Observation of time-reversal symmetry breaking in the band structure of altermagnetic RuO$_2$

Understand

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.