Weyl semimetals host topological quasiparticles — Weyl fermions — at special points in momentum space where two electronic bands cross. Each crossing point carries a topological charge, and the total charge must be zero: Weyl points come in pairs of opposite charge, like magnetic monopoles and antimonopoles in momentum space. The minimum possible number of Weyl points is two — one pair. But time-reversal symmetry, present in all nonmagnetic materials, maps each Weyl point to another at the opposite momentum, doubling the count. The minimum in nonmagnetic systems has been four.
This is why all experimentally realized minimal Weyl semimetals have been magnetic. Breaking time-reversal symmetry with magnetism lifts the doubling constraint, allowing a single pair. The price is that magnetic order introduces its own complexity — domain walls, thermal fluctuations, hysteresis — that complicates the topological physics.
Zheng, Gao, Ge, Liu, and Lu (arXiv 2602.22622, February 2026) identify two chiral boron allotropes that achieve the minimum — a single pair of Weyl points — without magnetism.
The mechanism uses crystallographic symmetry to circumvent the time-reversal constraint. Both boron structures belong to chiral space groups with high-order rotation axes (C4 or C3). Time-reversal symmetry still exists and still maps Weyl points to partners at opposite momenta. But the rotation symmetry places two Weyl points of equal charge at the same high-symmetry point in the Brillouin zone — the center and the zone corner along the rotation axis. These double Weyl points carry topological charge two instead of one. A pair of charge-two points, one at the center and one at the corner, satisfies both the charge-neutrality requirement (total charge zero) and the time-reversal constraint (each point is its own time-reversal partner, located at a time-reversal-invariant momentum). Two points total. The minimum.
The charge-two character creates distinctive electronic structure: linear dispersion along the rotation axis but quadratic dispersion perpendicular to it. This anisotropy produces surface Fermi arcs — the hallmark of Weyl semimetals — that span the entire surface Brillouin zone, much longer than typical Fermi arcs and easier to detect experimentally with angle-resolved photoemission spectroscopy.
The materials are two stable allotropes of pure boron, composed of icosahedral or cage-like building blocks arranged in chiral structures. They are energetically competitive with known boron phases and predicted to be dynamically stable. The chirality is essential — it is the handedness of the crystal structure that enables the rotation symmetry responsible for the Weyl point doubling. An achiral version of the same lattice would not work.
The result is a minimal topological semimetal made of a single element, without magnetic order, at ambient conditions. The minimum was always allowed by topology. It took the right symmetry — chirality plus rotation — to realize it without the brute force of breaking time reversal.