After nearly a century of exploring the nature of tiny ephemeral objects called excitons, researchers finally managed to imagine the structure, hinting at the true location of an electron. The findings could end up helping physicists create new states of matter or new quantum technologies.
Excitons occur inside semiconductors and other materials 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. Electrons and holes orbit each other, forming an exciton, essentially the entire regime of an electron and the hole. Because the choice has a negative charge and the hole a positive charge, the exciton itself is neutral. Excitons are momentary, as electrons almost always stick back into their holes. When electrons fall again, they emit a photon.
“Scientists first discovered excitons about 90 years ago,” co-author Keshav Dani, head of the femtosecond spectroscopy unit at the Okinawa Institute of Science and Technology, told a university Press release. “But until very recently, in general, only the optical signatures of excitons could be accessed, for example, the light emitted by an exciton when it was extinguished. Other aspects of their nature, such as their momentum, and how the electron and the hole orbit each other, could only be described theoretically.
Because electrons act as particles and waves, their location and momentum cannot be pinpointed simultaneously. The “probability cloud” of an exciton — the sphere of influence it constitutes — is the best indicator of where the electron can be placed around the hole.
The researchers tried to map the wave functions of the excitons, which would directly define the shape and size of the structure. The play comes to light in recent history research of the same equipment, which described a method for detecting the excitation impulse. For current work, published today in the journal Science Advances, the team fired light from a laser at a semiconductor, catalyzing photon absorption. The semiconductor was extremely thin, a two-dimensional wafer of matter only a few atoms thick.
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When the excitons formed, the team shattered them with high-energy photons, causing the electrons to disappear. They used an electron microscope to trace the output of the electrons.
“The technique has some similarities to high-energy physics collision experiments, where particles break together with intense amounts of energy, breaking them,” Dani said. “Here we are doing something similar: we use photons of extreme ultraviolet light to break excitons and measure the trajectories of electrons to imagine what is inside.”
By measuring how electrons came out of the semiconductor, researchers could unite 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 represents the probability cloud of the exciter; in other words, the spaces where the electron is most likely to rotate through the hole it left behind.
“This work represents a major breakthrough in the field,” lead author Julien Madeo, a staff scientist in the OIST Femtosecond Spectroscopy Unit, told the OIST version. “Being able to visualize the internal orbits of particles as they form larger composite particles could allow us to understand, measure and ultimately control composite particles in unprecedented ways. This could allow us to create new quantum states of matter and technology based on these concepts. “
What is it for you and me that is a forest eye on a honeycomb background? It is a blessing for scientists who want to know more about quantum physics at play in semiconductors, perhaps improving the designs of these technologies in the future. Now almost a century since the first prediction of excitation in 1931, we have come close to representing how the subatomic structure really manifests itself. Observations have yet to occur in very cold states, although the temperature was raised a couple of years ago. The newly described excitations lead us to a more complete understanding of this quantum mechanics, and further developments will surely take place by the time the exciton reaches its centenary.
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