Researchers think they’ve stumbled upon a new kind of particle. Known as Type-II Weyl fermions, the material they are thought to hide in responds dynamically to nearby magnetic fields, causing it to switch between an electrical conductor and an insulator depending on which direction the induced current is traveling in. This adaptability suggests that the particles could be used to create highly efficient transistor components in electronics worldwide. Their predictions are described in the journal Nature.
Fermions are a type of particle that can be either elementary, as in not composed of any other particles (such as the electron, neutrino or quark), or composite, meaning those composed of two or more smaller elementary particles, like the proton. They are similar to bosons, which are the force-carrying particles, but they have a different “spin,” or value of angular momentum.
The Weyl fermion was first hypothesized by German mathematician and physicist Hermann Weyl in 1929. They are massless particles, initially wrongly identified as neutrinos until it was discovered that neutrinos actually do have an incredibly small, but significant, mass. Bizarrely, Weyl fermions were thought to behave as both matter and antimatter – physical opposites in almost every manner but for their identical mass – only when contained within a crystal.
A pair of studies published earlier this year in the journal Science revealed for the first time direct evidence of Weyl fermions, describing them as quasiparticles: a recorded disturbance in the properties of a medium. These “disturbances” are actually stable despite their name; they don’t change direction or lose their charge or momentum until they encounter another Weyl fermion – the only other thing they interact with.
This new study details the mathematical evidence for a second type of Weyl fermion in a material called tungsten ditelluride, a “semimetal.” Compared to the first type, the type-II Weyl fermion makes this semimetal an excellent conductor and a phenomenal insulator of electricity depending on which direction the current is flowing. Unlike the type-I, this variant can reside in a wide range of thermodynamic states at zero-point energy, the lowest possible energy that any physical system or particle can have. This allows to it to imbue its host material with superconductivity and magnetic properties, among others.
These new quasiparticles are comparable to electrons. Electrons are elementary particles within our universe; inside special crystals, these type-II Weyl fermions are considered elementary particles of the solid, important for their composition but generally non-interacting. The authors of this study consider their host semimetal crystals to represent a microcosm of a universe, full of particles than cannot exist outside of them, including these Weyl fermions. These crystals can be grown at relatively little expense in a laboratory, allowing researchers to study the behavior of these incredibly elusive, exotic particles.
Although not directly “observed,” the team’s mathematical prediction paves the way for the experimental detection of the type-II Weyl fermions, much in the same manner that the type-I fermions were detected in several recent studies.