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 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 interesting phenomena and functionalities in fields ranging from topological matter to spintronics, which are based on the unconventional time-reversal symmetry breaking in altermagnets.

Femtosecond time-resolved core-level photoemission spectroscopy with a free-electron laser is used to measure the atomic-site specific charge-order dynamics of the charge-density~wave in the Mott insulator 1T-TaS2. After strong photoexcitation, a prompt loss of charge order and subsequent fast equilibration dynamics of the electron-lattice system are observed. On the time scale of electron-phonon thermalization, about 1~ps, the system is driven across a phase transition from a long-range charge ordered state to a quasiequilibrium state with domainlike short-range charge and lattice order. The experiment opens the way to study the nonequilibrium dynamics of condensed matter systems with full elemental, chemical, and atomic-site selectivity.

Angle-resolved photoemission spectroscopy at a temperature of 1 K is used to determine the wave-vector dependence of the spectral gap and band renormalization due to electron-phonon coupling in the layered charge-density-wave superconductor $2H$-NbSe${}_2$. The measured gap size and coupling parameter are Fermi-surface-sheet dependent and anisotropic. The largest energy gap, highest coupling strength, and strongest variation in both quantities are found on the double-walled Nb $4d$-derived Fermi-surface sheet that is centered on the corners of the hexagonal Brillouin zone. On this sheet, the spectral gap has two distinct anticorrelated components associated with superconductivity and the charge-density wave. The results establish $2H$-NbSe${}_2$ as a moderately correlated intermediate-coupling anisotropic multigap superconductor.

Empirical tight-binding simulations of the band structure of $1T\text{−}{\mathrm{TaS}}_2$ reveal that the combination of spin-orbit coupling and the low-temperature $\sqrt{13}\times \sqrt{13}$ reconstruction generates a distinct and very narrow band at the Fermi level. Spin-orbit interaction is therefore important in any understanding of the correlation effects in this material. The states within the split-off band are primarily of $d_{x^2-y^2}$, $d_{xy}$ character and reside preferentially on the central atom of the “Star-of-David” cluster.

Photo-induced switching between collective quantum states of matter is a fascinating rising field with exciting opportunities for novel technologies. Presently, very intensively studied examples in this regard are nanometer-thick single crystals of the layered material 1T-TaS₂, where picosecond laser pulses can trigger a fully reversible insulator-to-metal transition (IMT). This IMT is believed to be connected to the switching between metastable collective quantum states, but the microscopic nature of this so-called hidden quantum state remained largely elusive up to now. Here, we characterize the hidden quantum state of 1T-TaS₂ by means of state-of-the-art x-ray diffraction and show that the laser-driven IMT involves a marked rearrangement of the charge and orbital order in the direction perpendicular to the TaS₂-layers. More specifically, we identify the collapse of interlayer molecular orbital dimers as a key mechanism for this non-thermal collective transition between two truly long-range ordered electronic crystals.