New Publications

Red shift properties, crystal field theory and nephelauxetic effect on Mn4+-doped SrMgAl10-yGayO17 red phosphor for plant growth LED light

Gu, Simin; Xia, Mao; Zhou, Cheng; Kong, Zihui; Molokeev, Maxim S.; et al. Chemical Engineering Journal. https://doi.org/10.1016/j.cej.2020.125208

The discovery of novel Mn4+-doped oxide red phosphor with suitable spectrum for plant growth is a hot issue in the recent years due to the characteristic red photoluminescence of 2Eg → 4A2 transition in Mn4+ ions. Generally, the emission position of Mn4+ is hard to tune because of specific crystal field in most phosphors. In this work, tunable luminescence property with obvious red shift in the spectra is observed in the Mn4+-doped SrMgAl10-yGayO17 red phosphor via simple substitution of Ga3+ for Al3+, and crystal field theory and nephelauxetic effect are employed to explain this phenomenon. Meanwhile, the Ga3+ dopant changes the shape of the spectra because Ga3+ dopant guides the replacement site of Mn4+ and changes the luminescence center. Improved emission intensity is obtained because appropriate Ga3+ doping leads to larger band gap and reduces non-radiative transitions. Phosphor-converted LED (pc-LED) devices fabricated with blue chip (470 nm) and the as-obtained SrMgAl10-yGayO17:1.0%Mn4+ phosphors emit bright blue and red light, which fit the absorption regions of plant pigments well, thus SrMgAl10-yGayO17:Mn4+ phosphor can be a candidate for plant growth LED light.

Single particle Raman spectroscopy analysis of the metal-organic framework DUT-8(Ni) switching transition under hydrostatic pressure

Krylov, Alexander; Senkovska, Irena; Ehrling, Sebastian; Maliuta, Mariia; Krylova, Svetlana; et al. Chemical Communications. Chem. Commun., 2020,56, 8269-8272. https://doi.org/10.1039/D0CC02491K

Experimental in situ observations of phase coexistence in switchable metal–organic frameworks are reported to provide a fundamental understanding of dynamic adsorbents that can change their pore structure in response to external stimuli. A prototypical flexible pillared layer framework DUT-8(Ni) (DUT = Dresden University of Technology) was studied under hydrostatic pressure by in situ Raman spectroscopy on single crystals. The closing transition of the open pore phase (op) containing DMF in the pores in silicon oil as a pressure transmitting fluid, as well as the closed pore phase (cp) to op transition under pressure in methanol, were studied. Phase coexistences during both transitions were observed.

Tailoring diffraction of light carrying orbital angular momenta

Ikonnikov, Denis A.; Vyunishev, Andrey M. Optics Letters Vol. 45, Issue 14, pp. 3909-3912 (2020) •https://doi.org/10.1364/OL.389019.

A unified approach to controlling the diffraction of light carrying orbital angular momenta (OAM) is developed and experimentally verified in this Letter. This approach allows one to specify not only the number of diffraction maxima, their spatial frequencies, and the intensity distribution between them, but also the OAM in each maximum. It is verified that the approach can be used for structuring both single and multiple beams carrying OAMs. Simulations reveal phase singularities in structured beams. In addition, the approach makes it possible to shape the light in regular and irregular two-dimensional arrays with addressing the OAMs at each site. This approach offers new opportunities for singular optics.

Data-Driven Photoluminescence Tuning in Eu2+-Doped Phosphors

Lai, Shunqi; Zhao, Ming; Qiao, Jianwei; Molokeev, Maxim S.; Xia, Zhiguo Journal Of Physical Chemistry Letters. https://doi.org/10.1021/acs.jpclett.0c01471

Discovery of rare earth phosphors has generally relied on the chemical intuition and time-intensive trial-and-error synthesis; therefore, finding new materials assisted by data-driven computations is urgent. Herein, we utilize a regression model to predict the emission wavelengths of Eu2+-doped phosphors by revealing the relationships between the crystal structure and luminescence property. The emission wavelengths of [Rb(1–x)K(x)]3LuSi2O7:Eu2+ (0 ≤ x ≤ 1) phosphors, as examples for the data-driven photoluminescence tuning, are successfully predicted on the basis of the existing data of only eight systems, also consistent with the experimental results. These phosphors can be excited by blue light and exhibit broad-band red and near-infrared emission ranging from 619 to 737 nm. These findings in Eu2+-doped silicate phosphors indicate that data-driven computations through the regression mode would have bright application in discovering novel phosphors with a target emission wavelengths.

