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We present experimental data illustrating how heat load affects the performance of a cryogenically cooled monochromator under intense X-ray Free-Electron Laser pulses. The measurements are in good agreement with a depth-uniform model of X-ray dynamical diffraction taking beam absorption and heat deformation of the crystals into account., Comment: 9 pages, 4 figures, to be published in Optics Express

The Yakutsk Extensive Air Shower Array has been continuously operating for more than 50 years (since 1970) and up until recently it has been one of world's largest ground-based instruments aimed at studying the properties of cosmic rays in the ultra-high energy domain. In this report we discuss results recently obtained at the array - on cosmic rays energy spectrum, mass composition and directional anisotropy - and how they fit into the world data. Special attention is paid to the measurements of muonic component of extensive air showers. Theoretical results of particle acceleration at shocks are also briefly reviewed. Future scientific and engineering plans on the array modernization are discussed., Comment: 24 pages, 7 figures. Accepted for publication in Physics of Atomic Nuclei. For color figures see ISCRA-2021 contribution: https://indico.nevod.mephi.ru/event/6/contributions/161/

The progress and the chronology in understanding the influence of electron correlations on the electronic structure of atoms and the dynamics of atomic processes are reviewed focusing on benchmark rare-gas atoms. The contributions and the chronological development of Photon-Induced Fluorescence Spectroscopy (PIFS), measuring {} dispersed-fluorescence emission cross sections upon excitation by single photons provided by monochromatized synchrotron radiation is described. Selected experimental results obtained by complementary techniques are also discussed for comparison. The basic suites of computer programs used for the investigation of the many-electron effects in atoms and the obtained results are analyzed. Special attention is paid to the Configuration Interaction Pauli-Fock approximation with Core Polarization (CIPFCP) method used to interpret the PIFS data., Comment: 88 pages, 54 figures, 344 bibliography items

The absolute generalized cross sections and angular distribution parameters of photoelectrons for the two-photon above threshold $3p$-ionization of Ar were calculated in the exciting photon energy range from 15.76 to 36 eV. The correlation function technique developed earlier was extended for the case when an intermediate-state function is of a continuum-type. We show that two-photon ionization of Ar near the $3p^{4}$ threshold to a large extent is determined by the $(3p\dashrightarrow\varepsilon d)^{2}$ two-photon absorption via the giant resonance. This many-electron correlation causes (i) an increase of the photoionization cross sections by more than a factor of 3; (ii) the appearance of resonances in the exciting-photon energy range of the doubly-excited states. The predictions are supported by a good agreement between length and velocity results obtained after taking into account of the higher-order perturbation theory corrections., Comment: 11 pages, 8 figures, 26 bibliography

A detailed theoretical analysis of the 1s photoionization of neon is presented. It is found that the most significant many-electron correlation in computing photoionization of inner shells is the rearrangement of the outer shells caused by the inner vacancy. Further noticeable effects are: (i) the polarization of the ion core by the outgoing photoelectron and (ii) the coherent effect of double excitation/ionization. The core polarization increases the photoionization cross section by about 10% at the 1s threshold, and the coherent excitation results in further increases by about 5%. Incoherent excitation of the satellite channel leads to an additional 10% increase in the photoabsorption cross section in the double-ionization threshold region., Comment: 6 pages, 3 figures, 22 bibliography

Harmonic lasing provides an opportunity to extend the photon energy range of existing and planned X-ray FEL user facilities. Contrary to nonlinear harmonic generation, harmonic lasing can generate a much more intense, stable, and narrow-band FEL beam. Another interesting application is Harmonic Lasing Self-Seeding (HLSS) that allows to improve the longitudinal coherence and spectral power of a Self-Amplified Spontaneous Emission (SASE) FEL. This concept was tested at FLASH in the range of 4.5 - 15 nm and at PAL XFEL at 1 nm. In this paper we present recent results from the European XFEL where we successfully demonstrated harmonic lasing at 5.9 Angstrom and 2.8 Angstrom. In the latter case we obtained both 3rd and 5th harmonic lasing and, for the first time, operated a harmonic lasing cascade (5th-3rd-1st harmonics of the undulator). These results pave the way for reaching very high photon energies, up to 100 keV., Comment: arXiv admin note: text overlap with arXiv:1612.03635

We verify a recently proposed method for obtaining a $\beta$-function of ${\cal N}=1$ supersymmetric gauge theories regularized by higher derivatives by an explicit calculation. According to this method, a $\beta$-function can be found by calculating specially modified vacuum supergraphs instead of a much larger number of the two-point superdiagrams. The result is produced in the form of a certain integral of double total derivatives with respect to the loop momenta. Here we compare the results obtained for the three-loop $\beta$-function of ${\cal N}=1$ SQED in the general $\xi$-gauge with the help of this method and with the help of the standard calculation. Their coincidence confirms the correctness of the new method and the general argumentation used for its derivation. Also we verify that in the considered approximation the NSVZ relation is valid for the renormalization group functions defined in terms of the bare coupling constant and for the ones defined in terms of the renormalized coupling constant in the HD+MSL scheme, both its sides being gauge-independent., Comment: 19 pages, 8 figures; title changed, minor corrections; the final version to appear in Nuclear Physics B

High hardness and toughness are generally considered mutually exclusive properties for single-crystal ceramics. Combining experiments and ab initio molecular dynamics (AIMD) atomistic simulations at room temperature, we demonstrate that both the hardness and toughness of single-crystal NaCl-structure VNx/MgO(001) thin films are simultaneously enhanced through the incorporation of anion vacancies. Nanoindentation results show that VN0.8, here considered as representative understoichiometric VNx system, is ~20% harder, as well as more resistant to fracture than stoichiometric VN samples. AIMD modeling of VN and VN0.8 supercells subjected to [001] and [110] elongation reveal that the tensile strengths of the two materials are similar. Nevertheless, while the stoichiometric VN phase systematically cleaves in a brittle manner at tensile yield points, the understoichiometric compound activates transformation-toughening mechanisms that dissipate accumulated stresses. AIMD simulations also show that VN0.8 exhibits an initially greater resistance to both {110}<1-10> and {111}<1-10> shear deformation than VN. However, for progressively increasing shear strains, the VN0.8 mechanical behavior gradually evolves from harder to more ductile than VN. The transition is mediated by anion vacancies, which facilitate {110}<1-10> and {111}<1-10> lattice slip by reducing activation shear stresses by as much as 35%. Electronic-structure analyses show that the two-regime hard/tough mechanical response of VN0.8 primarily stems from its intrinsic ability to transfer d electrons between 2nd-neighbor and 4th-neighbor (i.e., across vacancy sites) V-V metallic states. Our work offers a route for electronic-structure design of hard materials in which a plastic mechanical response is triggered with loading.