. For current work, published today in the journal Science Advances, the team shot light from a laser at a semiconductor, catalyzing the absorption of photons. The semiconductor was extremely thin – a two-dimensional slice of matter just a few atoms thick.
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When the excitons formed, the team broke them apart with high-energy photons, blowing the electrons away. They used an electron microscope to map the output of the electrons.
“The technique is similar to the collision experiments of high-energy physics, where particles are compressed with intense amounts of energy, causing them to break open,” said Dani. “Here we do something similar: we use photons from extreme ultraviolet light to separate excitons and measure the trajectories of the electrons to get a picture of what’s inside.”
By measuring how the electrons exited the semiconductor, the researchers were able to piece together the locations, shapes, and sizes of the excitons. The image at the top of this article looks a bit like the sun in a clear sky, but it shows the exciton’s probability cloud; in other words, the spaces where the electron is likely to flutter around the hole it left behind.
“This work is a significant advance in the field,” said lead author Julien Madeo, a staff scientist at the OIST Femtosecond Spectroscopy Unit, in the OIST release. “By visualizing the internal trajectories of particles as they form larger composite particles, we can understand, measure and ultimately control the composite particles in unprecedented ways. This could enable us to create new quantum states of matter and technology based on these concepts. “
What is a rose on a honeycomb background to you and I is a boon to scientists eager to learn more about the quantum physics at play in semiconductors, and perhaps improve the designs of such technologies in the future. Now, nearly a century since the exciton’s first prediction in 1931, we have come closer to picturing how the subatomic structure actually manifests itself. The observations are still to take place in very cold states, although that temperature was raised a few years ago. The newly described excitations lead us to a fuller understanding of these quantum mechanics – and more developments are sure to take place by the time the exciton reaches its centenary.
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