Публикации 2017

We present propagating Bloch bound states in the radiation continuum with orbital angular momentum in an infinite linear periodical array of dielectric spheres. The bound states in the continuum demonstrate a giant Poynting vector spiraling around the array. They can be excited by a plane wave with incident linear polarization with a small tilt relative to the axis of the array.

We report bound states in the radiation continuum (BICs) in a single infinitely long dielectric rod with periodically stepwise modulated permittivity alternating from ${\epsilon }_1$ to ${\epsilon }_2$. For ${\epsilon }_2=1$ in air the rod is equivalent to a stack of dielectric disks with permittivity ${\epsilon }_1$. Because of rotational and translational symmetries the BICs are classified by orbital angular momentum $m$ and the Bloch wave vector $\beta$ directed along the rod. For $m=0$ and $\beta =0$ the symmetry protected BICs have definite polarization and occur in a wide range of the radius of the rod and the dielectric permittivities. More involved BICs with $m\ne 0,\beta =0$ exist only for a selected radius of the rod at a fixed dielectric constant. The existence of robust Bloch BICs with $\beta \ne 0,m=0$ is demonstrated. Asymptotic limits to a homogeneous rod and to very thin disks are also considered.

A multilayered, multi-conductor stripline resonator is proposed. It is shown that increase in the number of conductors leads to increase the unloaded Q-factor and decrease the resonant frequency, with increasing the effects for thinner dielectric interlayer. Application of the resonator to the bandpass filter enables designing bandpass filters with very wide and deep stopbands. To prove this, a 4-pole bandpass filter, having central frequency 360 MHz and an overall size of 15.5 × 31.75 × 8.45 mm³, was designed and fabricated.

• Original article: Fiz. Nizk. Temp. 43, 738-747

• Original article: Fiz. Nizk. Temp. 43, 764-771

• Original article: Fiz. Nizk. Temp. 43, 789-794

We analyze microscopic models of the particle source or sink which consist of a one- or two-site Bose-Hubbard model (the system) weakly coupled to a many-site Bose-Hubbard model (the reservoir). Assuming unequal filling factors for the system and reservoir, we numerically study equilibration dynamics and compare it with the solution of the master equation on the reduced density matrix of the system. Necessary conditions for the validity of the master equation approach are formulated.

The article reports on light enhancement by structural resonances in linear periodic arrays of identical dielectric elements. As the basic elements both subwavelength spheres and rods with circular cross section have been considered. In either case it has been demonstrated numerically that high-Q structural resonant modes originated from bound states in the continuum enable near-field amplitude enhancement by factor of 10–25 in the red-to-near infrared range in lossy silicon. The asymptotic behavior of the Q-factor with the number of elements in the array is explained theoretically by analyzing quasi-bound states propagation bands.

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The lithium(I) catena-diaquabarbiturate complex [Li(H₂O)₂(HBA–O,O′)]_n (I), where Н₂ВА is barbituric acid, has been structurally characterized by X-ray diffraction (CIF file CCDC no. 1447689), and its thermal decomposition and IR spectrum have been studied. Crystals of complex I are monoclinic, a = 6.4306(7) Å, b = 16.720(1) Å, c = 7.1732(8) Å, β = 108.253(4)°, V = 732.5(1) ų, space group P2₁/c, and Z = 4. One independent μ₂-bridging HBA^– ligand is coordinated to two Li(I) ions via the two oxygen atoms of C₄₍₆₎=O carbonyl groups. Each Li⁺ ion is linked with two μ₂-HBA^– ions and two terminal water molecules at tetrahedron vertices. μ₂-HBA^– ions link tetrahedra into a chain. The structure is stabilized by multiple hydrogen bonds and π–π-interaction between HBA^–. The shift of ν(C=O) vibration bands in the IR spectrum of complex I in comparison with Н₂ВА towards lower frequencies agrees with the coordination of HBA^– via oxygen atoms. The dehydration of complex I occurs in two stages in the regions of 100–150 and 150–240°C.

