Luminescence properties of Li6GdB3O9:Ce crystal fibers upon their excitation in the range of 4d → 4f core transitions

We have investigated UV luminescence with a subnanosecond time resolution of Li₆GdB₃O₉:Ce crystal fibers upon their ultrasoft X-ray selective excitation at 10 and 293 K in the range of 4d → 4f core transitions. We have revealed an intense fast-decaying subnanosecond luminescence component, which is caused by a high local density of electronic excitation and Auger core hole relaxation processes, and modulation of the luminescence excitation spectrum by an absorption band of the 4d–4f photoionization giant resonance in the energy range 135–160 eV.     

Efficient Approximation of the Capacitated Vehicle Routing Problem in a Metric Space of an Arbitrary Fixed Doubling Dimension

Doklady Mathematics - In this paper, for the first time, we provide a quasi-polynomial time approximation scheme for the well-known capacitated vehicle routing problem formulated in metric spaces...     

Characterization of 1T-TiSe2 surface by means of STM and XPD experiments and model calculations

Scanning tunneling microscopy (STM) and X-ray photoelectron diffraction (XPD) are applied to study the surface of layered dichalcogenide 1T-TiSe₂. XPD pattern simulation for the 1T-TiSe₂ surface is performed in the approach of electron multiple scattering within the EDAC code: considered are models of structural defects in the 1T-TiSe₂ lattice, relaxation contraction (expansion) of surface layers and van der Waals gap, and deviation of the 1T-TiSe₂ surface geometry from the basal plane (001). The atomic structure of 1T-TiSe₂ surface layers is reconstructed from the XPD pattern on Se(LMM) and Ti2p core level using the photoelectron holography scattering pattern extraction algorithm with maximum entropy method (SPEA-MEM). The results of the 3D reconstruction are in agreement with the XPD pattern simulation data. In both cases, the TiSe₂ surface corresponds to 1T polytype; an increase is observed in the parameter a₀ and in the van der Waals gap between two surface slabs. It is assumed that similar structural distortions of the 1T-TiSe₂ lattice lead to the formation of an energy gap between the valence band and the conduction band of titanium diselenide, which was observed earlier by photoemission spectroscopy and follows from the theoretical calculations.     

Kinetics of electron tunneling stimulated by mobility of lithium cations in lithium gadolinium borate crystals

The kinetics of electron tunneling between electron and hole sites, stimulated by mobility of lithium cations in lithium gadolinium orthoborate (Li₆Gd(BO₃)₃) crystals has been studied. A mathematical model has been proposed to describe the kinetics in the wide time interval of 10⁻⁸–100 s after pulsed irradiation. The calculation results have been compared with experimental data on the decay kinetics of transient optical absorption (TOA) in Li₆Gd(BO₃)₃ crystals in the visible and UV spectral regions. The nature of radiation defects responsible for TOA is discussed, as well as the dependence of the TOA kinetics on temperature, excitation power, and other experimental conditions.     

Luminescence and thermally stimulated recombination processes in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>:Ce<Superscript>3+</Superscript> crystals

The luminescence and thermally stimulated recombination processes in lithium borate crystals Li₆Gd(BO₃)₃ and Li₆Gd(BO₃)₃:Ce have been studied. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence), temperature dependences of the intensity of steady-state X-ray luminescence (XL), and thermally stimulated luminescence (TSL) spectra of these compounds have been investigated in the temperature range of 90–500 K. The intrinsic-luminescence 312-nm band, which is due to the ⁶ P _J → ⁸ S _7/2 transitions in Gd³⁺ matrix ions, dominates in the X-ray luminescence spectra of these crystals; in addition, there is a wide complex band at 400–420 nm, which is due to the d → f transitions in Ce³⁺ impurity ions. It is found that the steady-state XL intensity in these bands increases several times upon heating from 100 to 400 K. The possible mechanisms of the observed temperature dependence of the steady-state XL intensity and their correlation with the features of electronic-excitation energy transfer in these crystals are discussed. The main complex TSL peak at 110–160 K and a number of minor peaks, whose composition and structure depend on the crystal type, have been found in all crystals studied. The nature of the shallow traps that are responsible for TSL at temperatures below room temperature and their relation with defects in the lithium cation sublattice are discussed.     

Kinetics of thermally stimulated recombination processes in lithium borate crystals

This paper reports on the results of the experimental and theoretical investigations of thermally stimulated recombination processes in crystals of the lithium borates Li₂B₄O₇ and LiB₃O₅. For both types of crystals, the measurements of thermally stimulated luminescence curves, spectra, and temperature dependences of the intensities of steady-state X-ray luminescence have been performed in a single experimental cycle. In the framework of a unified model for the Li₂B₄O₇ and LiB₃O₅ crystals, the thermally stimulated recombination processes have been calculated and the obtained results have been interpreted talking into account all available experimental data.     

Structural defects on a 1T-TiSe2 surface: Experiment and model calculation of photoelectron diffraction

A 1T-TiSe₂ surface is studied by means of scanning tunneling microscopy (SMT) and X-ray photoelectron diffraction (XPD). The diffraction patterns were modeled in the multiple electron scattering approximation using the EDAC code. Variants of the point and structural defects on the 1T-TiSe₂ surface are considered. The experimental and theoretical XPD patterns are compared on the basis of a convergence analysis of their R factors.     

Quasi-isentropic compressibility of a strongly nonideal deuterium plasma at pressures of up to 5500 GPa: Nonideality and degeneracy effects

We report on the experimental results on the quasi-isentropic compressibility of a strongly nonideal deuterium plasma that have been obtained on setups of cylindrical and spherical geometries in the pressure range of up to P ≈ 5500 GPa. We describe the characteristics of experimental setups, as well as the methods for the diagnostics and interpretation of the experimental results. The trajectory of metal shells that compress the deuterium plasma was detected using powerful pulsed X-ray sources with a maximal electron energy of up to 60 MeV. The values of the plasma density, which varied from ρ ≈ 0.8 g/cm³ to ρ ≈ 6 g/cm³, which corresponds to pressure P ≈ 5500 GPa (55 Mbar), were determined from the measured value of the shell radius at the instant that it was stopped. The pressure of the compressed plasma was determined using gasdynamic calculations taking into account the actual characteristics of the experimental setups. We have obtained a strongly compressed deuterium plasma in which electron degeneracy effects under the conditions of strong interparticle interaction are significant. The experimental results have been compared with the theoretical models of a strongly nonideal partly degenerate plasma. We have obtained experimental confirmation of the plasma phase transition in the pressure range near 150 GPa (1.5 Mbar), which is in keeping with the conclusion concerning anomaly in the compressibility of the deuterium plasma drawn in [1].