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 crystallize in the centrosymmetric $Pnma$ structure with G-type collinear magnetic order. Symmetry analysis within the magnetic space group $Pnm'a'$ (BNS No.~62.447) predicts a nonrelativistic spin splitting proportional to $k_yk_z$, with symmetry-enforced degeneracy on the $k_y$ and $k_z$ nodal planes, consistent with first-principles calculations. On the $k_x=0$ plane, the momentum-averaged spin splitting reaches 88.4, 52.3, and 70.6~meV at the Fermi level in TaCoB$_2$, ZrCoB$_2$, and HfCoB$_2$, respectively, with maxima exceeding 200~meV in all three compounds. Exchange analysis shows that the altermagnetic magnon splitting originates from symmetry-inequivalent sixth-neighbour inter-sublattice interactions. The chirality splitting reaches 1.51, 2.03, and 3.10 meV below 50 meV in TaCoB$_2$, ZrCoB$_2$, and HfCoB$_2$, respectively, making it accessible to inelastic neutron scattering. Monte Carlo simulations yield N'eel temperatures of $67\pm1.1$, $330\pm5.7$, and $307\pm5.0$~K, placing ZrCoB$_2$ and HfCoB$*2$ above room temperature. With spin-orbit coupling included, all three compounds host symmetry-protected Weyl points near the Fermi level, Fermi-arc surface states and sizable intrinsic anomalous Hall conductivities $σ*{zx}$ of $+342$, $-392$, and $-221$~S/cm at the Fermi level, reaching maximum magnitudes of $763$, $942$, and $924$~S/cm for TaCoB$_2$, ZrCoB$_2$, and HfCoB$_2$, respectively. $X$CoB$_2$ therefore provides a single compensated platform carrying both magnonic and electronic chirality, one in the spin waves and the other in the Berry curvature, without any stray field.