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A polymeric coordination compound, [Ba(H₂O) ₂(Hba)₂] (1) (H₂ba – barbituric acid, C₄H₄N₂O₃), was obtained. The structure of 1 was solved using powder X-ray diffraction methods. The Ba²⁺ ion in 1 formed a three-capped trigonal prism. The BaO₉ polyhedra, connected with each other by the edges and faces, formed a chain. Several 4- and 12-membered cycles due to the bridging μ₂-H₂O and bridging μ₃-Hba^– also formed implementing a 3-D polymer structure. The structures of 1 and other thiobarbiturate complexes were compared. The replacement of a S atom by an O atom in the heterocyclic ligand Htba⁻ (thiobarbiturate ion) of the compound Ba(H₂O)₂(Htba)₂ resulted in changes of the coordination number Ba(II) and supramolecular structure. The intermolecular hydrogen bonds O–H⋯O and N–H⋯O formed a 3-D net where pronounced 2-D layers of Hba^– ions could be found. A new topological net in 1 was observed. The IR and thermal stability were investigated.

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We have shown significant suppression of resonant properties of metallic nanoparticles at the surface plasmon frequency during the phase transition “solid–liquid” in the basic materials of nanoplasmonics (Ag, Au). Using experimental values of the optical constants of liquid and solid metals, we have calculated nanoparticle plasmonic absorption spectra. The effect was demonstrated for single particles, dimers and trimers, as well as for the large multiparticle colloidal aggregates. Experimental verification was performed for single Au nanoparticles heated to the melting temperature and above up to full suppression of the surface plasmon resonance. It is emphasized that this effect may underlie the nonlinear optical response of composite materials containing plasmonic nanoparticles and their aggregates.

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The employment of colloid quantum dots in a number of applications is limited by their instability under light irradiation. Additional methods of photostability enhancement of UV+visible-irradiated TGA-stabilized CdTe quantum dots are investigated. Photostability enhancement was observed via either addition of sodium sulphite in the role of chemical oxygen absorber or addition of 1% gelatin, or, finally, by additional stabilization by bovine serum albumine (BSA). The latter method is the most promising, since it not only enhances the quantum dots’ photostability but also makes them more biocompatible and extends the possibilities of their biological applications.

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We consider Bloch bound states in the radiation continuum in periodic arrays of dielectric spheres. It is demonstrated that the bound states are associated with phase singularities of the quasimode coupling strength. That makes the bound states topologically protected and, therefore, robust against any variation of parameters preserving the periodicity and rotational symmetry about the array axis. It is shown that under variation of parameters the bound states can only be destroyed by either annihilation of the topological charge or by migration to the sector of the parametric space where the second radiation channel is open.

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The formation of the atomically ordered L1₀-FePd structure during the solid-state reaction in Fe/Pd bilayer thin films is in situ investigated by electron microscopy and electron diffraction analysis. The initial iron and palladium layers were mainly coherently oriented crystallites with the orientation relationship α-Fe (001)[110] || Pd(001)[100]. It is established that the solid-state reaction between the iron and palladium layers upon heating at a rate of 4–8°C/min starts with the formation of the FePd solid solution at 390°C; at 430°C, the formation of the atomically ordered L1₀-FePd structure is observed. It is shown that at the low heating rate (4–8°C/min), the L1₀-FePd structure with the [001] orientation relative to the film plane forms, while at the high heating rate (50°C/min) it forms with the [100], [010], and [001] orientations.

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The direct and indirect measurements of intensive electrocaloric effect in a triglycine sulfate ferroelectric crystal are performed under equilibrium and nonequilibrium thermodynamic conditions implemented in the adiabatic calorimeter. The effect of the electric field parameters (frequency, profile, and strength) on the value of the effect and degree of its reversibility are studied. The difference between the temperature variation values in a switched-on and switched-off dc field under quasi-isothermal conditions is established. The low-frequency periodic electric field induces the temperature gradient along the electrocaloric element and heat flux from its free end to the thermostated base. A significant excess of the field switching-off rate over the switching-on rate leads to a noticeable intensification of the cooling effect.

