First-ever discovery: Unique particle only has mass when moving in one direction

For the first time, scientists have directly observed a semi-Dirac fermion, a quasiparticle with massless behavior in one direction and mass in another. This groundbreaking discovery, made within a crystal of a semi-metal material called ZrSiS, was recently published in the journal Physical Review X by researchers from Penn State and Columbia University.

“This was totally unexpected,” said Yinming Shao, a Penn State assistant professor of physics and lead author of the study. “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.”

Quasiparticles such as semi-Dirac fermions emerge from the collective behavior of electrons within solid materials. Unlike individual particles, quasiparticles exhibit unique properties, which in this case manifest as massless behavior along one direction and mass-like behavior in another. Such phenomena arise from the material's electronic structure, where the interactions among electrons and atoms create novel quantum behaviors.

Semi-Dirac fermions were first theorized over 16 years ago. Researchers predicted that these quasiparticles could emerge in specific systems, behaving masslessly in one direction while possessing mass in another. However, this prediction had remained unverified until now.

The experimental breakthrough came via magneto-optical spectroscopy, a method that involves exposing materials to infrared light under a powerful magnetic field. The technique revealed peculiar patterns in ZrSiS’s electron behavior.

The research was conducted at the National High Magnetic Field Laboratory in Florida, home to the world's most powerful sustained magnetic field. This facility’s hybrid magnet generates a field 900,000 times stronger than Earth’s magnetic field.

The ZrSiS samples were cooled to -452 degrees Fahrenheit, near absolute zero, to eliminate thermal noise. Exposing these samples to the intense magnetic field and infrared light uncovered surprising quantum interactions.

Shao explained that in the presence of a magnetic field, electrons in a material form discrete energy levels called Landau levels. The spacing of these levels typically depends on the electrons’ mass and the field’s strength.

However, the levels in ZrSiS followed an unusual pattern, aligning with a theoretical “B^(2/3) power law” that had been predicted as a signature of semi-Dirac fermions. The experiment thus provided definitive evidence of these quasiparticles.

Theoretical physicists partnered with the experimental team to model ZrSiS’s electronic structure. They found that the material’s electrons moved along specific pathways, or “tracks,” where their behavior shifted depending on direction.

“Imagine the particle is a tiny train confined to a network of tracks,” Shao said. “At certain intersections, the train switches from a fast track to a slower one, experiencing resistance and gaining mass.” This directional dependence is the defining feature of semi-Dirac fermions.

The observed behavior of electrons in ZrSiS reflects the material’s underlying structure, which includes planar nodal squares connected by vertical nodal lines. These features form a chainlike configuration in momentum space, creating conditions for semi-Dirac fermions to emerge.

Ab initio calculations confirmed that these quasiparticles originated at specific crossing points within the nodal lines. Near these points, electrons exhibited linear energy dispersion in one direction and quadratic dispersion in the perpendicular direction, matching the theoretical predictions for semi-Dirac behavior.

The potential applications of this discovery extend far beyond basic physics. Materials exhibiting semi-Dirac behavior could revolutionize technologies that rely on advanced electronic properties, such as batteries, sensors, and quantum devices.

Shao highlighted that ZrSiS, like graphite, has a layered structure, making it a candidate for further refinement. By isolating single layers, researchers could precisely control the material’s properties, much like graphene.

“It is a layered material, which means once we can figure out how to have a single-layer cut of this compound, we can harness the power of semi-Dirac fermions,” Shao said. “The most thrilling part of this experiment is that the data cannot be fully explained yet. There are many unsolved mysteries in what we observed, so that is what we are working to understand.”

This pioneering research involved collaboration among experts from Penn State, Columbia University, and other institutions, including Temple University, Florida State University, and Radboud University in the Netherlands. The U.S. National Science Foundation, the Department of Energy, and the Simons Foundation provided funding for the project.

The discovery of semi-Dirac fermions marks a significant advance in condensed matter physics. While theoretical studies had long predicted their existence, their experimental realization within ZrSiS opens new avenues for exploring topological and correlated phases of matter. It also underscores the power of collaborative, interdisciplinary approaches in uncovering fundamental quantum phenomena.

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