Altermagnets are an emerging elementary class of collinear 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 time-reversal 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 RuO2 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 interesting phenomena and functionalities in fields ranging from topological matter to spintronics, which are based on the unconventional time-reversal symmetry breaking in altermagnets.

Deutsches Elektronen Synchrotron DESY 22607 Hamburg Germany

Institut für Experimentalphysik Universität Hamburg 22761 Hamburg Germany

Institut für Experimentelle und Angewandte Physik Christian Albrechts Universität zu Kiel 24098 Kiel Germany

Institut für Physik Johannes Gutenberg Universität Mainz Staudingerweg 7 D 55128 Mainz Germany

Institute of Physics Academy of Sciences of the Czech Republic Cukrovarnick'a 10 Praha 6 Czech Republic

Linac Coherent Light Source SLAC National Accelerator Laboratory Menlo Park CA 94025 USA

Ruprecht Haensel Laboratory Deutsches Elektronen Synchrotron DESY 22607 Hamburg Germany

School of Physics and Astronomy University of Nottingham NG7 2RD Nottingham UK

Sumy State University Rymski Korsakov 2 40007 Sumy Ukraine

Universität Kaiserslautern Department of Physics 67663 Kaiserslautern Germany

University of West Bohemia New Technologies Research Center Plzen 30100 Czech Republic

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