ARPES Studies of Spin-Splitting Antiferromagnets

Altermagnetism has recently emerged as a third fundamental branch of magnetism, distinct from conventional ferromagnetism and antiferromagnetism (AFM). Characterized by zero net magnetization yet exhibiting time-reversal symmetry breaking and non-relativistic spin splitting in momentum space, altermagnets offer a unique platform for next-generation spintronics. This review highlights the pivotal role of angle-resolved photoemission spectroscopy (ARPES) in experimentally validating this phase by directly visualizing its hallmark spin-split electronic bands. We critically examine recent ARPES investigations across a spectrum of representative candidate materials, ranging from the debated d-wave prototype RuO₂, where spin splitting remains elusive, to the robust observation of crystal-symmetry-paired spin-valley locking in layered systems such as KV₂Se₂O and Rb_1-δV₂Te₂O. Furthermore, we discuss the direct detection of giant spin splitting in g-wave compounds like MnTe and high-temperature CrSb, alongside the interplay of altermagnetism with correlated orders in CoNb₄Se₈. On the other hand, we also discuss the observation of AFM-induced spin splitting in MnTe₂, which is not an altermagnet but belongs to an extended category of spin-split antiferromagnet. Finally, we provide an outlook on emerging opportunities, including the use of nano-ARPES for domain imaging, the search for new candidate materials, and the potential for realizing topological superconductivity in altermagnetic heterostructures.