Broken Kramers degeneracy in altermagnetic MnTe

Altermagnetism is a newly identified fundamental class of magnetism with vanishing net magnetization and time-reversal symmetry broken electronic structure. Since first proposed, altermagnetism has attracted broad interests for its analogy to unconventional superconductivity and potential applications in the emerging field of “antiferromagnetic spintronics”. Characterization of its unique electronic structure with momentum-dependent spin splitting would not only be a direct experimental verification of an altermagnetic phase, but also provide valuable insights into the origins and further utilizations of novel macroscopic phenomena. In this regard, intensive efforts have recently been put into spectroscopic studies of several prototypical altermagnets.

By combining high-quality film growth and in situ angle-resolved photoemission spectroscopy, we report the electronic structure of an altermagnetic candidate, α-MnTe. Temperature-dependent study reveals the gradual lifting of Kramers degeneracy accompanied by a magnetic phase transition at T_N ~ 267 K with a large band splitting of up to 370 meV which we attribute to the non-relativistic spin splitting of altermagnetism, providing direct spectroscopic evidence for altermagnetism in MnTe.