Scientists report the first known observation of a variety of quasiparticle that exhibits a very peculiar behavior: it appears to have mass, but only while moving in one direction.
Scientists at Pennsylvania State University recently succeeded in detecting the unusual quasiparticle while conducting studies involving a semi-metallic crystalline material. Known as a semi-Dirac fermion, this unique formation of particles was first theorized more than a decade ago, but until now had never been directly observed.
The discovery potentially paves the way toward future advances in a range of emerging technologies that include power storage and novel forms of sensor technologies.
Detecting a Novel Quasiparticle
Quasiparticles are small collections of particles that normally appear within crystal lattices or under other special conditions, which generally possess both momentum and position, and under certain conditions may also be considered particles.
Discovering a novel quasiparticle like a semi-Dirac fermion had not been something Yinming Shao, assistant professor of physics at Penn State and lead author of a new paper revealing the discovery, had anticipated when he and his colleagues began experimenting with ZrSiS, a semi-metal crystal material that became the focus of their efforts.
“We weren’t even looking for a semi-Dirac fermion when we started working with this material, but we were seeing signatures we didn’t understand—and it turns out we had made the first observation of these wild quasiparticles that sometimes move like they have mass and sometimes move like they have none.”
Particles Without Mass
More than a century ago, Einstein’s theory of general relativity predicted that anything moving at the speed of light will have no mass. Because of this, physicists already recognize that a particle can essentially be massless under certain circumstances, namely when its energy comes entirely from its motion. Under such conditions, particles are recognized as manifestations of energy moving at the speed of light, such as in the case of photons.
However, quasiparticles moving through solid materials like crystalline structures can sometimes behave differently. In the observations of the Penn State research team, this apparently resulted in the appearance of particles that have mass in only one direction.
Beginning in 2008, it was initially predicted that mass-shifting properties might be observed in certain kinds of quasiparticles, which provided the theoretical framework for semi-Dirac fermions. Based on these initial predictions by scientists with the University of California, Davis and Université Paris Sud in France, such quasiparticles would seemingly be massless when moving in one direction but would almost paradoxically appear to possess mass when moving in another direction.
Shao and the Penn State team happened upon the discovery of such bizarre quasiparticle behavior while utilizing what is known as magneto-optical spectroscopy, which allows researchers to observe infrared light reflected off materials that are placed under the influence of strong magnetic fields.
A Little Help from a Powerful Magnetic Field
Specifically, Shao and his team hoped to glean insights into the properties of quasiparticles held within the silvery crystalline structure of the novel ZrSiS material during experiments using the most powerful sustained magnetic field in existence, produced at the National High Magnetic Field Laboratory in Florida.
Generating a field estimated to exceed that of the Earth’s magnetic field by as much as 900,000 times, the sustained magnetic field the National High Magnetic Field Lab produces is strong enough to cause some tiny objects to levitate.
Chilling a sample of ZrSiS to an incredibly cold temperature of around -452°F, the material was then exposed to the National High Magnetic Field’s powerful influence. At temperatures approaching absolute zero, the sample was then subjected to magneto-optical spectroscopy in order to reveal information about the behavior of quantum interactions occurring within.
“We were studying optical response, how electrons inside this material respond to light, and then we studied the signals from the light to see if there is anything interesting about the material itself, about its underlying physics,” Shao said of the team’s observations.
“In this case, we saw many features we’d expect in a semi-metal crystal and then all of these other things happening that were absolutely puzzling,” he added.
Unraveling Odd Behavior
Working with theoretical physicists, the team examined the behavior of electrons, and how they might move and intersect in order to help account for their apparently massless qualities while moving in certain directions. Shao likens the observed behavior of the particles to tiny trains moving at light speed that are confined to specific “tracks” or pathways, which intersect at certain points. When the particles hit these crossroads and are required to change direction, they experience resistance, causing them to have mass.
“The particles are either all energy or have mass depending on the direction of their movement along the material’s ‘tracks’,” Shao said.
Ultimately, the results of the team’s experiments led to the first known observation of the theorized semi-Diract fermion and its odd behaviors, which Shao said came as a surprise to the Penn State team.
“This was totally unexpected,” Shao said in a statement, describing the perplexing signatures exhibited by the unusual quasiparticle.
For Shao, however, the most exciting aspect of the team’s discovery is that the data they uncovered cannot be presently be fully explained.
“There are many unsolved mysteries in what we observed,” Shao said, “so that is what we are working to understand.”
Shao and his colleagues recently published their findings in a new paper, “Semi-Dirac Fermions in a Topological Metal,” that appeared in the journal Physical Review X.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.