Symmetry-Enforced Single-Pair Charge-Two Triple-Point Semimetals

The Nielsen-Ninomiya theorem mandates that the net chiral charge of topological nodes in a crystal Brillouin zone must vanish, requiring them to appear in pairs. While realizing the absolute minimum of a single pair of topological nodes has been achieved for Weyl semimetals, hosting the minimal configuration of chiral charge-two (|C| = 2) triple degenerate points (C-2 TPs) in a realistic electronic material remains an open challenge. By combining little-group constraints with both possible two-node orbit structures across all 1651 magnetic space groups (MSGs), we identify 21 MSGs without spin-orbit coupling (SOC) and 3 MSGs with SOC that are compatible with an isolated single pair of C-2 TPs. Guided by these symmetry criteria, we predict that the cubic boron allotrope P23-B₃₆ acts as a premier electronic platform for this exotic phase. Owing to the negligible SOC of light boron atoms, the system behaves as a spinless semimetal where the three crossing bands form exactly two symmetry-enforced C-2 TPs at the high-symmetry points Γ and R with chiralities C = +2 and C = -2, respectively. First-principles calculations reveal maximally extended double Fermi arcs on both the (001) and (110) surfaces, providing a distinct spectroscopic fingerprint for experimental detection. Our work establishes a rigorous symmetry-guided design framework for minimal multifold semimetals and identifies an elemental platform for exploring their intrinsic quantum transport phenomena.