Unconventional crystal structure of the high-pressure superconductor La3_3Ni2_2O7_7

The discovery of high-temperature superconductivity in La$_3$Ni$_2$O$_7$ at pressures above 14 GPa has spurred extensive research efforts. Yet, fundamental aspects of the superconducting phase, including the possibility of a filamentary character, are currently subjects of controversial debates. Conversely, a crystal structure with NiO$_6$ octahedral bilayers stacked along the $c$-axis direction was consistently posited in initial studies on La$_3$Ni$_2$O$_7$. Here we reassess this structure in optical floating zone-grown La$_3$Ni$_2$O$_7$ single crystals that show signs of filamentary superconductivity. Employing scanning transmission electron microscopy and single-crystal x-ray diffraction under high pressures, we observe multiple crystallographic phases in these crystals, with the majority phase exhibiting alternating monolayers and trilayers of NiO$_6$ octahedra, signifying a profound deviation from the previously suggested bilayer structure. Using density functional theory, we disentangle the individual contributions of the monolayer and trilayer structural units to the electronic band structure of La$_3$Ni$_2$O$_7$, providing a firm basis for advanced theoretical modeling and future evaluations of the potential of the monolayer-trilayer structure for hosting superconductivity

The DLR Transport and the Environment Project - Building competency for a sustainable mobility future

This article describes the thematic and organizational approach of the DLR Transport Program’s research project “Transport and the Environment” (Verkehrsentwicklung und Umwelt, VEU). It illuminates the research approach, which employs scenario techniques to achieve a common understanding and a framework for the project. Establishing a platform for scientific exchange, VEU clearly facilitates the interdisciplinary integration of research on mobility within the DLR and its partner institutes. A set of transport scenarios for Germany and Europe (time horizon 2040), which are presented in greater detail in this article, is used in this respect and aims to provide answers for pressing societal questions

Spin-orbit excitons in a correlated metal: Raman scattering study of Sr2RhO4

Using Raman spectroscopy to study the correlated 4$d$-electron metal Sr$_2$RhO$_4$, we observe pronounced excitations at 220 meV and 240 meV with $A_\mathrm{1g}$ and $B_\mathrm{1g}$ symmetries, respectively. We identify them as transitions between the spin-orbit multiplets of the Rh ions, in close analogy to the spin-orbit excitons in the Mott insulators Sr$_2$IrO$_4$ and $\alpha$-RuCl$_3$. This observation provides direct evidence for the unquenched spin-orbit coupling in Sr$_2$RhO$_4$. A quantitative analysis of the data reveals that the tetragonal crystal field $\Delta$ in Sr$_2$RhO$_4$ has a sign opposite to that in insulating Sr$_2$IrO$_4$, which enhances the planar $xy$ orbital character of the effective $J=1/2$ wave function. This supports a metallic ground state, and suggests that $c$-axis compression of Sr$_2$RhO$_4$ may transform it into a quasi-two-dimensional antiferromagnetic insulator.Comment: 6 (main text) + 7 (supplementary) pages, published versio

The DLR VEU-Project Transport and the Environment - building competency for a sustainable mobility future

The poster describes the thematic and organizational approach of the DLR Transport Program’s research project “Transport and the Environment” (Verkehrsentwicklung und Umwelt, VEU). VEU clearly facilitates the interdisciplinary integration of research on mobility within the DLR and its partner institutes. A set of transport scenarios for Germany and Europe (time horizon 2040) is used in this respect and aims to provide answers for pressing societal questions

Coherent propagation of spin-orbit excitons in a correlated metal

Abstract Collective excitations such as plasmons and paramagnons are fingerprints of atomic-scale Coulomb and exchange interactions between conduction electrons in metals. The strength and range of these interactions, which are encoded in the excitations’ dispersion relations, are of primary interest in research on the origin of collective instabilities such as superconductivity and magnetism in quantum materials. Here we report resonant inelastic x-ray scattering experiments on the correlated 4d-electron metal Sr2RhO4, which reveal a spin-orbit entangled collective excitation. The dispersion relation of this mode is opposite to those of antiferromagnetic insulators such as Sr2IrO4, where the spin-orbit excitons are dressed by magnons. The presence of propagating spin-orbit excitons implies that the spin-orbit coupling in Sr2RhO4 is unquenched, and that collective instabilities in 4d-electron metals and superconductors must be described in terms of spin-orbit entangled electronic states