Single Pair of Charge-two Weyl Fermions in Chiral Boron Allotropes

Abstract

The realization of a minimal Weyl semimetal (WSM) hosting a single pair of Weyl points (WPs) has thus far been restricted to magnetic systems, since time-reversal symmetry generally enforces a minimum of four WPs in nonmagnetic materials. Here, combining first-principles calculations with symmetry analysis, we identify two stable boron allotropes, chiral HDSBC-B$_{20}$ and cage-like CR-B$_{12}$, as the first nonmagnetic electronic materials realizing a single pair of WPs in the spinless regime. We show that the interplay between time-reversal symmetry and crystallographic rotation symmetry ($C_4$ or $C_3$) stabilizes exactly one pair of $C=2$ WPs pinned at time-reversal-invariant momenta, thereby circumventing the conventional node-quartet constraint. These double-WPs exhibit linear dispersion along the rotation axis and quadratic dispersion in the perpendicular plane. In HDSBC-B$_{20}$, the sign of the topological charge is directly correlated with structural chirality. Both materials host exceptionally long double Fermi arcs spanning the surface Brillouin zone, providing experimentally accessible signatures. Our findings establish nonmagnetic material platforms for minimal double-Weyl fermions and broaden the landscape of unconventional WSMs.