Weyl semimetal
/ VILE SEM-ee-MET-ul /
Imagine two tiny whirlpools in the ocean, one spinning clockwise and the other counter-clockwise. They always come in pairs, and each has a fixed handedness that you can't change without merging it back with its partner. In a Weyl semimetal, the electrons behave like these paired, opposite-handed whirlpools — except the whirling is in the abstract space of their quantum states, not in water.
A Weyl semimetal is a material whose energy bands touch at isolated points called Weyl points, near which electrons behave like massless particles, much as in a Dirac semimetal. The crucial twist is that Weyl points always come in pairs of opposite 'chirality,' or handedness — one acts as a tiny source and the other as a tiny sink for a kind of abstract quantum field, and each is a robust knot that can't be undone on its own. Their handedness gives the electrons a definite left- or right-spinning character tied to their motion.
This matters because the paired Weyl points produce striking, measurable signatures — strange surface arcs of electrons that connect the projections of the two points, and unusual currents that appear when electric and magnetic fields are aligned. These features are robust because a single Weyl point cannot simply disappear; it must find its opposite partner and annihilate. A common confusion is to lump Weyl and Dirac semimetals together: a Dirac point needs a protecting symmetry, whereas a Weyl point is sturdier and survives even when that symmetry is broken.
Tantalum arsenide was the first confirmed Weyl semimetal. Experiments using ARPES mapped its surface and found the predicted 'Fermi arcs' — open curves of electron states that begin at one Weyl point's projection and end at its partner's, a signature impossible in ordinary metals.
Fermi arcs are open-ended curves — a surface fingerprint unique to Weyl semimetals.
The chirality of Weyl points is itself a topological charge: each point carries a Chern number of plus or minus one, and the rule that they sum to zero over the whole crystal is why they must come in opposite-handed pairs.