There is a fantastic new achievement in particle physics.
For the first time, scientists have been able to image the orbits of electrons in a brush particle, known as an exciton – a result that enabled them to finally measure the excitonic wave function that the spatial distribution of electron momentum within the quasi-particle described.
This achievement has been sought after since the discovery of excitons in the 1930s, and while it may sound abstract at first, it may help in the development of various technologies, including quantum technology applications.
“Excitons are really unique and interesting particles; they are electrically neutral, which means that they behave very differently in materials than other particles. Their presence can change the way a material reacts to light,” said physicist Michael Man of Okinawa. Institute of Science and Technology (OIST) Femtosecond spectroscopy unit in Japan.
“This work brings us closer to understanding the nature of excitons.”
The electron probability distribution of an exciton indicates where the electron is likely to be. (OIST)
An exciton is not a real particle, but a brush particle – a phenomenon that arises when the collective behavior of particles causes them to behave in a particle-like manner. Involutions appear in semiconductors, materials that are more conductive than an insulator, but not quite enough to count as conductors.
Semiconductors are useful in electronics as they allow a finer degree of control over the flow of electrons. It is difficult to observe, and excitons play an important role in this material.
Excitations can occur when the semiconductor absorbs a photon (a particle of light) that elevates negatively charged electrons to a higher energy level; the photon ‘excites’ the electron, leaving a positively charged gap called an electron hole. The negative electron and its positive hole are bound together in a mutual orbit; an exciton is this electron-electron-hole pair that spins.
But excitons are very short-lived and very fragile, as the electron and its hole can reassemble within a fraction of a second, so it is no significant thing to see it.
“Scientists first discovered excitons about 90 years ago,” said physicist Keshav Dani of the Femtosecond Spectroscopy Unit at OIST.
“But until recently, one 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 the hole each revolves around another, can only be described theoretically. “
This is a problem that the researchers were working on. In December last year, they published a method to directly observe the moment of the electrons. Now they have used the method. And it worked.
The technique uses a two-dimensional semiconductor material called tungsten diselenide, which is housed in a vacuum chamber that is 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, and the information can then be used to work out the initial paths of the particles at the point at which 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 collision experiments of high-energy physics, where particles with intense amounts of energy are crushed and broken open. By measuring the orbits of the smaller internal particles produced during the collision, scientists can start pieces together the internal structure of the original intact particles, ”Dani explained.
“Here we do something similar – we use extreme ultraviolet light photons to break up excitons and measure the orbits of the electrons to represent what is in them.”
Although it was delicate, 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 so that physicists can accurately predict the position of the electron.
With a little tweaking, the team’s research could be a big leap forward for exciton research. It can be used to measure the wave function of different excitation states and configurations, and to investigate the excitation physics of different semiconductor materials and systems.
“This work is an important advancement in the field,” said physicist Julien Madeo of the OIST Femtosecond Spectroscopy Unit.
“If we can visualize the internal orbits of particles as they form larger composite particles, we can understand, measure and ultimately control the constituent particles in unprecedented ways. This can enable us to create new quantum states of matter and technology based on these concepts. “
The team’s research was published in Scientific progress.