Scientists have accidentally discovered a particle that has mass when it’s traveling in one direction, but no mass while traveling in a different direction. Known as semi-Dirac fermions, particles with this bizarre behavior were first predicted 16 years ago.
The discovery was made in a semi-metal material called ZrSiS, made up of zirconium, silicon and sulfur, while studying the properties of quasiparticles. These emerge from the collective behavior of many particles within a solid material.
“This was totally unexpected,” said Yinming Shao, lead author on 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.”
It sounds like an impossible feat – how can something gain and lose mass readily? But it actually comes back to that classic formula that everyone’s heard of but many might not understand – E = mc2. This describes the relationship between a particle’s energy (E) and mass (m), with the speed of light (c) squared.
According to Einstein’s theory of special relativity, nothing that has any mass can reach the speed of light, because it would take an infinite amount of energy to accelerate it to that speed. But a funny thing happens when you flip that on its head – if a massless particle slows down from the speed of light, it actually gains mass.
And that’s what’s happening here. When the quasiparticles travel along one dimension inside the ZrSiS crystals, they do so at the speed of light and are therefore massless. But as soon as they try to travel in a different direction, they hit resistance, slow down and gain mass.
“Imagine the particle is a tiny train confined to a network of tracks, which are the material’s underlying electronic structure,” said 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.’”
The researchers originally set out to study the quantum interactions in the material by watching how the electrons in it responded to light. They found that the energy level of the electrons followed an unexpected pattern as the strength of the magnetic field increases, which turned out to be a key prediction of semi-Dirac fermions.
While it’s pretty high-concept physics, the team says the discovery could eventually open up a range of applications for ZrSiS similar to that of 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, control its properties with the same precision as graphene,” said Shao. “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.”
The research was published in the journal Physical Review X.
Source: Penn State
