An analysis of the coherence properties of the fourth-generation high-energy storage rings with emittance values of 10 pm rad is performed. It is presently expected that a storage ring with these low emittance values will reach diffraction limit at hard X-rays. Simulations of coherence properties were performed with the XRT software and an analytical approach for different photon energies from 500 eV to 50 keV. It was demonstrated that a minimum photon emittance (diffraction limit) reached at such storage rings is λ/2π. Using mode decomposition it is shown that, for the parameters of the storage ring considered in this work, the diffraction limit will be reached for soft X-ray energies of 500 eV. About ten modes will contribute to the radiation field at 12 keV photon energy and even more modes give a contribution at higher photon energies. Energy spread effects of the electron beam in a low-emittance storage ring were analysed in detail. Simulations were performed at different relative energy spread values from zero to 2 × 10⁻³. A decrease of the degree of coherence with an increase of the relative energy spread value was observed. This analysis shows that, to reach the diffraction limit for high photon energies, electron beam emittance should go down to 1 pm rad and below.

The discovery of superconductivity above 250 K at high pressure in LaH₁₀ and the prediction of overcoming the room temperature threshold for superconductivity in YH₁₀ urge for a better understanding of hydrogen interaction mechanisms with the heavy atom sublattice in metal hydrides under high pressure at the atomic scale. Here we use locally sensitive X-ray absorption fine structure spectroscopy (XAFS) to get insight into the nature of phase transitions and the rearrangements of local electronic and crystal structure in archetypal metal hydride YH₃ under pressure up to 180 GPa. The combination of the experimental methods allowed us to implement a multiscale length study of YH₃: XAFS (short-range), Raman scattering (medium-range) and XRD (long-range). XANES data evidence a strong effect of hydrogen on the density of 4d yttrium states that increases with pressure and EXAFS data evidence a strong anharmonicity, manifested as yttrium atom vibrations in a double-well potential.

The parent compound ${\mathrm{BaBiO}}_3$ of bismuthate high-temperature superconductors (HTSCs) $\mathrm{BaBi}\left(\mathrm{Pb}\right){\mathrm{O}}_3$ and $\mathrm{Ba}\left(\mathrm{K}\right){\mathrm{BiO}}_3$ with perovskitelike structure exhibits unusual electronic and structural properties, which can be satisfactorily explained if we assume that all charge carriers are in the paired state. However, the prior experiments and the first-principle calculations only indirectly indicate the existence of paired charge carriers in ${\mathrm{BaBiO}}_3$. In this work, we report the direct evidence of initially paired electrons and holes in the upper antibonding Bi $6s-\mathrm{O}2p_{{\sigma }^{*}}$ orbital of the neighboring octahedral complexes in the ground state of ${\mathrm{BaBiO}}_3$ using the time-resolved x-ray absorption spectroscopy (XAS) to monitor the electron dynamics after the femtosecond resonant 633 nm laser excitation. We observe strong changes in the oxygen $K$-edge XAS preedge region, defined by the $\mathrm{Bi}6s-\mathrm{O}2p_{{\sigma }^{*}}$ orbitals. We interpret them as a fast ($\le 0.3$ ps) breaking of charge carrier pairs and slower (0.3–0.8 ps) lattice rearrangement from the distorted monoclinic structure into the new metastable state with a cubic lattice, which persists at least up to 60 ps after the excitation. Analysis of the intermediate state at the fast excitation shows that the bond disproportionation and monoclinic distortion of ${\mathrm{BaBiO}}_3$ structure are energetically favorable due to the charge carrier pairing. Thus the compound ${\mathrm{BaBiO}}_3$ forms a new quantum state that we define as a local pair density wave. Taking into account a large number of similarities between bismuthate and cuprate high-temperature superconductors, we believe that our work will give a new impetus to understanding the nature of superconductivity in perovskite HTSCs.

Temperature dependent comparative EXAFS studies of Ba 1– x K x BiO 3 superconducting oxides and copper based superconductors with hole (La 2– x Sr x CuO 4 ) and electron (Nd 2– x Ce x CuO 4– δ ) doping showed the common dynamical local structure nonuniformity observed as the double-well potential of oxygen ion oscillations. Obtained data evidence that these local structure nonuniformity arise from the local dynamic charge ordering due to different electronic structure of the neighbouring BiO 6 or CuO n ( n = 4, 6) complexes in perovskite-like lattice. Based on the experimental results, the phenomenological description was proposed which allowed us to establish the correlations between the local and the macroscopic properties and to explain the insulator-metal phase transition and the appearance of the superconductive state with both hole and electron doping of parent BaBiO 3 , La 2 CuO 4 and Nd 2 CuO 4 insulators. The presented results makes clear the role of electron-phonon coupling in the mechanism of high temperature superconductivity.

Second-order intensity interferometry was employed to study the spatial and temporal properties of the European X-ray Free-Electron Laser (EuXFEL). Measurements