  We performed angle-resolved photoemission spectroscopy (ARPES) on hexagonal
MnTe, a candidate for an altermagnet with a high critical temperature (TN=307
K). By utilizing photon-energy-tunable ARPES in combination with
first-principles calculations, we found that the band structure in the
antiferromagnetic phase exhibits a strongly anisotropic band-splitting
associated with the time-reversal-symmetry breaking, providing the first direct
experimental evidence for the altermagnetic band-splitting. The magnitude of
the splitting reaches 0.8 eV at non-high-symmetry momentum points, which is
much larger than the spin-orbit gap of ~0.3 eV along the GK high-symmetry cut.
The present result paves the pathway toward realizing exotic physical
properties associated with the altermagnetic spin-splitting.
