Kurt Kummer

Altermagnetism has been recognized as a magnetic phase that bridges the properties of ferro- and antiferromagnetism. The momentum-dependent spin splitting observed in these materials reflects their distinctive symmetry characteristics, which also establish the conditions for chiral magnons to emerge. Here we provide direct experimental evidence for a chiral magnon in the altermagnetic candidate manganese telluride (MnTe) revealed by circular-dichroism resonant inelastic x-ray scattering. This mode, which we term chiral altermagnon, exhibits a distinct momentum dependence of its spin polarization consistent with the altermagnetic g -wave symmetry of MnTe. Our polarization-resolved data corroborate the existence of a discrete class of magnetic excitations, demonstrating how altermagnetic order shapes spin dynamics and paving the way for advances in spintronic and quantum technologies.

Lattice vibrations carrying angular momentum, known as chiral phonons, offer a route to couple lattice, electronic and magnetic degrees of freedom, but their deterministic control has remained elusive. Here we demonstrate reversible electric-field switching of phonon angular momentum in the technologically relevant ferroelectric BaTiO ₃ . Using circularly dichroic resonant inelastic X-ray scattering, we directly resolve phonon chirality via angular momentum transfer between circularly polarized X-rays and the lattice. We observe a momentum-dependent dichroic response that switches with ferroelectric polarization, establishing a robust and non-volatile gyroelectric effect. The measured dichroism is in quantitative agreement with first-principles calculations. These results establish phonon angular momentum as an electrically controllable degree of freedom and provide a pathway towards phonon-based information and energy technologies.

Chromium thiophosphate ( CrPS 4 ) is a long-known material: A layered semiconducting antiferromagnet. Its recently discovered gate-tunable metamagnetic phase transitions, the remarkable positive and oscillating magnetoresistance as a tunnel barrier, and its Fano-resonance luminescence, elusive among the multitude of Cr 3 + compounds, call for revisiting the understanding of its electronic structure, especially regarding how it relates to magnetic order. Here, we employ X-ray magnetic circular dichroism, implemented in both absorption and resonant inelastic X-ray spectroscopies, together with quantum many-body calculations, to unveil the role of metal-ligand covalency in mediating the metamagnetic transitions in CrPS 4 , using crystal-field and charge-transfer excitations as fingerprints of the evolving magnetic order. We reveal the role of extended superexchange paths involving P and S atoms, coupling interactions between the Cr spins across the different magnetic phases—antiferromagnetic, canted, and ferromagnetic. Our results elucidate the electronic states involved in these phases and provide prescriptions for engineering the metamagnetic phase diagram of CrPS 4 .

Lattice vibrations carrying angular momentum, known as chiral phonons, offer a route to couple lattice, electronic, and magnetic degrees of freedom, but their deterministic control has remained elusive. Here we demonstrate reversible electric-field switching of phonon angular momentum in the technologically-relevant ferroelectric BaTiO3. Using circularly dichroic resonant inelastic X-ray scattering, we directly resolve phonon chirality via angular momentum transfer between circularly polarised X-rays and the lattice. We observe a momentum dependent dichroic response that switches with ferroelectric polarisation, establishing a robust and non-volatile gyroelectric effect. The measured dichroism is in quantitative agreement with first-principles calculations. These results establish phonon angular momentum as an electrically controllable degree of freedom and provide a pathway toward phonon-based information and energy technologies.

We use resonant x-ray scattering at the nickel L 2 , 3 edges to investigate the interplay between orbital degrees of freedom and charge density waves (CDWs) in the superconductor BaNi 2 ( As 1 − x P x ) 2 . Both the incommensurate and commensurate CDWs in this system exhibit strong resonant enhancement with distinct energy and polarization dependencies, indicative of orbital ordering. Azimuthal-angle-dependent measurements reveal a lowering of the local Ni site symmetry, consistent with monoclinic or lower point group symmetry. The scattering signatures of both CDWs are dominated by contributions from Ni d x z , y z orbitals, with similar orbital character despite their distinct wave vectors. These findings point to a shared orbital-driven formation mechanism and provide new insight into the symmetry breaking and orbital and nematic fluctuations in the high-temperature regime of the superconductor BaNi 2 ( As 1 − x P x ) 2 .

We employ several X-ray based techniques, including X-ray diffraction, X-ray absorption spectroscopy and resonant inelastic X-ray scattering, to disentangle the contributions of individual chemical species to the structural, electronic and magnetic properties of high-entropy oxides. In the benchmark compound Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O and related systems, we unambiguously resolve a sizable Jahn–Teller distortion at the Cu sites, more pronounced in the absence of Ni²⁺ and Mg²⁺, suggesting that these ions promote positional order, whereas Cu²⁺ ions act to destabilize it. Moreover, we detect magnetic excitations and estimate the strength of the interactions between pairs of different magnetic elements. Our results provide valuable insights into the role of various chemical species in shaping the physical properties of high-entropy oxides.

At the heart of Li-rich layered materials, Li-ion cathodes which exhibit the highest energy density, is the notoriously unstable anionic redox in general, and in the first charge in particular, manifesting itself in a characteristic long high-voltage plateau. This instability leads to a huge irreversibility, lowers overall capacity, and degrades long cycling. The literature abounds 25 in studies which characterize this phenomenon, but there is little or no reported effort which touches on eliminating this plateau altogether. Using a model material, we demonstrate how to remove the plateau and make anionic redox stable. Leveraging advanced X-ray techniques, we show that the oxygen redox in this material follows a new oxygen redox reaction path and that an expanded lattice is the required condition to enable this path.

Understanding high-temperature superconductivity in cuprates requires knowledge of the metallic phase it evolves from, particularly the pseudogap profoundly affecting the electronic properties at low carrier densities. A key question is the influence of chemical disorder, which is ubiquitous but exceedingly difficult to model. Using resonant X-ray scattering, we identified two-dimensional charge order in stoichiometric YBa₂Cu₄O₈ (T _c = 80 K), which is nearly free of chemical disorder. The charge order amplitude shows a concave temperature dependence and vanishes sharply at T* = 200 K, the onset of a prominent pseudogap previously determined by spectroscopy, suggesting a causal link between these phenomena. The gradual onset of charge order in other cuprates is thus likely attributable to an inhomogeneous distribution of charge ordering temperatures due to disorder induced by chemical substitution. The relationship between the pseudogap and the disorder-induced gradual freeze-out of charge carriers remains a central issue in research on high-T _c superconductors.