Symmetry-driven spin splitting in altermagnets: an angle-resolved photoemission spectroscopy perspective.

UNLABELLED: Altermagnetism arises from composite real-space and spin-space symmetries, combining zero net magnetization with pronounced momentum-dependent spin splitting. This review highlights the pivotal role of angle-resolved photoemission spectroscopy (ARPES)—along with its spin-resolved (SARPES) and circular-dichroism (CD-ARPES) variants, in directly visualizing the nonrelativistic band splitting and spin textures of altermagnets. Within the spin-group framework, we distinguish ferromagnetic, antiferromagnetic, and altermagnetic orders and elucidate the symmetry origin of spin polarization. We then systematically review representative systems: the debated [Formula: see text]-wave prototype RuO, layered [Formula: see text]-wave altermagnets KVSeO and Rb VTeO, and a series of [Formula: see text]-wave compounds, including MnTe (domain-tunable) and CrSb (topological), together with the noncoplanar antiferromagnet MnTe and other emerging and prospective candidates and platforms. Overall, ARPES has become a key microscope for resolving symmetry-driven spin splitting. Future advances in micro/nano-beam and spectroscopies, combined with strain and domain engineering, heterostructure design, and the exploration of broader unconventional magnetic states, are expected to drive the joint evolution of altermagnetism and photoemission spectroscopy, paving the way for spintronic and correlated quantum research. GRAPHICAL ABSTRACT: [Image: see text]