semi dirac fermions zrsis physics breakthrough

Physicists Observe Directional Mass-Only Particle for the First Time

Introduction to the Discovery of Semi-Dirac Fermions

Researchers have successfully identified, for the first time, a semi-Dirac Fermion-a quasiparticle characterized by being massless in one direction and possessing mass in the other. Though hypothesized 16 years ago, it was only recently observed within a ZrSiS semi-metal crystal. This breakthrough holds promise for transformative applications in technologies such as sensors and batteries.

Researchers from Penn State and Columbia University recently unveiled their findings in Physical Review X.

The Surprise Discovery and Its Significance

Unanticipated Findings

"This discovery came as a complete surprise," said Yinming Shao, assistant professor of physics at Penn State and the study's lead author. "Our initial research was not focused on semi-Dirac ferminos, but unexpected signatures in the data led us to realize we had made the first observation of these remarkable quasiparticles, which alternately behave as if they have mass or are massless."

Understanding Massless Particles

Particles are considered massless when their energy originates entirely from motion, making them pure energy traveling at the speed of light. For instance, photons, the particles of light, are massless as they always move at light speed. Albert Einstein's special relativity states that no object with mass can achieve this velocity.

"In solid materials, the collective dynamics of numerous particles, referred to as quasiparticles, can exhibit properties distinct from those of individual particles. In this case, this phenomenon resulted in particles possessing mass in only one direction," Shao explained.

The Concept of Semi-Dirac Ferminos and Their Theoretical Origins

Theoretical Predictions

The existence of Semi-Dirac ferminos was first proposed between 2008 and 2009 by multiple research teams, including scientists from Université Paris Sud in France and the University of California, Davis. Theoretical models suggested that these quasiparticles would exhibit directional mass-shifting behavior, appearing massless along one axis while possessing mass along another.

Unexpected Observation

After 16 years, Shao and his collaborators accidentally identified the predicted quasiparticles through magneto-optical spectroscopy, a technique that uses infrared light and a strong magnetic field to study reflected light. Their goal to examine quasiparticle properties in silver-colored ZrSiS crystals.

Landau level spectroscopy provides insights into semi-Dirac fermions at the inter-section of two nodal lines within a semi-metal material. (Left: Fermi surface of a nodal-line crossing model; Right: Band structure of the material). Credit: Yinming Shao.

The Experiment and Methods Used

Magneto-Optical Spectroscopy at the National High Magnetic Field Laboratory

The experiments were carried out at the National High Magnetic Field Laboratory in Florida, which houses the world's most powerful hybrid magnet. This magnet generates a sustained magnetic field approximately 900,000 times stronger than Earth's magnetic field—strong enough to levitate small objects like water droplets.

Conducting the Experiments at Extremely Low Temperatures

The researchers reduced the temperature of a ZrSiS sample to -452°F, just a few degrees above absolute zero, before subjecting it to the lab's strong magnetic field and illuminating it with infrared light to investigate the quantum interactions within the material.

Analyzing the Unusual Results

"We were investigating how the material's electrons respond to light, focusing on their optical response," said Shao. "By analyzing the light signals, we hoped to uncover any intriguing aspects of the material's underlying physics. What are found was a mix of expected features typical of a semi-metal crystal, along with surprising phenomena that left us completely puzzled."

Uncovering the Semi-Dirac Fermion Behavior

Unexpected Magnetic Field Behavior

"When a magnetic field is applied to a material, the energy levels of its electrons are quantized into distinct Landau levels," Shao explained. "These levels are discrete, similar to climbing stairs with no intermediate steps. The spacing between them is determined by the electron's mass and the strength of the magnetic field. As the magnetic field intensifies, the energy levels should increase in fixed increments based on the electron's mass—yet, in this case, that didn't happen."

Identifying the Power Law and Collaboration with Theoretical Physicists

By utilizing the powerful magnet in Florida, the researchers discovered that the energy transitions of the Landau levels in the ZrSiS crystal exhibited an entirely unexpected relationship with the strength of the magnetic field. This distinct pattern, identified by theorists years ago as the 'B2/3 power law,' is a hallmark characteristic of semi-Dirac fermions.

Explaining the Particle Behavior

A Model for Electron Behavior in ZrSiS

To unravel the unusual behavior they observed, the experimental physicists collaborated with theoretical physicists to create a model that explained the electronic structure of ZrSiS. Their focus was on the possible paths along which electrons could move and intersect, in order to understand how electrons in the material were losing mass when traveling in one direction but not in another.

Understanding the Massless and Massive Transitions

"Picture the particle as a miniature train confined to a network of tracks, which represent the material's fundamental electronic structure," explained Shao. "At specific intersections, the particle train moves along a fast track at light speed, but when it encounters an intersection and switches to a perpendicular track, it suddenly gains resistance and mass. In this way, the particles either remain pure energy or acquire mass depending on the direction they travel along the material's 'tracks.'"

Implications for Future Technologies

The Role of ZrSiS in Emerging Technologies

The team's analysis revealed the presence of semi-Dirac fermions at the crossing points. These particles were massless when moving along a linear path but acquired mass when traveling in a perpendicular direction. Shao further explained that ZrSiS is a layered material, similar to graphite, composed of carbon atom layers that can be exfoliated into graphene sheets just one atom thick. Graphene plays a vital role in the development of emerging technologies, such as batteries, supercapacitors, solar cells, sensors, and biomedical devices.

Looking Ahead: Unresolved Questions and Future Research

"This is a layered material," said Shao. "Once we figure out how to isolate a single layer of this compound, we could harness the unique properties of semi-Dirac fermions and control them with the same precision that we do with graphene. However, the most exciting aspect of this experiment is that the data we collected cannot yet be fully explained. There are still many unresolved questions, and our focus is on understanding them."

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