Physicists have for the first time captured electron orbits in an exciton quasiparticle

An incredibly new breakthrough in particle physics has occurred.

For the first time, scientists were able to display electron orbits within a quasiparticle known as an exciton – a result that allowed them to finally measure the function of the exciton wave that describes the spatial distribution of electron momentum within a quasiparticle.

This achievement has been sought since the discovery of excitons in the 1930s, and although it sounds abstract at first, it could help in the development of various technologies, including quantum technological applications.

“Excitons are really unique and interesting particles; they are electrically neutral which means they behave very differently in materials than other particles like electrons. Their presence can really change the way a material reacts to light,” said physicist Michael Man of the Okinawa Institute. Science and Technology Spectroscopy Unit (OIST) in Japan.

“This paper brings us closer to a full understanding of the nature of excitons.”

probability of excitonThe electron distribution of the exciton probability shows where the electron is 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 particle-like manner. Excitons occur in semiconductors, materials that are more conductive than insulators, but insufficient enough to count among their own conductors.

Semiconductors are useful in electronics because they provide a finer degree of control over the flow of electrons. Hard to spot, excitons play an important role in these materials.

Excitons can be created when a semiconductor absorbs a photon (a particle of light) that raises negatively charged electrons to a higher energy level; that is, the photon ‘excites’ an electron, which leaves a positively charged void called an electron hole. The negative electron and its positive hole become interconnected in orbit; exciton is this orbital pair of electrons-electron holes.

But excitons are very short-lived and very fragile, because an electron and its hole can return in just a split second, so actually looking at them is no feat.

“Scientists first discovered excitons about 90 years ago,” said physicist Keshav Dani of the FEMtosecond Spectroscopy Unit at OIST.

“But until recently, mostly only exciton’s optical signatures could be accessed – for example, the light emitted by an exciton when it goes out. Other aspects of their nature, such as momentum and how an electron and a hole 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 of direct observation of electron pulses. Now they used that method. And it worked.

The technique uses a two-dimensional semiconductor material called tungsten diesel, located 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 so that the excitons do not overheat.

The laser pulse creates excitons in this material; the second ultra high energy laser then ejects the electrons completely into the vacuum chamber cavity, which is monitored by an electron microscope.

This instrument measures the velocities and trajectories of electrons, by which information can calculate the initial trajectories of particles at the point at which they are ejected from their excitons.

exciton wave functionSquare exciton wave function. (Man et al., Sci. Adv., 2021)

“The technique has some similarities to collision experiments in high-energy physics, where particles break up along with intense amounts of energy, breaking them down. By measuring the trajectories of smaller internal particles produced in a collision, scientists can begin to tear together the internal structure of original intact particles,” Dani explained. .

“Here, we’re doing something similar – we use photons of extreme ultraviolet light to break excitons and measure the paths of electrons to capture what’s inside.”

Although it was a sensitive, time-consuming job, the team was finally able to measure the exciton wave function, which describes its quantum state. This description includes its orbit with an electron hole, allowing physicists to accurately predict the position of the electron.

With certain tweaks, the research team could be a big leap forward for exciton research. It could be used to measure the wave function of different states and exciton configurations and to examine the exciton physics of different semiconductor materials and systems.

“This work is an important advancement in the field,” said physicist Julien Madeo of the OIST unit for femtosecond spectroscopy.

“The ability to visualize the inner orbits of particles as they form larger composite particles could allow us to understand, measure and ultimately control composite particles at unprecedented levels. This could allow us to create new quantum states of matter and technology based on these concepts.”

The research team was published in Scientific progress.

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