Scientists accidentally discovered a particle that gains or loses mass depending on the direction it moves

A group of scientists from the University of Pennsylvania and Columbia University for the first time experimentally discovered signs of semi-Dirac fermions – quasiparticles theoretically predicted 16 years ago. Their amazing feature is that they completely lose mass when changing direction of movement perpendicular to the previous one. Particles without mass are capable of accelerating to the speed of light, which in our Universe is only possible for photons.

Model of the behavior of a semi-Dirac fermion. Image source: Penn State University

Signs of semi-Dirac fermions were discovered when studying the quantum properties of a number of topological materials, in particular the compound zirconium, silicon and sulfur (ZrSiS), a popular semimetal among researchers. In a sense, its structure resembles graphene, but no one has yet succeeded in peeling off sheets of atomic thickness from it. If this succeeds and the compound demonstrates the ability to control semi-Dirac fermions, then this material will find many applications, from batteries to sensors.

«This was completely unexpected,” said Yinming Shao, assistant professor of physics at the University of Pennsylvania and lead author of the paper. “We weren’t even looking for semi-Dirac fermions when we started working with this material.” However, we observed signs that we could not explain, and it turned out that for the first time we saw these unusual quasiparticles that sometimes move as if they have mass, and sometimes as if they do not.”

By and large, semi-Dirac fermions are not individual independent particles, but the group behavior of particles, in other words, quasiparticles. In this case, the group behavior of electrons led to the fact that when moving in one direction the quasiparticle had mass, but when moving in the other direction it did not.

The researchers used magneto-optical spectroscopy to study ZrSiS. This method involves shining a material with infrared light and simultaneously exposing it to a strong magnetic field, after which the reflected light is analyzed. The applied magnetic field exceeded the strength of the Earth’s magnetic field by 900 thousand times. Such a field is capable of lifting small non-magnetic objects, such as drops of water, into the air. The sample was pre-cooled to a temperature of -268.89 °C, which is only slightly above absolute zero.

«”We looked at the optical response, which is how the electrons inside the material react to light, and then analyzed the light signals to see if there was anything interesting about the fundamental physics of the material,” Shao explained. “In our case, we saw many expected features characteristic of a semimetal. But then completely mysterious properties appeared.”

To analyze the experimental data, the scientists involved theorists, who jointly built a model of the behavior of quasiparticles. This model was consistent with predictions of semi-Dirac fermions made in theoretical papers from 2008–2009. The discovery opens up new perspectives for studying quantum properties in an area that has until now remained unexplored.