Altermagnetism represents an emergent collinear magnetic phase with compensated order and an unconventional alternating even-parity wave spin order in the non-relativistic band structure. We investigate directly this unconventional band splitting near the Fermi energy through spin-integrated soft X-ray angular resolved photoemission spectroscopy. The experimentally obtained angle-dependent photoemission intensity, acquired from epitaxial thin films of the predicted altermagnet CrSb, demonstrates robust agreement with the corresponding band structure calculations. In particular, we observe the distinctive splitting of an electronic band on a low-symmetry path in the Brilliouin zone that connects two points featuring symmetry-induced degeneracy. The measured large magnitude of the spin splitting of approximately 0.6 eV and the position of the band just below the Fermi energy underscores the significance of altermagnets for spintronics based on robust broken time reversal symmetry responses arising from exchange energy scales, akin to ferromagnets, while remaining insensitive to external magnetic fields and possessing THz dynamics, akin to antiferromagnets.

Center for Quantum Science and Engineering George Mason University Fairfax VA 22030 USA

Centre for Quantum Spintronics Norwegian University of Science and Technology NTNU 7491 Trondheim Norway

Department of Physics and Astronomy George Mason University Fairfax VA 22030 USA

Department of Physics Texas A and M University College Station TX 77843 4242 USA

Ernst Ruska Centre for Microscopy and Spectroscopy with Electrons Forschungszentrum Jülich 52425 Jülich Germany

Inst of Physics Academy of Sciences of the Czech Republic Cukrovarnická 10 Praha 6 Czech Republic

Institut für Physik Johannes Gutenberg Universität Mainz 55099 Mainz Germany

Paul Scherrer Institut CH 5232 Villigen PSI Switzerland

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