Characterization of the iron oxide phases formed during the synthesis of core-shell FexOy@C nanoparticles modified with Ag

Petrov, D. A.; Lin, C. -R; Ivantsov, R. D.; Ovchinnikov, S. G.; Zharkov, S. M.; et al. Nanotechnology, Volume 31, Number 39.

Core–shell FexOy@C nanoparticles (NPs) modified with Ag were studied with x-ray diffraction, transmission electron microscopy, energy dispersive elemental mapping, Mössbauer spectroscopy, static magnetic measurements, and optical magnetic circular dichroism (MCD). FexOy@C NPs synthesized by the pyrolysis process of the mixture of Fe(NO3)3 centerdot 9H2O with oleylamine and oleic acid were added to a heated mixture of oleylamine and AgNO3 in different concentrations. The final product was a mixture of iron oxide crystalline NPs in an amorphous carbon shell and Ag crystalline NPs. The iron oxide NPs were presented by two magnetic phases with extremely close crystal structures: Fe3O4 and γ-Fe2O3. Ag is shown to form crystalline NPs located very close to the iron oxide NPs. An assumption is made about the formation of hybrid FexOy@C-Ag NPs. Correlations were obtained between the Ag concentration in the fabricated samples, their magnetic properties and the MCD spectrum shape. Introducing Ag led to a approximately linear decrease of the NPs saturation magnetization depending upon the Ag concentration, it also resulted into the MCD spectrum shift to the lower light wave energies. MCD was also studied for the Fe3O4@C NPs synthesized earlier with the same one-step process using different heat treatment temperatures, and MCD spectra were compared for two series of NPs. A possible contribution of the surface plasmon excitation in Ag NPs to the MCD spectrum of the FexOy@C-Ag NPs is discussed.

Corner excitations in the 2D triangle-shaped topological insulator with chiral superconductivity on the triangular lattice

Fedoseev, A. D. Journal Of Physics-condensed Matter. Journal of Physics: Condensed Matter, Volume 32, Number 40.

The 2D triangle-shaped C 3-symmetric topological insulator with the chiral superconducting coupling on the triangular lattice is investigated. While such a system cannot provide the topologically protected corner excitations, we report the presence of the nontopological corner excitations with energy value to lie in the first-order edge spectrum gap. Though these excitations are not topologically protected, they appear for a rather wide range of the parameters values and are robust against the boundary defects and weak disorder. We reveal the presence of the Majorana corner states, which appear along the line in the parameter space.

Solid-state synthesis, magnetic and structural properties of interfacial B2-FeRh(001) layers in Rh/Fe(001) films

Myagkov, V. G.; Ivanenko, A. A.; Bykova, L. E.; Zhigalov, V. S.; Volochaev, M. N.; et al. Scientific Reports. DOI: https://doi.org/10.1038/s41598-020-67837-2

