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

Abstract

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 $Pn{m}^{\prime }{a}^{\prime }$ (BNS No.~62.447) predicts a nonrelativistic spin splitting proportional to $k_y{k}_z$, 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 $67\pm 1.1$, $330\pm 5.7$, and $307\pm 5.0$~K, placing ZrCoB${}_2$ and HfCoB$\ast 2$ 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 $\sigma \ast zx$ 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.