The intermediate valence of formally tetravalent compounds has been detected by $L_3$ x-ray-absorption near-edge structure (XANES) in ${\mathrm{CeO}}_2$ and in ${\mathrm{PrO}}_2$ but not in ${\mathrm{UO}}_2$, which have the same ${\mathrm{CaF}}_2$ structure and large f and ligand mixing. The intermediate valence has been found both in ${\mathrm{CeO}}_2$ and in Ce(${\mathrm{SO}}_4$ ${)}_2$⋅${4\mathrm{H}}_2$O, which have similar local structure but different crystal structure. We show that $L_3$ XANES final states are a direct probe of configuration interaction between 4$f^n$ and 4$f^{n+1\mathrm{}}$L configurations in the ground state and that the weight of the 4$f^{n+1\mathrm{}}$L in the ground state can be deduced. The many-body final states arise from the characteristic properties of these materials: (i) the presence of localized 4f level above the oxygen 2p band separated by a gap δɛ with relevant correlation energy $U_{\mathrm{ff}}$ ($U_{\mathrm{ff}}$≥δɛ) and (ii) mixing of 4f localized states with ligand valence orbitals such that the hybridization energy V is of the same order of magnitude as the energy separation ΔE between the many-body configuration 4$f^n$ and 4$f^{n+1\mathrm{}}$L (V≥ΔE). These insulating materials, which cannot be classified as standard mixed-valence systems, are called here interatomic intermediate-valence systems.

Polarized Cu $L_3$-edge x-ray absorption spectra of a single crystal of Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.8}$ have been measured to determine the symmetry of the orbital of the electronic holes induced by doping. The states induced by doping in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ ($0<\mathrm{}\delta <\sim 0.5$) have been found to be ligand holes ${\mathit{L}}^{*}$, giving the ${3d}^9{\mathit{L}}^{*}$ configuration, and their weight is correlated with $T_c$. The polarization dependence shows that the hole ${\mathit{L}}^{*}$, induced by doping, is found to be mainly in the Cu-O orbital oriented along the $\mathbf{c}$ axis involving the $\mathrm{O} {2p}_z$ orbital of the O(4) atom in the Ba plane and the $\mathrm{Cu} {3d}_{z^2}$ orbitals. This result indicates the important role of BaO layers and shows that theories of high-$T_c$ superconductivity have to take account of a second band with different symmetry, beyond the single band (the two-dimensional band derived from the $\mathrm{Cu} {3d}_{x^2-y^2}$ and O ${2p}_x$, ${2p}_y$ orbitals in the $\mathbf{a}-\mathbf{b}$ plane of the Cu${\mathrm{O}}_2$ layers) usually considered in models of the electronic structure of high-$T_c$ superconductors.

By making a comparison between the Mn K-edge absorption of (${\mathrm{MnO}}_4$ ${)}^{\mathrm{-}}$ and [Mn(${\mathrm{H}}_2$O${)}_6$ ${]}^{2+}$ complexes in aqueous solution we obtain an experimental determination of the energy extent of the type-II multiple-scattering (MS) regime that is substantially wider than expected. Theoretical calculations based on the MS formalism support this conclusion. We also recognize three energy regions in the absorption spectra of these complexes: a full MS region, where numerous or an infinite number of MS paths of high order contribute (depending on whether the MS series converges or not), an intermediate MS region, where only a few MS paths of low order are relevant, and a single-scattering region where the photoelectron is backscattered only once by the ligands [extended x-ray-absorption fine-structure (EXAFS) regime]. Theoretical considerations show that this must be a general situation in x-ray-absorption spectra and opens the way to a unified scheme for their interpretation. The energy extent of the three regions is obviously system dependent. We also show how to generalize to MS contributions the usual EXAFS analysis using curved-wave propagators and indicate how to extract geometrical information from the spectra of the two clusters investigated. In particular the method is used to derive the Mn—O—O—Mn path length in the (${\mathrm{MnO}}_4$ ${)}^{\mathrm{-}}$ complex.

Monitoring of vibrational eigenmodes of an elastic body excited by gravitational waves was one of the first concepts proposed for the detection of gravitational waves. At laboratory scale, these experiments became known as resonant bar detectors first developed by Joseph Weber in the 1960s. Due to the dimensions of these bars, the targeted signal frequencies were in the kHz range. Weber also pointed out that monitoring of vibrations of Earth or the Moon could reveal gravitational waves in the mHz band. His Lunar Surface Gravimeter experiment deployed on the Moon by the Apollo 17 crew had a technical failure, which greatly reduced the science scope of the experiment. In this article, we revisit the idea and propose a Lunar Gravitational-Wave Antenna (LGWA). We find that LGWA could become an important partner observatory for joint observations with the space-borne, laser-interferometric detector LISA and at the same time contribute an independent science case due to LGWA’s unique features. Technical challenges need to be overcome for the deployment of the experiment, and development of inertial vibration sensor technology lays out a future path for this exciting detector concept.

Electronic functionalities in materials from silicon to transition metal oxides are, to a large extent, controlled by defects and their relative arrangement. Outstanding examples are the oxides of copper, where defect order is correlated with their high superconducting transition temperatures. The oxygen defect order can be highly inhomogeneous, even in optimal superconducting samples, which raises the question of the nature of the sample regions where the order does not exist but which nonetheless form the “glue” binding the ordered regions together. Here we use scanning X-ray microdiffraction (with a beam 300 nm in diameter) to show that for La ₂ CuO _{4+ y} , the glue regions contain incommensurate modulated local lattice distortions, whose spatial extent is most pronounced for the best superconducting samples. For an underdoped single crystal with mobile oxygen interstitials in the spacer La ₂ O _{2+ y} layers intercalated between the CuO ₂ layers, the incommensurate modulated local lattice distortions form droplets anticorrelated with the ordered oxygen interstitials, and whose spatial extent is most pronounced for the best superconducting samples. In this simplest of high temperature superconductors, there are therefore not one, but two networks of ordered defects which can be tuned to achieve optimal superconductivity. For a given stoichiometry, the highest transition temperature is obtained when both the ordered oxygen and lattice defects form fractal patterns, as opposed to appearing in isolated spots. We speculate that the relationship between material complexity and superconducting transition temperature T _c is actually underpinned by a fundamental relation between T _c and the distribution of ordered defect networks supported by the materials.