2020, Kumar, Nitesh, Yao, Mengyu, Nayak, Jayita, Vergniory, Maia G., Bannies, Jörn, Wang, Zhijun, Schröter, Niels B.M., Strocov, Vladimir N., Müchler, Lukas, Shi, Wujun, Rienks, Emile D.L., Mañes, J.L., Shekhar, Chandra, Parkin, Stuart S.P., Fink, Jörg, Fecher, Gerhard H., Sun, Yan, Bernevig, B. Andrei, Felser, Claudia

Multifold degenerate points in the electronic structure of metals lead to exotic behaviors. These range from twofold and fourfold degenerate Weyl and Dirac points, respectively, to sixfold and eightfold degenerate points that are predicted to give rise, under modest magnetic fields or strain, to topological semimetallic behaviors. The present study shows that the nonsymmorphic compound PdSb2 hosts six-component fermions or sextuplets. Using angle-resolved photoemission spectroscopy, crossing points formed by three twofold degenerate parabolic bands are directly observed at the corner of the Brillouin zone. The group theory analysis proves that under weak spin–orbit interaction, a band inversion occurs. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2016, Johnston, Steve, Monney, Claude, Bisogni, Valentina, Zhou, Ke-Jin, Kraus, Roberto, Behr, Günter, Strocov, Vladimir N., Málek, Jiři, Drechsler, Stefan-Ludwig, Geck, Jochen, Schmitt, Thorsten, van den Brink, Jeroen

Strongly correlated insulators are broadly divided into two classes: Mott–Hubbard insulators, where the insulating gap is driven by the Coulomb repulsion U on the transition-metal cation, and charge-transfer insulators, where the gap is driven by the charge-transfer energy Δ between the cation and the ligand anions. The relative magnitudes of U and Δ determine which class a material belongs to, and subsequently the nature of its low-energy excitations. These energy scales are typically understood through the local chemistry of the active ions. Here we show that the situation is more complex in the low-dimensional charge-transfer insulator Li2CuO2, where Δ has a large non-electronic component. Combining resonant inelastic X-ray scattering with detailed modelling, we determine how the elementary lattice, charge, spin and orbital excitations are entangled in this material. This results in a large lattice-driven renormalization of Δ, which significantly reshapes the fundamental electronic properties of Li2CuO2.

2022, Zhang, Wenliang, Agrapidis, Cliò Efthimia, Tseng, Yi, Asmara, Teguh Citra, Paris, Eugenio, Strocov, Vladimir N., Giannini, Enrico, Nishimoto, Satoshi, Wohlfeld, Krzysztof, Schmitt, Thorsten

The nature of the spin excitations in superconducting cuprates is a key question toward a unified understanding of the cuprate physics from long-range antiferromagnetism to superconductivity. The intense spin excitations up to the over-doped regime revealed by resonant inelastic X-ray scattering bring new insights as well as questions like how to understand their persistence or their relation to the collective excitations in ordered magnets (magnons). Here, we study the evolution of the spin excitations upon hole-doping the superconducting cuprate Bi2Sr2CaCu2O8+δ by disentangling the spin from the charge excitations in the experimental cross section. We compare our experimental results against density matrix renormalization group calculations for a t-J-like model on a square lattice. Our results unambiguously confirm the persistence of the spin excitations, which are closely connected to the persistence of short-range magnetic correlations up to high doping. This suggests that the spin excitations in hole-doped cuprates are related to magnons—albeit short-ranged.