Exchange-Driven Chiral Magnons and Weyl States in Room-Temperature Metallic XCoB$_2$ (X= Ta, Zr and Hf) Altermagnets

Metallic altermagnets remain rare, particularly in low-symmetry three-dimensional crystals where spin-split electronic bands and chiral magnon excitations can coexist. We report a family of metallic $d_{yz}$-wave altermagnets in the orthorhombic ternary borides XCoB$_2$ (X = Ta, Zr, Hf), which cryst