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For the first time, scientists have observed a collection of particles that's massless when moving one direction but has mass in the other direction. The quasiparticle, called a semi-Dirac fermion, was spotted inside a crystal of semi-metal material called ZrSiS. The observation of the quasiparticle opens the door to future advances in a range of emerging technologies from batteries to sensors, according to the researchers.
“This was totally unexpected,” said Yinming Shao, assistant professor of physics at Penn State and lead author on the paper. “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.”

Semi-Dirac fermions were first theorized in 2008 and 2009 by several teams of researchers, including scientists from the Université Paris Sud in France and the University of California, Davis. The theorists predicted there could be quasiparticles with mass-shifting properties depending on their direction of movement — that they would appear massless in one direction but have mass when moving in another direction.

Sixteen years later, Shao and his collaborators accidentally observed the hypothetical quasiparticles through a method called magneto-optical spectroscopy. The technique involves shining infrared light on a material while it's subjected to a strong magnetic field and analyzing the light reflected from the material. Shao and his colleagues wanted to observe the properties of quasiparticles inside silver-colored crystals of ZrSiS.

https://journals.aps.org/prx/abstract/10.1103/PhysRevX.14.041057