Scientists have made a groundbreaking discovery by spotting a unique collection of particles, known as a quasiparticle, which behaves as massless in one direction while possessing mass in the opposite direction. This quasiparticle, identified as a semi-Dirac fermion, was initially theorized 16 years ago but has only recently been detected within a semi-metal crystal named ZrSiS. According to the researchers, this finding paves the way for future advancements across various emerging technologies, ranging from batteries to sensors.
The research team, led by experts from Penn State and Columbia University, has shared their findings in the journal Physical Review X.
“This discovery was completely unexpected,” commented Yinming Shao, an assistant professor of physics at Penn State and the lead author of the study. “We weren’t specifically looking for a semi-Dirac fermion while working with this material. However, we observed patterns that puzzled us, leading to the first detection of these unusual quasiparticles that sometimes exhibit mass and at other times behave as if they were massless.”
A particle is deemed massless when its energy is purely kinetic, effectively making it a form of energy that travels at light speed. For instance, a photon, or light particle, is considered massless since it moves at this speed. According to Albert Einstein’s special relativity theory, anything moving at light speed cannot carry mass. Within solid materials, the collective behavior of groups of particles, referred to as quasiparticles, can differ from that of individual particles, resulting in the phenomenon of particles having mass in only one direction, as explained by Shao.
Originally theorized in 2008 and 2009 by several research teams, including those from the Université Paris Sud and the University of California, Davis, semi-Dirac fermions were predicted to possess mass-altering characteristics depending on their movement’s direction—being massless in one direction while having mass when moving in another.
Sixteen years later, Shao and his team stumbled upon these hypothesized quasiparticles while employing a technique known as magneto-optical spectroscopy. This method involves directing infrared light onto a material under a strong magnetic field and analyzing the reflected light. The researchers aimed to investigate the properties of quasiparticles present in the silver-colored ZrSiS crystals.
The experiments conducted at the National High Magnetic Field Laboratory in Florida utilized the most powerful sustained magnetic field globally, approximately 900,000 times more intense than Earth’s magnetic field. This incredible field strength is capable of levitating small objects like water droplets.
The researchers cooled a sample of ZrSiS to -452 degrees Fahrenheit—just a few degrees above absolute zero—and subjected it to the lab’s potent magnetic field while illuminating it with infrared light to explore the quantum interactions within the material.
“We focused on how electrons within the material respond to light, then examined the resulting signals to identify interesting aspects regarding the material and its underlying physics,” Shao explained. “While we observed many expected features of a semi-metal crystal, we were also faced with puzzling results that were beyond our initial understanding.”
When a magnetic field is applied to any material, the energy levels of electrons become quantized into defined levels known as Landau levels, according to Shao. These levels behave like fixed steps on a staircase, meaning they can only occupy specific values without any intermediate steps. The spacing between these energy levels relies on the mass of the electrons and the magnetic field’s strength. Hence, as the magnetic field intensifies, the energy levels should increase based on the electrons’ mass—but that wasn’t the case here.
Using the powerful magnet in Florida, the researchers noted a distinct pattern in the energy transitions of the Landau levels within the ZrSiS crystal, differing markedly from the expected dependence on the magnetic field strength. Years earlier, theorists had designated this pattern as the “B^(2/3) power law,” a defining indicator of semi-Dirac fermions.
To decode the peculiar behavior they encountered, the experimental physicists collaborated with theoretical physicists to create a model explaining ZrSiS’s electronic structure. They concentrated on the potential paths for electron movement and intersections to delve into why the electrons were shedding mass in one direction but retaining it in another.
Consider the particle as a small train navigating a track network, which symbolizes the material’s electronic structure,” Shao stated. “At certain points, the tracks cross; hence, our particle train speeds along its path at light speed but encounters an intersection requiring it to switch direction. At this moment, it confronts resistance, thus acquiring mass. The particles can exhibit either pure energy or mass, contingent upon their movement direction within the material’s ‘tracks.’
The analysis revealed the presence of semi-Dirac fermions at these intersection points. They were massless when traveling straight but gained mass when shifting direction. Shao pointed out that ZrSiS is a layered material, similar to graphite, which comprises layers of carbon atoms that can be split into one-atom-thick graphene sheets. Graphene plays a crucial role in various emerging technologies, including batteries, supercapacitors, solar cells, sensors, and biomedical devices.
“Being a layered material allows us to potentially isolate a single layer of this compound, hence harnessing the capabilities of semi-Dirac fermions and controlling its properties with precision akin to that of graphene,” Shao remarked. “The thrilling aspect of this experiment is that our data remains partially unexplained. Many mysteries linger from what we observed, and that’s what we’re delving into.”
Other contributors from Penn State to the paper include Seng Huat Lee, an assistant research professor specializing in bulk crystal growth; Yanglin Zhu, a postdoctoral researcher; and Zhiqiang Mao, a professor of physics, materials science and engineering, and chemistry. Dmitri Basov, the Higgins Professor of Physics at Columbia University, also shared the lead authorship. Additional co-authors are Jie Wang from Temple University; Seongphill Moon from Florida State University and the National High Magnetic Field Laboratory; Mykhaylo Ozerov, David Graf, and Dmitry Smirnov from the National High Magnetic Field Laboratory; A. N. Rudenko and M. I. Katsnelson from Radboud University in the Netherlands; Jonah Herzog-Arbeitman and B. Andrei Bernevig from Princeton University; Zhiyuan Sun from Harvard University; as well as Raquel Queiroz and Andrew J. Millis from Columbia University.
This research received funding from the U.S. National Science Foundation, the U.S. Department of Energy, and the Simons Foundation for the Penn State aspects.
