Robust spin splitting and fermiology in a layered altermagnet

Altermagnetism defies conventional classifications of collinear magnetic phases, combining compensated magnetization with momentum-dependent, nonrelativistic spin splitting. While altermagnetic band splitting has been established in three-dimensional compounds such as MnTe and CrSb, layered altermagnets remain far less explored spectroscopically. Here, combining termination-resolved micro-ARPES, fine photon-energy mapping, spin-ARPES, and density functional theory, we map the three-dimensional fermiology of the layered g-wave altermagnet CoNb4⁢Se8 , obtained by ordered Co intercalation of 2H-NbSe2 . We resolve the spin–split branches of the altermagnetic pairs directly in the raw photoemission spectra and, with spin-ARPES at these experimentally identified momenta, show the reversal of the spin polarization at opposite momenta and between two regions of the sample, consistent with opposite time-reversal domains, and its absence along the symmetry-protected nodal Γ –K direction. We further report a weak low-temperature anomaly near T0 ≈ 50–60 K, detected by magnetometry, temperature-dependent micro-ARPES, and ultrafast reflectivity, whose microscopic origin remains open. These results provide a momentum-resolved benchmark of the electronic structure of a layered altermagnet.