Magnetic circular dichroism in two-photon photoemission

Observation of time-reversal symmetry breaking in the band structure of altermagnetic RuO2

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.

Direct observation of antiferromagnetic parity violation in the electronic structure of Mn2Au

Using momentum microscopy with sub-µm spatial resolution, allowing momentum resolved photoemission on individual antiferromagnetic domains, we observe an asymmetry in the electronic band structure,E(k)≠E(-k), in Mn₂Au. This broken band structure parity originates from the combined time and parity symmetry,PT, of the antiferromagnetic order of the Mn moments, in connection with spin-orbit coupling. The spin-orbit interaction couples the broken parity to the Néel order parameter direction. We demonstrate a novel tool to image the Néel vector direction,N, by combining spatially resolved momentum microscopy withab-initiocalculations that correlate the broken parity with the vectorN.

Magnetic dichroism from optically excited quantum well states

We demonstrate magnetic dichroism from optically excited states in two-photon photoemission. Using ultrathin cobalt films grown on Cu(001), we observe unoccupied quantum well states which give rise to a sizable intensity change in photoemission under magnetization reversal. The simultaneous comparison of both circular and linear magnetic dichroism in the same system permits us to check fundamental symmetry requirements and allows us to explicitly elucidate the common origin of both effects. Based on our observations we argue that the observed effect is related to spin-orbit coupling in the intermediate quantum well states.