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Thermal and dielectric studies of (ND₄)₂MoO₂F₄ crystals undergoing successive phase transitions at T₁ = 272 K and T₂ = 181 K showed that deuteration is accompanied by an increase in the chemical pressure in the crystal lattice (Δp ≈ 0.02 GPa), which shifts the Cmcm ↔ Pnma transformation for the first order to the tricritical point. The direct participation of ammonium groups in the mechanism of structural distortions is demonstrated by a decrease in the entropy of the high-temperature phase transition (ΔS₁ = R ln 6.0). An external hydrostatic pressure leads to an expansion of the temperature interval of the intermediate antiferroelectric Pnma phase. The triple point on the T–p phase diagram, where the Cmcm, Pnma, amd Pnma* phases coexist, can be realized at a negative pressure of p_trp ≈ −0.8 GPa.

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In this paper, we present a study of the electron spin resonance (ESR) of nanoparticles (NPs) of Cu_1−xFe_xCr₂Se₄ chalcogenides with x = 0, 0.2, and 0.4. NPs were synthesized via the thermal decomposition of metal chloride salts and selenium powder in a high-temperature organic solvent. According to the XRD and HRTEM data, the NPs were single crystalline nearly hexagonal plates with the structure close to CuCr₂Se₄ (Fd-3m, a = 10.337 Å). For x = 0 and 0.2, the NPs tend to form long stacks consisting of the plates “face to face” attached to each other due to the magnetostatic interparticle interaction. Only separate NPs were observed in the case of x = 0.4. Peculiarities were revealed in the ESR temperature behavior for the NPs with x = 0 and 0.2 consistent with the features in the temperature dependences of the NPs magnetization. The non-monotonous dependence of the resonance field H_res on the temperature with a kink near 130 K and the energy gap in the resonance spectrum depending on the type of nanoparticle compacting are the distinct peculiarities. One of the main factors is discussed in order to explain the peculiarities: the coexistence of two types of anisotropy in the Cu_1−xFe_xCr₂Se₄ NPs, in-plain shape anisotropy and magnetocrystalline anisotropy with four easy axes, which increases strongly with the temperature decrease.

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Taking into account the space inhomogeneity of the light electromagnetic wave, a new quantum mechanical expression for the natural optical activity (NOA) of electron transitions has been obtained, which is consistent with the phenomenological theory. It is shown that properties of the NOA of parity forbidden f–f transitions substantially differ from those of allowed transitions. The experimentally observed large NOA of the f–f transitions and extremely large (close to unity) NOA of vibrational repetitions of the f–f transitions are qualitatively explained on the basis of the obtained theoretical expression.

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The investigation of mixed Co–Fe ludwigite single crystals shows that their magnetic properties are close to the magnetic properties of Fe₃BO₅ despite the predominance of cobalt ions. The magnetic properties of Co_{3 – x}Fe_xBO₅ single crystals with x = 0.10 are studied in detail. Magnetometric measurements demonstrate a strong magnetic anisotropy with easy magnetization axis b, and the orbital magnetic moment of cobalt is in a frozen state. The detected temperature dependence of the absorption of Mössbauer spectra allowed us to determine the magnetic ordering temperature, which agrees with the results of magnetization measurements (T_C = 84 K).

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The spin–fermion model, which is an effective low-energy realization of the three-band Emery model after passing to the Wannier representation for the p_x and p_y orbitals of the subsystem of oxygen ions, reduces to the generalized Kondo lattice model. A specific feature of this model is the existence of spin-correlated hoppings of the current carriers between distant cells. Numerical calculations of the spectrum of spin-electron excitations highlight the important role of the long-range spin-correlated hoppings.

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The (Y,Ce)₂BaAl₄SiO₁₂ phosphor, a garnet-structured blue-to-yellow color convertor for WLED, exhibits an interesting “microcrystal-glass powder” feature, which can be regarded as the 4th form for phosphor, in addition to the “ceramic powder phosphor”, the “single crystal phosphor” and the “glass-ceramic phosphor”. The structure exhibits luminescent microcrystals embedding in non-luminescent glass matrix: the spherical crystals are mainly arranged around the glass phase forming a “necklace” pattern, while the individual crystals show a “core-shell” architecture regarding the luminescence intensity variation. Further combining the phase evolution evidence evaluated by Rietveld refinement, we propose the formation mechanism for such unique morphology/structure as a two-stage process, including an initial nucleation by solid state reaction and following liquid-assisted crystal growth, instead of a precipitation-crystallization process.

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