By Amit Malewar 16 Dec, 2024
Collected at: https://www.techexplorist.com/particle-massless-moving-direction-mass-other-direction/94677/
Scientists first suggested a type of particle called a semi-Dirac fermion 16 years ago. They thought these particles could change their mass depending on their moving direction. In one direction, they would act like they had no mass, but in another, they would have mass.
For the first time, scientists have seen these particles in a special crystal material called ZrSiS. They were surprised that the particles gained mass when moving in one direction.
The scientists discovered this by accident. At first, they weren’t trying to find these semi-Dirac fermions. Instead, they noticed strange behavior they couldn’t explain, which turned out to be these unusual particles that sometimes act like they have mass and sometimes don’t.
This accidental discovery was made using a technique called magneto-optical spectroscopy. In this method, scientists shine infrared light onto a material while placed in a strong magnetic field and study the light that bounces back. The scientists were trying to study the behavior of quasiparticles inside silver-colored ZrSiS crystals.
During the experiment at the National High Magnetic Field Laboratory in Florida, the team cooled a piece of ZrSiS to -452 degrees Fahrenheit. They then exposed the material to the lab’s strong magnetic field and hit it with infrared light to study the quantum interactions inside it.
Yinming Shao, assistant professor of physics at Penn State and lead author of the paper, said, “We were studying the optical response, how electrons inside this material respond to light, and then we studied the signals from the light to see if anything is interesting about the material itself, about its underlying physics. In this case, we saw many features we’d expect in a semi-metal crystal and all of these other puzzling things happening.”
“When a magnetic field is applied to any material, the energy levels of electrons inside that material become quantized into Landau levels. The levels can only have fixed values, like climbing a set of stairs with no little steps in between. The spacing between these levels depends on the mass of the electrons and the strength of the magnetic field, so as the magnetic field increases, the energy levels of the electrons should increase by set amounts based entirely on their mass — but in this case, they didn’t.”
The team used a strong magnet to study how the energy in the ZrSiS crystal changed when the magnetic field was applied. They found that the energy followed a different pattern than expected, known as the “B^(2/3) power law.” This pattern is a key sign of the semi-Dirac fermions.
To better understand this strange behavior, the team worked with theoretical physicists to create a model of ZrSiS’s electronic structure. They focused on how electrons move and interact with each other, trying to figure out why electrons in the material lose their mass when moving in one direction but not in another.
Shao said, “Imagine the particle is a tiny train confined to a network of tracks, which are the material’s underlying electronic structure. Now, the tracks intersect at certain points, 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 team’s analysis found that the semi-Dirac fermions appeared at the points where the electrons crossed paths. These particles acted like they had no mass when moving in a straight line but gained mass when moving in a direction at a right angle to the first.
Shao explained that ZrSiS is a layered material similar to graphite. Graphite comprises layers of carbon atoms that can be peeled into very thin sheets, just one atom thick. Graphene is essential in new technologies, such as batteries, supercapacitors, solar cells, sensors, and medical devices.
“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 mysteries in what we observed, so that is what we are working to understand.”
Journal Reference:
- Yinming Shao, Seongphill Moon, et al. Semi-Dirac Fermions in a Topological Metal. Physical Review X. DOI: 10.1103/PhysRevX.14.041057
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