In art, negative space in a painting is as important as the painting itself, and a similar situation exists in insulating materials, where the empty space left by missing electrons plays a crucial role in determining the material's properties. When a negatively charged electron is excited by light, it leaves behind a positively charged hole. Since holes and electrons are oppositely charged, they attract each other and form bonds. The resulting pair is short-lived and is called an exciton (pronounced exit-tawn).
Excitons in technology
Excitons are an integral part of many technologies, such as solar panels, photodetectors and sensors. They are also a key part of the light-emitting diodes found in televisions and digital displays. In most cases, exciton pairs are bound by electric or electrostatic forces, also known as Coulomb interactions.
Now, in a new study published in Nature Physics, Caltech researchers report that the detected excitons are bound not by Coulomb forces, but by magnetism. This is the first experiment to examine how these so-called Hubbard excitons (named after the late physicist John Hubbard) form in real time.
In a material called an antiferromagnetic Mott insulator, the electrons (spheres) are organized in an atomic lattice structure such that their spins move upward (blue) or downward (pink) in an alternating pattern. This is the energy-minimized stable state. When a material is hit by light, the electron jumps to a nearby atomic site, leaving a positively charged hole (dark ball) where it once resided. If the electron and hole are further away from each other, the alignment of their spins is disturbed—the spins no longer point in the opposite direction to their neighbors, as shown in the second panel—and this consumes energy. To avoid this loss of energy, electrons and holes tend to stay close to each other. This is the magnetic binding mechanism behind Hubbard excitons. Image source: Caltech
"Using advanced spectroscopic probes, we were able to observe the production and decay of magnetically bound excitons (Hubbard excitons) in real time," said the study's lead author Omar Mehio (PhD '23), a recent Caltech graduate student who worked in collaboration with Caltech Professor of Physics David Hsieh. Mechio is now a postdoctoral fellow at Cornell University’s Kaveri Institute.
"In most insulators, oppositely charged electrons and holes interact, just like electrons and protons combine to form hydrogen atoms," explains Mehio. "However, in a special material called a Mott insulator, photoexcited electrons and holes are combined through magnetic interactions."
Potential applications and experiments
The findings could be used to develop new exciton-related technologies, or excitonics, in which excitons will be manipulated through their magnetic properties.
"Hubbard excitons and their magnetic binding mechanisms are radically different from the traditional excitonology paradigm, creating opportunities to develop an entire ecosystem of new technologies that are simply not possible with traditional exciton systems," said Mehio. "Having excitons and magnetism closely intertwined in a single material could lead to new technologies that take advantage of both properties."
To generate Hubbard excitons, the researchers shined light on an insulating material called an antiferromagnetic Mott insulator. These are magnetic materials in which electron spins are arranged in repeating, stable patterns. The light excites the electrons, which jump to other atoms, leaving holes behind.
"In these materials, when electrons or holes travel through the crystal lattice, they leave behind a trail of magnetic excitations," said Mehio. "Imagine that you tie one end of a bungee cord to your friend and the other end to yourself. If your friend runs away from you, you feel the cord pulling you in that direction, and you start to follow. This is similar to what happens between a light-excited electron and the hole it leaves behind in a Mott insulator. For Hubbard excitons, the string of magnetic excitations between pairs of excitons serves the same purpose as the cord connecting you to your friend."
To prove the existence of Hubbard excitons, the researchers used a method called ultrafast time-domain terahertz spectroscopy, which allowed them to look for very brief signatures of excitons at very low energy scales.
"The exciton is unstable because the electron wants to go back into the hole," Xie explains. "We have a way to detect the short time window before this recombination occurs, which allows us to see that Hubbard exciton fluids are transiently stable."