This Week's Other Big Particle Discovery: The Weyl Point

Imagine 3D graphene and manufacturing massless electrons.

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Jul 18 2015, 12:00pm

Image: Ling Lu and Qinghui Yan

Physicists were all kinds of excited this week as scientists at CERN announced the first observation of the elusive, demonic pentaquark particle, a long-predicted combination of four quarks and one antiquark. The unveiling wasn't alone, however. On Thursday, two teams of researchers, one at MIT and the other at Princeton, published independent papers in Science describing the first direct observations (again, independent) of another long-predicted particle: the Weyl point.

What is a Weyl point? It's a bit trickier to imagine than a pentaquark (which probably isn't the greatest comparison in the first place). It's a bit like an electron, but it has no mass. When they first showed up as a solution to the Dirac equation—which links the bizarro world of quantum physics to the more conventional "macro" one of space and time—physicists thought they might be even better building blocks of subatomic particles than electrons for the simple reason that they're more basic or primitive. Because a Weyl point is massless and because it has both a right-handed and left-handed spin, it's also very mobile.

An upshot of this is that a Weyl point trapped in a crystal is able to behave as a magnetic monopole, a theoretical construct that can be imagined as half of a magnet where instead of the new half-magnet having its own north and south polarizations, it has only a north or south polarization. Which is pretty weird. The resulting behavior is as if the points were made up of a mixture of the two components.

The crystal-trapped Weyl fermion (to be more specific), is thus able to hang out with monopoles of the same charge side by side. This is another way of looking at the Weyl particle's enhanced mobility.

Image: Su-Yang Xu and M. Zahid Hasan, Princeton Department of Physics

The resulting crystal (called a gyroid, photographed above) behaves a lot like graphene only in three dimensions instead of two and it comes with a whole laundry list of neat and highly desireable properties. Like graphene, it offers the possibility of near-perfect electrical conductivity. The Princeton researchers imagine Weyl particles as potential alternatives to electrons in future blanktronics applications, while the MIT group imagines endlessly scale-able lasers.

"The physics of the Weyl fermion are so strange, there could be many things that arise from this particle that we're just not capable of imagining now," offered M. Zahid Hasan, a professor of physics who led the Princeton research team. Hasan imagines using Weyl particles to create new massless forms of electrons that are perfectly efficient, generating no heat, thanks to their capability for tunneling through obstructions (other particles).

In other words, there's no backscattering. "It's like they have their own GPS and steer themselves without scattering," Hasan said. "They will move and move only in one direction since they are either right-handed or left-handed and never come to an end because they just tunnel through. These are very fast electrons that behave like unidirectional light beams and can be used for new types of quantum computing."