Observation of Altermagnetic Spin-Splitting in an Intercalated Transition Metal Dichalcogenide

Altermagnetism is a magnetic phase combining characteristics of both antiferromagnetism and ferromagnetic ordering. Despite growing theoretical interest in altermagnetic materials, reports of experimentally verified high-Néel temperature layered compounds are limited or remain to be firmly established. Here, we present an angle-resolved photoemission spectroscopy and density functional theory study of Co_1/4TaSe₂, a compound we identify as a layered altermagnetic material. Magnetic susceptibility measurements confirm type-A antiferromagnetic ordering with a Néel temperature of 178 K. Our spin-resolved and spin-integrated angle-resolved photoemission spectroscopy measurements reveal an electronic band structure in excellent agreement with density functional theory calculations, demonstrating clear signatures of altermagnetic spin splitting at the Fermi surface. Furthermore, temperature-dependent angle-resolved photoemission spectroscopy reveals a reconstructed valence band structure, with observable band shifts upon heating above the Néel temperature, consistent with the suppression of altermagnetic order. These findings establish Co_1/4TaSe₂ as a promising platform for exploring altermagnetic phenomena.