In solids, Weyl fermions lead to unusual behavior. “Experimentally, this work really is a nice example,” Soluyanov says. But Hasan and colleagues peered inside the material. With surface measurements alone, it’s hard to confirm the type-II Weyl fermions are there, says physicist Alexey Soluyanov of ETH Zurich. Although previous experiments have shown hints of the unusual quasiparticles, those measurements were skin-deep, assessing particles only on the surface of the material, Hasan says. In the new study, Hasan and colleagues measured the relationship between the energy and momentum of the quasiparticles, showing that they were consistent with type-II Weyl fermions. Soon, scientists realized that their Lorentz-violating relatives, type-II Weyl fermions, might likewise pop up in solids. But the quasiparticle version of Weyl fermions burst onto the scene in 2015, when scientists first discovered them in a compound made of tantalum and arsenic ( SN: 8/22/15, p. No examples have been found in particle physics. Weyl fermions are a massless variety of fermion. Neutrinos, notoriously lightweight and elusive particles, could be either Majorana or Dirac fermions scientists aren’t yet sure which. Majorana fermions are their own antiparticles. Dirac fermions are the garden-variety type and include electrons and quarks. There are three different types of fermions: Dirac, Majorana and Weyl. For a real particle, violating Lorentz symmetry would be an unallowable faux pas, but for quasiparticles, the rules are looser, so type-II Weyl fermions can behave in a way a normal particle wouldn’t.įermions are a class of elementary particle that includes quarks, which make up protons and neutrons, and electrons. Lorentz symmetry is the foundation of Einstein’s special theory of relativity, which details the physics of observers zipping along near the speed of light.
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