Theoretical study of the X-ray Kβ emission spectrum of an argon atom

The influence of many-particle effects on absolute values and the shape of the doubly differential cross section of resonant inelastic scattering of a linearly polarized X-ray photon by a free Ar atom close to the K- and KM ₂₃-ionization thresholds is studied theoretically. The evolution of the spatially extended profile of the scattering cross section to the principal Kβ_1,3 and satellite Kβ₅ structures of the Ar X-ray Kβ emission spectrum is demonstrated. The calculations are performed in the nonrelativistic Hartree-Fock approximation for the wave functions of one-electron states and in the dipole approximation for the anomalous-dispersion amplitude of the scattering probability. The effects of the radial relaxation of electron shells, the correlation loosening, vacuum correlations, the spin-orbit and multiplet splitting, as well as the Auger and radiative decays of the vacancies produced are taken into account. The calculation results have a predictive character and, for the case of the incident photon energies of 3199.2 and 3245.9 eV, they are in good agreement with the results of the synchrotron experiment on measuring the X-ray Kβ emission spectrum of a free Ar atom.     

New approach to the defibrillation problem: Suppression of the spiral wave activity of cardiac tissue

A model of an excitable medium is considered for describing the development of fibrillation (i.e., spatiotemporal chaos) in cardiac tissue through the generation of a set of coexisting spiral waves. It is shown that a weak external point action on such a medium leads to the suppression of all spiral waves and, correspondingly, to the stabilization of the system dynamics. After reaching the regular regime, only the external source exists in the medium. The frequencies and amplitudes at which such stabilization occurs are determined. The case of the action of several point sources is considered. Analysis is performed using the Bray method to identify the number of spiral waves.     

Polarization dynamics of unidirectional optical pulse evolution in a laser amplifier

Polarization dynamics of optical pulses in an isotropic two-level medium is analyzed by solving an integrable system of evolution equations without using the slowly varying envelope approximation. The analysis is focused on the regime of unidirectional pulse generation in an initially inverted medium. Qualitative difference in polarization dynamics is revealed between few-cycle and quasi-monochromatic pulse propagation     

Dielectric relaxation of Cr<Superscript>3+</Superscript>-Li<Superscript>+</Superscript> pair centers in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> crystals

The temperature-frequency dependences of the permittivity of crystals of lithium heptagermanate Li₂Ge₇O₁₅ doped with Cr³⁺ ions are investigated. A dielectric response to reorientation of the dipole moments of the chromium impurity centers is revealed. Anomalies of the permittivity are described within the model of a Debye relaxator.     

Holographic interference microscope for laboratory studies

A digital holographic interference microscope is designed that allows real-time 3D imaging of phase microobjects and measurement of their morphological parameters. The instrument may find application in medical institutions and research laboratories.     

Luminescence VUV spectroscopy of cerium-and europium-doped lithium borate crystals

The paper reports on a study of the luminescence of lithium borate crystals (Li₆Gd(BO₃)₃ doped by Eu³⁺ and Ce³⁺ ions, Li_5.7Mg_0.15Gd(BO₃)₃: Eu, and Li₆Eu(BO₃)₃) initiated by selective excitation by synchrotron radiation at excitation energies of 3.7–27 eV at 10 and 290 K. Efficient energy transfer between the rare-earth ions Gd³⁺ → Ce³⁺ and Gd³⁺ → Eu³⁺ was found to proceed by the resonance mechanism, as well as by electron-hole recombination. Fast decay kinetics of luminescence of the Ce³⁺ activator centers was studied under intracenter photoexcitation and excitation in the interband transition region. The mechanisms involved in luminescence excitation and radiative relaxation of electronic states of rare-earth ions are analyzed, and the energy transfer processes operating in these crystals are discussed.     

Polarization properties of Li2−x Na x Ge4O9 (0.2 ≤ x ≤ 0.3) crystals

The polarization switching in sinusoidal fields and the pyroelectric properties of Li_2?x Na _x Ge₄O₉ (0.2 ≤ x ≤ 0.3) crystals are measured in the temperature range T _c ?T ≤ 40 K. The behavior of the P?E hysteresis loops with variations in temperature is investigated for crystals with phase transition temperatures T _c < 300 K and T _c > 300 K. It is shown that, for crystals with phase transition temperatures T _c < 300 K, the temperature dependence of the hysteresis loop exhibits a behavior typical of crystals with second-order phase transitions. The crystals with phase transition temperatures T _c > 300 K are characterized by double hysteresis loops in the temperature range T _c ?T ₁ ≈ 30 K. The correlation between the polarization properties and possible structural transformations of the Li_2?x Na _x Ge₄O₉ crystals due to the change in the concentration ratio of Na and Li ions is discussed.     

Diffraction mechanism of specular reflection of light from photonic crystals

The spectra of specular reflection and diffraction of light for a resonant two-dimensional photonic crystal consisting of semiconductor cylinders embedded in a dielectric matrix, as well as the exciton-polariton band structure of this crystal, are studied theoretically. It is shown that specular reflection of light from a photonic crystal can be considerably enhanced by diffraction in the photonic crystal and reflection from its inner boundary with vacuum.     

Suppression of magnetic relaxation by a transverse alternating magnetic field

The evolution of the spatial distribution of the magnetic induction in a superconductor after the action of the alternating magnetic field perpendicular to the trapped magnetic flux has been analyzed. The observed stabilization of the magnetic induction profile is attributed to the increase in the pinning force, so that the screening current density becomes subcritical. The last statement is corroborated by direct measurements.