  Altermagnets are an emerging third elementary class of magnets. Unlike
ferromagnets, their distinct crystal symmetries inhibit magnetization while,
unlike antiferromagnets, they promote strong spin polarization in the band
structure. The corresponding unconventional mechanism of timereversal symmetry
breaking without magnetization in the electronic spectra has been regarded as a
primary signature of altermagnetism, but has not been experimentally visualized
to date. We directly observe strong time-reversal symmetry breaking in the band
structure of altermagnetic RuO$_2$ by detecting magnetic circular dichroism in
angle-resolved photoemission spectra. Our experimental results, supported by ab
initio calculations, establish the microscopic electronic-structure basis for a
family of novel phenomena and functionalities in fields ranging from
topological matter to spintronics, that are based on the unconventional
time-reversal symmetry breaking in altermagnets.
