Back in 1929, a mathematician named Hermann Weyl dreamed up a weird particle. It would have no mass but could still carry an electric charge. For decades, physicists wondered if such a thing could actually exist outside the equations. Then in 2015, researchers found them hiding inside chunky 3D crystals. The particles were real.
Now scientists have done something even stranger. They’ve squeezed these exotic particles into a sheet just one atom thick. The work appears in Nature Communications, and it marks the first time anyone has created a two-dimensional Weyl semimetal. The team grew a single layer of bismuth on top of a tin-sulfide crystal, creating a material where massless particles can zip around in a flat plane. This isn’t just miniaturization. It changes the rules entirely.
When Flat Means Different
The shift from 3D to 2D matters because electrons behave differently when you trap them in a pancake. The researchers made their material from bismuthene, which has a honeycomb pattern like graphene. But the bismuth atoms don’t sit perfectly flat. They buckle up and down like a crumpled egg carton. To coax out the Weyl fermions, the team used a technique called molecular beam epitaxy. They basically spray-painted atoms onto the tin-sulfide surface one layer at a time.
“The discovery of Weyl semimetals, which host spin-split massless quasiparticles in three-dimensional (3D) crystals, is particularly thrilling as it represents an experimental realization of Weyl fermions, a concept proposed long ago in the realm of particle physics.”
The tin-sulfide underneath does more than just hold things up. It messes with the symmetry of the bismuth lattice in useful ways. Combined with something called spin-orbit coupling, this creates special points where electrons lose their mass. At these Weyl nodes, the particles can move absurdly fast.
Strings at the Edge
Here’s where it gets visual. In 3D Weyl materials, the surface states form curved lines called Fermi arcs. Flatten everything into 2D and those arcs collapse into straight lines. The researchers call them Fermi strings, and they live right on the edge of the material. Using a scanning tunneling microscope, the team actually saw these edge channels. Current flows through them with almost no resistance.
This matters for spintronics. Regular electronics use an electron’s charge to move information around. That generates heat and slows things down. Spintronics uses the electron’s spin instead, which is way more efficient. The Fermi strings in this 2D material are protected by topology. Defects and dirt that would normally wreck the signal just bounce off.
“Our work opens the door for the experimental study of Weyl fermions in low-dimensional materials.”
The fact that these researchers pulled this off means Weyl physics can work in thin films. Since everything happens in a single atomic layer, you could stack these materials into devices. Imagine transistors that switch on and off in a fraction of the time it takes silicon, using a sliver of the power. That’s the promise here.
Of course, building actual gadgets from this stuff is still years away. But the 85-year wait for Weyl fermions is over. Now we’re just waiting to see what we can do with them.
Nature Communications: 10.1038/s41467-024-50329-6
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