After nearly a century of researching the nature of small, ephemeral objects called excitons, researchers have finally managed to imagine the structure, suggesting the true location of an electron. The discoveries could eventually help physicists create new states of matter or new quantum technologies.
Excitons occur inside 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 their place. Electrons and holes orbit each other, forming an exciton – essentially the entire regime of an electron and the hole. Because the election has a negative charge and the hole has a positive charge, the exciter itself is neutral. The excitons are momentary, as the electrons almost always attach immediately back into their holes. When electrons fall back, they emit a photon.
“Scientists first discovered excitons about 90 years ago,” said co-author Keshav Dani, head of the Femtosecond Spectroscopy Unit at the Okinawa Institute of Science and Technology at a university. Press release. “But until recently, only the optical signatures of excitons could generally be accessed – for example, the light emitted by an exciton when extinguished. Other aspects of their nature, such as their momentum and how the electron and the orifice orbit each other, could only be described theoretically. ”
Because electrons act both as particles and as waves, their location and momentum cannot be identified simultaneously. The “probability cloud” of an exciton – the sphere of influence it constitutes – is the best indicator of where the electron might be around the hole.
The researchers tried to map the wave functions of the exciters, which would directly define the shape and size of the structure. The work follows in the footsteps of the recent ones research by the same team, which described a method for detecting the exciton pulse. For the current paper, published today, in the journal Science Advances, the team drew light from a laser to a semiconductor, catalyzing photon absorption. The semiconductor was extremely thin – a two-dimensional plate of matter with only a few thick atoms.
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When excitations formed, the team then broke them with high-energy photons, dropping electrons. They used an electron microscope to map the electron output.
“The technique has some similarities to high-energy physics collision experiments, where particles are broken up with intense amounts of energy, breaking them open,” Dani said. “Here, we do something similar – we use photons with extreme ultraviolet light to break excitations and measure the trajectory of electrons to imagine what is inside.”
By measuring how electrons left the semiconductor, researchers could divide the locations, shapes and sizes of excitons. The image at the top of this article looks a bit like the Sun in a clear sky, but describes the probability cloud of exciton; in other words, the spaces where the electron most likely flies around the hole it left behind.
“This work is an important step forward in the field,” said lead author Julien Madeo, a scientist in the OIST Femtosecond Spectroscopy Unit, in the OIST version. “Being able to visualize the internal orbits of particles as they form larger composite particles would allow us to finally understand, measure, and control composite particles in unprecedented ways. This would allow us to create new quantum states of matter and technology based on these concepts. ”
What is for you and me a cheek on a honeycomb background is a blessing for scientists eager to learn more about quantum physics at play in semiconductors, perhaps improving the projects of such technologies in the future. Nearly a century after the first prediction of exciton in 1931, we came closer to describing how the subatomic structure actually manifests. Observations still need to take place in very cold conditions, although the temperature has risen a few years ago. The newly described excitations lead us to a more complete understanding of this quantum mechanics – and there will certainly be more evolutions until the exciton reaches its centenary.
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