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Transfer of energy and linear momentum between lattice vibrations via anharmonic coupling is an important concept in solid-state physics. However, it remained difficult to directly observe how angular momentum is exchanged and conserved among lattice modes, even though these processes are thought to play an important role in achieving magnetization equilibrium and in spin relaxation effects like the Einstein-de Haas effect. Here we demonstrate and coherently control angular momentum transfer between two lattice modes using the inverse process of anharmonic decay. The observed rotational phonon-phonon Umklapp scattering enforces the conservation of quantized crystal angular momentum, as dictated by the discrete rotational symmetry of the crystal. We thereby experimentally confirm the fundamental analogy between linear and angular momentum conservation in solids. Moreover, we establish axial nonlinear phononics as a promising handle for the ultrafast control of material properties.
Density-functional theory (DFT) is a widely used method to compute properties of materials, which are often collected in databases and serve as valuable starting points for further studies. In this article, we present the Materials Cloud Three-Dimensional Structure Database (MC3D), an online database of computed three-dimensional (3D) inorganic crystal structures. Close to a million experimentally reported structures were imported from the COD, ICSD and MPDS databases; these were parsed and filtered to yield a collection of 72 589 unique and stoichiometric structures, of which 95% are, to date, classified as experimentally known. The geometries of structures with up to 64 atoms were then optimized using DFT with automated workflows and curated input protocols. The procedure was repeated for different functionals and computational protocols, generating three methodology-based MC3D subdatabases: PBE-v1, PBEsol-v1, and PBEsol-v2, with the latest containing 32 013 unique structures. All subdatabases of the MC3D are made available on the Materials Cloud portal, which provides a graphical interface to explore and download the data. The database includes the full provenance graph of all the calculations driven by the automated workflows, thus establishing full reproducibility of the results and more-than-FAIR procedures.
The unique structural properties of interfacial water are at the heart of many important processes in electrochemistry, climate science, and biophysics. At interfaces, water molecules exhibit preferential orientations and an altered intermolecular H-bond connectivity. Characterizing this layer-dependent anisotropic structure for such a thin molecular boundary, however, is a veritable challenge, with many important details remaining unknown. Here, we combine a novel depth-resolved second-order spectroscopy with molecular dynamics simulations to study the anisotropic structure at the air-water interface through the H─O─H bending vibration. We first uncover the elusive anisotropic interfacial response by removing the bulk-like (quadrupolar) term that is found to dominate the spectrum and has hampered previous experimental investigations of the interfacial structure. Thereafter, we reveal that the molecular structure at the interface shows a pronounced layering of alternating tilt-twist motifs. This highlights the often-disregarded anisotropy in the molecular twist angle and offers a revised picture of aqueous interfaces.
Capturing the structural changes that molecules undergo during chemical reactions in real space and time is a long-standing dream and an essential prerequisite for understanding and ultimately controlling femtochemistry. A key approach to tackle this challenging task is Coulomb explosion imaging, which has benefited decisively from recently emerging high-repetition-rate X-ray free-electron laser sources. With this technique, information on the molecular structure is inferred from the momentum distributions of the ions produced by the rapid Coulomb explosion of molecules. Retrieving molecular structures from these distributions poses a highly nonlinear inverse problem that remains unsolved for molecules consisting of more than a few atoms. Here, we address this challenge using a diffusion-based Transformer neural network. We show that the network reconstructs unknown molecular geometries from ion-momentum distributions with a mean absolute error below one Bohr radius, which is half the length of a typical chemical bond.
Cu/ZnO/Al2O3 catalysts are the industrial standard for methanol synthesis. Their high activity stems from the synergy between Cu and Zn, but their precise structure under CO2 hydrogenation conditions remains unknown. Here we show, using operando transmission electron microscopy, that the formation of ZnOx overlayers and CuZn surface alloys on Cu surfaces can be reversible and is mediated by the operating temperature and the chemical potential of the gas phase. Lower temperatures and more oxidative conditions lead to thicker ZnOx overlayers. At elevated temperatures, the overlayer coverage opens, exposing Cu nanoparticle surfaces to the feed and enabling CO2 activation. Furthermore, we show that CuZn alloys are transient species and are re-oxidized by H2O formed during the reaction. This implies that, in CO2 hydrogenation conditions, CuZn and Cu-ZnO surface states may coexist and continuously convert into one another as the local chemical potential oscillates throughout steady-state operation. Maintaining this fluctuation might be critical to the lifetime and performance of the catalyst.
