A fantastic new achievement has been made in particle physics.
For the first time, scientists have managed to image the orbits of electrons in a quasi-particle known as an exciton – a result that has enabled them to finally measure the excitonic wave function that defines the spatial distribution of electrons. momentum within the quasi-particle.
This feat has been sought after since the discovery of excitons in the 1930s, and while it may sound abstract at first, it could aid in the development of a variety of technologies, including quantum technology applications.
“Excitons are really unique and interesting particles; they are electrically neutral, which means that they behave very differently within materials than other particles such as electrons. Their presence can really change the way a material reacts to light,” said physicist Michael Man of the Okinawa Institute of Science and Technology (OIST) Femtosecond Spectroscopy Unit in Japan.
“This work brings us closer to fully understanding the nature of excitons.”
The electron probability distribution of an exciton shows where the electron is most likely to be located. (OIST)
An exciton is not a real particle, but a quasi-particle – a phenomenon that occurs when the collective behavior of particles causes them to act in a particle-like manner. Excitons are created in semiconductors, materials that are more conductive than an insulator, but not enough to be considered actual conductors.
Semiconductors are useful in electronics because they allow a finer degree of control over electron flow. Difficult as they may be to detect, excitons play an important role in these materials.
Excitons can arise when the semiconductor absorbs a photon (a light particle) that lifts negatively charged electrons to a higher energy level; that is, the photon “excites” the electron, leaving behind a positively charged gap called an electron hole. The negative electron and its positive hole become connected in a mutual orbit; an exciton is this electron-electron-hole pair orbiting the earth.
But excitons are very short-lived and very fragile, since the electron and its hole can get back together in just a fraction of a second, so actually seeing them is no mean feat.
“Scientists first discovered excitons about 90 years ago,” said physicist Keshav Dani of the Femtosecond Spectroscopy unit at OIST.
“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 every other, can only be described theoretically. “
This is a problem the researchers have been working on to fix. In December last year they published a method for directly observing the momenta of the electrons. Now they have used that method. And it worked.
The technique uses a two-dimensional semiconductor material called tungsten diselenide housed in a vacuum chamber cooled to a temperature of 90 Kelvin (-183.15 degrees Celsius or -297.67 degrees Fahrenheit). This temperature must be maintained to prevent the excitons from overheating.
A laser pulse creates excitons in this material; a second ultra-high energy laser then kicks the electrons completely out into the void of the vacuum chamber, which is monitored by an electron microscope.
This instrument measures the velocities and trajectories of the electrons, which information can then be used to calculate the first trajectories of the particles at the point where they were kicked out of their excitons.
Square wave function of an exciton. (Man et al., Sci. Adv., 2021)
“The technique has some similarities with the collider experiments of high-energy physics, in which particles with intense amounts of energy are compressed, causing them to break open. By measuring the orbits of the smaller internal particles produced by the collision, scientists can begin to share together the internal structure of the original intact particles, ”Dani explained.
“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.”
Although it was delicate and time-consuming work, the team was finally able to measure the wave function of an exciton, which describes the quantum state. This description includes its orbit with the electron hole, which allows physicists to accurately predict the electron’s position.
With some adjustments, the team’s research could be a quantum leap for exciton research. It could be used to measure the wave function of different exciton states and configurations, and to investigate the exciton physics of different semiconductor materials and systems.
“This work is a significant advance in the field,” said physicist Julien Madeo of the OIST Femtosecond Spectroscopy Unit.
“By visualizing the internal orbits of particles as they form larger composite particles, we can understand, measure and ultimately control the composite particles in unprecedented ways. This could allow us to create new quantum states of matter and technology based on these concepts. . “
The team’s research is published in Science Advances.