"Quasiparticle" that only has mass moving in one direction is seen for the first time

For the first time, researchers have observed a type of quasiparticle that behaves in an unusual way. In one direction, it acts like it has no mass, zipping around as if it were made of pure energy.

But turning another way, it carries mass, slowing down and resisting motion.

This mysterious entity, called a semi-Dirac fermion, had appeared in theory about 16 years ago but had never been seen inside a real material.

After careful experiments, a team made up of scientists from Penn State and Columbia University recently announced its discovery in the journal Physical Review X.

Unexpected outcomes in a crystal

The strange quasiparticle was spotted within a crystal known as ZrSiS, which is a type of semi-metal. The team had not set out searching for semi-Dirac fermions but instead stumbled upon them.

“This was totally unexpected,” exclaimed lead author on the paper is Yinming Shao, assistant professor at Penn 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 — 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,” Shao explained.

These findings highlight how some materials harbor unusual particle behavior that does not match everyday expectations.

Inside solid substances, many particles can clump together to form emergent behavior. It was within these interactions that the researchers identified the odd switch between massless and massive motion.

The discovery follows predictions made in 2008 and 2009, when theoretical physicists outlined the possibility of particles that behave differently depending on their direction of travel.

Quasiparticles: From theory to reality

Sixteen years after those predictions, the team caught the semi-Dirac fermion through magneto-optical spectroscopy.

They shone infrared light on the crystal while applying a powerful magnetic field and then examined the reflected light to uncover the quasiparticle’s properties.

“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 noted.

“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.”

Running the experiment in the National High Magnetic Field Laboratory in Florida made it possible to tap into one of the strongest sustained magnetic fields on Earth.

The team exposed their sample of ZrSiS to a field around 900,000 times stronger than the Earth’s own.

They also chilled the material down to around -452 degrees Fahrenheit, which helped reduce unwanted motion of particles and revealed the subtle patterns of energy levels inside the crystal.

Massless along just one path

Quasiparticles that have no mass move at the speed of light. A photon, for instance, is massless and sprints along at light speed.

Under certain conditions, a particle’s behavior can shift, so its effective mass emerges only along certain directions within the structure of a crystal.

Inside ZrSiS, these shifting conditions became clear through direct observation of energy levels known as Landau levels.

The researchers found that instead of following the standard pattern expected from massive particles, these levels showed a relationship known as the B^(2/3) power law, considered a signature of semi-Dirac fermions.

Electrons trapped in the crystal’s structure move along certain pathways. When the magnetic field forces their energy states into Landau levels, the spacing between these steps would normally depend on the electron mass.

Instead, something entirely different took place. The observed shifts matched the earlier theoretical predictions for semi-Dirac fermions rather than the standard pattern seen in conventional materials.

Quasiparticles changing directions and identity

It is as if these tiny entities shed their mass when moving in one direction, only to pick it up again when forced along another route. That directional change disrupts the normal flow of energy and produces distinctive signals.

“Imagine the particle is a tiny train confined to a network of tracks, which are the material’s underlying electronic structure,” Shao implored.

“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.’”

The structure of ZrSiS creates these intersections. Instead of particles freely moving in all directions, they are guided by the arrangement of atoms. At certain crossing points, the character of the quasiparticle changes.

Why does any of this matter?

ZrSiS is made up of layers that resemble other well-known materials such as graphite. Researchers can isolate very thin sheets that show off unusual properties.

“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, control its properties with the same precision as graphene,” Shao said.

“But 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.”

Such layered materials have drawn interest for a broad range of uses. Graphene has been studied for cutting-edge energy storage, advanced electronics, and sensitive detectors.

ZrSiS, now known to host semi-Dirac fermions, could follow a similar path toward future devices.

With careful efforts, it might become a valuable piece of future sensors, energy devices, or other emerging technologies.

New direction for quantum research

The existence of these quasiparticles sparks new questions. The semi-Dirac fermion represents a new twist on what seemed like established physics.

While the basic idea of mass and massless particles is well-understood, this finding adds a fresh layer of complexity that challenges standard thinking.

Although researchers made an important observation, they acknowledge that there are still puzzles that must be addressed. New theoretical models might help pin down exactly why these quasiparticles behave as they do.

With these experimental results now on record, Shao and his collaborators are eager to learn more. Each piece of data adds to the understanding of how electrons interact, how they pick up or lose mass, and what this means for future materials research.

The first direct observation of semi-Dirac fermions marks an intriguing moment. There may be more oddities yet to find, and those answers will require further studies, but the door is now open.

The full study was published in the journal Physical Review X.

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