Nearly 16 years after its possibility was first predicted, scientists have made a satisfying and intriguing physical discovery: a quasiparticle (a group of particles that behaves like a single particle) that has effective mass only when it moves in one direction. In physics, mass generally refers to properties of particles that relate to things like their energy and resistance to motion. However, not all mass is created equal; While some of these describe the energy of a particle at rest, for example, mass can also take into account the energy of the particle’s motion.

In this case, the effective mass describes the quasi-particle’s response to changing forces depending on whether the motion is up-down or back-and-forth through the material. While ordinary quasiparticles have the same mass regardless of their direction of motion, the quasi-Dirac fermion studied here (to give it a technical name) does not appear to behave according to normal rules.

This is a discovery that could fundamentally change the situation in fields such as quantum physics and electronic sensors. The new quasiparticle was discovered by an international team of scientists inside a semimetallic ZrSiS crystal cooled to -452 degrees Fahrenheit (or -269 degrees Celsius); This is an extreme set of conditions for an extremely rare quasiparticle.

In general, particles can be described as bosons or fermions depending on the measure of a property called spin. Dirac fermions – in both typical and quasi-particle forms – have properties that have opposite particle and antiparticle forms. This quasi-Dirac fermion, detailed in a new study, is a strange beast that has so far only existed in theory, operating in very different energy directions in orthogonal directions.

“This was completely unexpected,” says Yinming Shao, a condensed matter physicist at Pennsylvania State University. “When we started working with this thing, we weren’t even looking for a half-Dirac fermion, but we were seeing signatures that we didn’t understand.”

“It turns out that this is the first time we’ve observed these wild quasiparticles that sometimes act as if they have mass, sometimes as if they don’t.”

The researchers used a scientific analysis method known as magneto-optical spectroscopy to make the discovery. Here, materials are examined using the reflection of infrared light they emit under the influence of a strong magnetic field.

We mean really strong: About 900,000 times stronger than the Earth’s magnetic field, courtesy of the National Strong Magnetic Field Laboratory in Florida. These are exotic conditions that scientists use to study the rarest interactions at the smallest scales.

From here, quasi-Dirac fermion activity was observed and described through some numerical simulations: In one direction it was massless (all its energy was defined by its motion), but in the other direction it had effective mass. Fortunately for non-physicists, researchers provide an analogy.

“Think of a particle as a small train bounded by a network of rails, which is the fundamental electronic structure of the material,” Shao says. “The paths now cross at certain points, so our particle train is traveling at the speed of light on its fast track, but then it hits an intersection and has to switch to a vertical path.

“Suddenly he feels resistance, he has mass. Particles have either full energy or mass depending on the direction in which they move along the “tracks” of the material.

This is a landmark in physics, including those who first proposed the phenomenon in 2008. However, there’s still a lot to discover here before we even start thinking about it, including figuring out how to remove individual layers from a ZrSiS multilayer crystal. all its consequences and all its practical uses.

“The most exciting part of this experiment is that the data cannot be fully explained yet,” Shao says. “There are a lot of unsolved mysteries in what we observe, so we’re trying to figure it out.” The study was published on: Physical Examination X.