Physicists discover quasiparticle that only sometimes has mass

Physicists have observed a “quasiparticle” that has mass when moving one way, and no mass when headed in another direction.

The quasiparticle was theorised 16 years ago, but this is the first time it’s been directly detected.

Called a “semi-Dirac fermion”, the quasiparticle was found by accident while the team was studying other quasiparticles.

“Imagine the particle is a tiny train confined to a network of tracks, which are the material’s underlying electronic structure,” says lead author Yinming Shao, an assistant professor of physics at Pennsylvania State University, USA.

“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 have published their findings in Physical Review X.

Quasiparticles are collections of interactions in system that behave like a single particle.

Semi-Dirac fermions were first theorised in 2008 and 2009 by physicists  who predicted that these quasiparticles could have mass or be massless depending on their direction of movement.

“This was totally unexpected,” says Shao, “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.”

The researchers were studying the quantum interactions inside a zirconium silicon sulphide crystal (ZrSiS) with a technique called magneto-optical spectroscopy.

This involves subjecting a material to a super-strong magnet – in this case, one of the most powerful magnets in the world at the US National High Magnetic Field Laboratory – and shining infrared light onto it.

The reflected and absorbed infrared light can tell researchers information about the structure of the material.

“We were studying 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,” says Shao.

“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.”

The team was expecting the electrons inside the material to sort themselves into energy levels in a specific pattern, related to their mass and the strength of the magnetic field.

But instead, they formed a completely different order – consistent with a semi-Dirac fermion. The team built a model with theoretical physicists to establish its identity.

Shao says that this discovery might make ZrSiS a useful material in future technology.

“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,” he says.

“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.”