Scientists have made a fascinating and rewarding discovery in physics, confirming a prediction made 16 years ago: a quasiparticle (a collection of particles acting as a single entity) that possesses an effective mass only when moving in a specific direction.
In physics, mass is generally considered a property of particles that influences aspects such as energy and resistance to motion. However, not all mass behaves identically—some relates to the energy of a particle at rest, while in other cases, mass factors in the energy of motion.
Here, the effective mass refers to how the quasiparticle responds to forces, changing based on whether its movement through the material is along one axis or another.
While ordinary quasiparticles maintain the same mass regardless of their travel direction, the semi-Dirac fermion (its technical designation) observed in this study defies conventional expectations.
This breakthrough could have profound implications in fields like quantum physics and advanced electronic sensors.
An international team of scientists detected this novel quasiparticle within a ZrSiS semi-metal crystal, cooled to a frigid -452 degrees Fahrenheit (-269 degrees Celsius)—an extreme environment fitting for such a rare phenomenon.
Particles are generally classified as bosons or fermions, depending on their spin properties. Dirac fermions—whether in their standard or quasiparticle forms—exhibit characteristics that appear in complementary particle-antiparticle pairs.
The semi-Dirac fermion described in this research is an unusual entity that, until now, had only existed in theory, behaving according to unique energy rules in perpendicular directions.
“This was totally unexpected,” says condensed matter physicist Yinming Shao from Pennsylvania State University. “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.”
“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.”
The discovery was made using a technique called magneto-optical spectroscopy, which examines materials by analyzing the infrared light they reflect under a strong magnetic field.
And when we say strong, we mean it: about 900,000 times more powerful than Earth’s magnetic field, made possible by the National High Magnetic Field Laboratory in Florida. Scientists rely on such extreme conditions to investigate rare interactions occurring at microscopic scales.
By combining these observations with numerical modeling, researchers identified the semi-Dirac fermion’s distinctive behavior: appearing massless in one direction (with all its energy derived from motion) while displaying effective mass in another. Fortunately, the scientists provide an analogy to clarify the concept.
“Imagine the particle is a tiny train confined to a network of tracks, which are the material’s underlying electronic structure,” says Shao.
“Now, at certain points the tracks intersect, so 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. The particles are either all energy or have mass depending on the direction of their movement along the material’s ‘tracks’.”
This marks a significant moment in physics, particularly for those who first theorized this phenomenon in 2008. However, much remains to be uncovered—such as finding a way to isolate single layers from the multi-layered ZrSiS crystal—before its full implications and potential applications can be realized.
“The most thrilling part of this experiment is that the data cannot be fully explained yet,” says Shao.
“There are many unsolved mysteries in what we observed, so that is what we are working to understand.”
The research has been published in Physical Review X.
