Cooperative down-conversion of UV light in disordered scheelitelike Yb-doped NaGd(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> and NaLa(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals

Concentration series of disordered scheelitelike Yb:NaGd(MoO₄)₂ and Yb:NaLa(MoO₄)₂ single crystals are grown by the Czochralski method. The actual concentrations of Yb³⁺ ions in the crystals are determined by optical-absorption spectroscopy. The luminescence of Yb³⁺ ions in these crystals in the region of 1 μm is studied under UV and IR excitation. In the case of UV excitation, this luminescence appears as a result of nonradiative excited state energy transfer from donor centers of unknown nature to ytterbium. The character of the concentration dependence of Yb³⁺ luminescence indicates that the energy transfer at high Yb concentrations occurs with active participation of a cooperative mechanism, according to which the excitation energy of one donor center is transferred simultaneously to two Yb³⁺ ions. In other words, the quantum yield of this transfer exceeds unity, which can be used to increase the efficiency of crystalline silicon (c-Si) solar cells.     

High-Frequency EPR Spectroscopy of Tb3+ Ions in Synthetic Forsterite

Paramagnetic centers formed by the impurity Tb³⁺ ions in synthetic forsterite Mg₂SiO₄ have been studied by high-frequency tunable electron paramagnetic resonance spectroscopy in the frequency range of 150–300 GHz. These centers represent single Tb³⁺ ions and dimer associates of the Tb³⁺ ions. It has been found that the integral intensity of resonance transitions belonging to the dimer centers is much higher than that to be expected for the statistical distribution of the impurity Tb ions in the forsterite host. Therefore, there is a mechanism favoring the self-organization of the Tb ions in dimer associates during the crystal growth. The parameters of the effective spin Hamiltonian describing the dependence of electron-nuclear sublevels on the magnetic field have been determined for all observed paramagnetic centers and the possible structures of the centers have been discussed.     

Colors of mixed-substituted double molybdate single crystals having scheelite structure

X-ray diffraction, neutron diffraction, optical and absorption spectroscopy, electron probe microanalysis, and temperature-dependent electrical conductivity measurements were used to study structural features and defect generation to be responsible for coloring of single crystals of nominal compositions (Na_0.5La _x Gd_{0.5 ? x} )MoO₄:Tm having scheelite structure which were Czochralski grown under a weakly oxidizing atmosphere followed by air annealing. In nonannealed crystals, the color was found to be due to oxygen vacancies and their compensator electrons (black color) and, most likely, electrons located on Mo6+ ions (lilac color); the concentration of these defects decreased upon air annealing, thereby discoloring the samples. The yellow color of crystals was interpreted as arising from possible formation of a color center (an association of a molybdenum vacancy and a hole).     

Scaling analysis of a charged jet formed upon electrospinning of a viscoelastic liquid

The dependences of the parameters of a jet and the intensity of the current generated in it during electrospinning of a viscoelastic liquid on the conductivity, viscosity, surface tension, and other characteristics of the liquid, as well as the strength of an external electric field and the liquid flow rate, are considered. The process is described in terms of the Maxwell liquid model, which is characterized by a single relaxation time and convective-type nonlinearity. Two regimes corresponding to high and low liquid flow rates are identified. The jet diameter is shown to decrease with a rise in the liquid conductivity coefficient and electric field strength. Moreover, at high flow rates, the current intensity almost linearly grows with the field strength.     

High-frequency tunable EPR spectroscopy of Tm3+ ions in synthetic forsterite

Paramagnetic centers formed by impurity Tm³⁺ ions in synthetic forsterite Mg₂SiO₄ were studied by high-frequency tunable electron paramagnetic resonance spectroscopy in the frequency range of 150–315 GHz. Crystals were grown from the melt by the Czochralski technique in slightly oxidizing atmosphere. Several centers distinguished by different zero-field splitting between the ground and first excited singlets were found and investigated. Parameters of the effective spin Hamiltonian for these centers describing the dependence of electron-nuclear sublevels on magnetic field were determined.     

Sur les propriétés-limites du module des fonctions entières d'ordre fini

Sans résumé     

Concept of the Fuel Cycle of the IGNITOR Tokamak

Physics of Atomic Nuclei - In 2015, the development of the conceptual design of IGNITOR, a Russian–Italian experimental thermonuclear tokamak, was completed, and the parameters of all the...