A team of researchers has now observed something physicists had only theorized: a quasiparticle that behaves like it has mass when moving one way and becomes massless when it travels another.
This peculiar particle, called a semi-Dirac fermion, was first predicted 16 years ago. Now, it’s been spotted inside a semi-metal crystal of zirconium silicon sulfide (ZrSiS). The discovery could pave the way for innovations in fields ranging from battery technology to advanced sensors.
Yinming Shao, a physicist at Penn State and the lead author of the study, admitted the discovery was anything but planned. “We weren’t even looking for a semi-Dirac fermion when we started working with this material,” he said. “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.”
A Wild Quasiparticle
Particles like photons, which make up light, are considered massless because they move at the speed of light. Einstein’s theory of special relativity explains this phenomenon. Anything that moves this fast can’t possess mass. But things get tricky when you consider solid materials.
Inside these materials, collective behaviors of many particles create what physicists call quasiparticles. These quasiparticles can mimic real particles but often with properties that defy everyday intuition. These “particles” aren’t real, standalone particles, but they behave as if they are. For example, the way electrons move together might create a quasiparticle that acts like it has no mass or moves faster than any individual electron. So, quasiparticles help scientists understand complex behaviors in solids, making it easier to study things like electricity, magnetism, and new materials for technology.
The semi-Dirac fermion is one such example. First predicted by researchers in 2008 and 2009, these quasiparticles were thought to switch between massless and massive states based on the direction they travel.
Shao and his colleagues used a powerful technique called magneto-optical spectroscopy. This involves shining infrared light on a material under a strong magnetic field and analyzing how the light reflects back. They carried out their experiment at the National High Magnetic Field Laboratory in Florida, home to a magnet 900,000 times stronger than Earth’s magnetic field.
Mass or No Mass, Depending on the Path
To detect the semi-Dirac fermions, the team chilled a ZrSiS crystal to -452°F — just a few degrees above absolute zero — and subjected it to the lab’s colossal magnetic field. They then bombarded the crystal with infrared light and observed how the electrons inside the material behaved.
What they saw defied expectations. Instead of electrons following the typical energy pattern dictated by their mass, they behaved differently. This unusual pattern of electron energy levels matched a theoretical prediction known as the B2/3 power law, a signature of semi-Dirac fermions.
Shao likened the quasiparticles’ movement to trains on intersecting tracks. “Imagine the particle is a tiny train confined to a network of tracks, which are the material’s underlying electronic structure,” he explained. “Now, at certain points the tracks intersect. Our particle train is moving along its fast track, at light speed, but then it hits an intersection and needs to switch to a perpendicular track. Suddenly, it experiences resistance, it has mass.”
This directional quirk gives the semi-Dirac fermions their strange dual nature. Along one path, they glide effortlessly like photons. Turn them in another direction, and they lumber along as if weighed down by mass.
Unlocking New Potential
ZrSiS, the material in which these quasiparticles were found, is a layered semi-metal. It bears similarities to graphite, the same all-carbon material used in pencils. When graphite is peeled down to a single atom layer, it becomes graphene which has incredible properties for use in electronics, batteries, and sensors.
Shao believes the same potential exists for ZrSiS. “Once we can figure out how to have a single-layer cut of this compound, we can harness the power of semi-Dirac fermions,” he said. “We can control its properties with the same precision as graphene.”
Yet, the discovery comes with new puzzles. “The most thrilling part of this experiment is that the data cannot be fully explained yet,” Shao said. “There are many unsolved mysteries in what we observed, so that is what we are working to understand.”
The semi-Dirac fermion might have been a long time coming, but its implications are just beginning to unfold. Whether it leads to next-generation sensors or better energy storage systems, one thing is clear: In the quantum world, the more you look, the stranger things get.
The findings were published recently in Physical Review X.
