There was a fabulous new achievement in particle physics.
For the first time, scientists were able to imagine the orbits of electrons in a quasi-particle known as the exciton – a result that allowed them to finally measure the function of the exciton wave by describing the spatial distribution of the electron pulse in the quasi-particle.
This achievement has been sought since the discovery of excitons in the 1930s and, although it may seem abstract at first, it could help develop various technologies, including quantum applications.
“Excitons are truly unique and interesting particles; they are electrically neutral, which means they behave very differently inside materials than other particles, such as electrons. Their presence can really change the way a material responds 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 electronic probability distribution of an exciton shows where the electron is most likely to be. (OIST)
An exciton is not a true particle, but a quasi-particle – a phenomenon that occurs when the collective behavior of particles causes them to act in a similar way to particles. Excitons occur in semiconductors, materials that are more conductive than an insulator, but not sufficient to be considered suitable conductors.
Semiconductors are useful in electronics because they allow a higher degree of control over the flow of electrons. As difficult to observe, excitons play an important role in these materials.
Excitons can form when the semiconductor absorbs a photon (a particle of light) that raises the negatively charged electrons to a higher energy level; that is, the photon “excites” the electron, which leaves a positively charged gap called the electron hole. The negative electron and its positive hole connect together in a mutual orbit; an exciton is this orbiting pair of electron-electron holes.
But excitons have a very short lifespan and are very fragile, as the electron and its orifice can come together in just a split second, so actually seeing them is not a wonderful thing.
“Scientists first discovered excitons about 90 years ago,” said physicist Keshav Dani of the Femtosecond Spectroscopy Unit at OIST.
“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 orbit orbit each other, could only be described theoretically. “
This is a problem that researchers have been working to solve. In December last year, they published a method for direct observation of electron moments. Now, they have used this method. And it worked.
The technique uses a two-dimensional semiconductor material called diselenid tungsten, 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 excitement in this material; a second laser with ultra-high energy then throws the electrons completely into the vacuum chamber, which is monitored by an electron microscope.
This instrument measures the speed and trajectory of electrons, which information can then be used to work out the initial orbits of the particles at the point where they were ejected from their excitons.
The 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, in which particles are broken together with intense amounts of energy, breaking them open. By measuring the trajectory of smaller internal particles produced by the collision, scientists can begin to enjoy together the internal structure of the original intact particles, “Dani explained.
“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.”
Although it was a delicate, time-consuming job, the team was finally able to measure the wave function of an exciton, which describes its quantum state. This description includes its orbit with the electron hole, allowing physicists to accurately predict the position of the electron.
With some modifications, the team’s research could be a huge leap forward for exciton research. It could be used to measure the wave function of different exciton states and configurations and to test the exciton physics of different semiconductor materials and systems.
“This work is an important step forward in the field,” said physicist Julien Madeo of the OIST Femtosecond Spectroscopy Unit.
“The ability 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. . “
The team ‘s research was published in Scientific advances.