Here we first report results of the start of the solid-state reaction at the Rh/Fe(001) interface and the structural and magnetic phase transformations in 52Rh/48Fe(001), 45Rh/55Fe(001), 68Rh/32Fe(001) bilayers from room temperature to 800 °C. For all bilayers the non-magnetic nanocrystalline phase with a B2 structure (nfm-B2) is the first phase that is formed on the Rh/Fe(001) interface near 100 °C. Above 300 °C, without changing the nanocrystalline B2 structure, the phase grows into the low-magnetization modification αlʹ (MSl ~ 825 emu/cm3) of the ferromagnetic αʹ phase which has a reversible αlʹ ↔ αʺ transition. After annealing 52Rh/48Fe(001) bilayers above 600 °C the αlʹ phase increases in grain size and either develops into αhʹ with high magnetization (MSh ~ 1,220 emu/cm3) or remains in the αlʹ phase. In contrast to αlʹ, the αhʹ ↔ αʺ transition in the αhʹ films is completely suppressed. When the annealing temperature of the 45Rh/55Fe(001) samples is increased from 450 to 800 °C the low-magnetization nanocrystalline αlʹ films develop into high crystalline perfection epitaxial αhʹ(001) layers, which have a high magnetization of ~ 1,275 emu/cm3. αhʹ(001) films do not undergo a transition to an antiferromagnetic αʺ phase. In 68Rh/32Fe(001) samples above 500 °C non-magnetic epitaxial γ(001) layers grow on the Fe(001) interface as a result of the solid-state reaction between the epitaxial αlʹ(001) and polycrystalline Rh films. Our results demonstrate not only the complex nature of chemical interactions at the low-temperature synthesis of the nfm-B2 and αlʹ phases in Rh/Fe(001) bilayers, but also establish their continuous link with chemical mechanisms underlying reversible αlʹ ↔ αʺ transitions.

Collective Lattice Resonances in All-Dielectric Nanostructures under Oblique Incidence

Utyushev, Anton D.; Zakomirnyi, Vadim, I; Ershov, Alexander E.; Gerasimov, Valeriy S.; Karpov, Sergey, V; et al. Photonics. https://doi.org/10.3390/photonics7020024

Collective lattice resonances (CLRs) emerging under oblique incidence in 2D finite-size arrays of Si nanospheres have been studied with the coupled dipole model. We show that hybridization between the Mie resonances localized on a single nanoparticle and angle-dependent grating Wood–Rayleigh anomalies allows for the efficient tuning of CLRs across the visible spectrum. Complex nature of CLRs in arrays of dielectric particles with both electric dipole (ED) and magnetic dipole (MD) resonances paves a way for a selective and flexible tuning of either ED or MD CLR by an appropriate variation of the angle of incidence. The importance of the finite-size effects, which are especially pronounced for CLRs emerging for high diffraction orders under an oblique incidence has been also discussed

Two-site Cr(3+)occupation in the MgTa2O6:Cr(3+)phosphor toward broad-band near-infrared emission for vessel visualization

Liu, Gaochao; Molokeev, Maxim S.; Lei, Bingfu; Xia, Zhiguo Journal Of Materials Chemistry C. J. Mater. Chem. C, 2020,8, 9322-9328. https://doi.org/10.1039/D0TC01951H

Near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) have great potential in photonic, optoelectronic and biological applications, while the discovery of a broad-band NIR phosphor still remains a challenge. Here, we report a novel Cr3+-activated MgTa2O6 phosphor with an asymmetrical emission band ranging from 700 to 1150 nm and a large full width at half maximum (FWHM) of 140 nm upon 460 nm blue light excitation. The broad spectrum is assigned to the overlap of two bands centered at 910 and 834 nm, which originate from the spin-allowed transition of 4T2 → 4A2 for different Cr3+ ions located in the two six-coordinated crystallographic sites of Mg2+ and Ta5+, respectively. The distribution of blood vessels and bones in human palm and wrist is observed with the assistance of a commercial NIR camera and a fabricated pc-LED, which demonstrates that the MgTa2O6:Cr3+ phosphor is promising in biological applications.

Structure analysis, tuning photoluminescence and enhancing thermal stability on Mn4+-doped La2-xYxMgTiO6 red phosphor for agricultural lighting

Gai, Shujie; Zhu, Haifeng; Gao, Peixin; Zhou, Cheng; Kong, Zihui; et al. Ceramics International. https://doi.org/10.1016/j.ceramint.2020.05.095

Currently, phosphor-converted LEDs (pc-LEDs) are revolutionizing the industry of plant growth lighting. To meet the requirements of this technology, phosphors with tunable photoluminescence, high thermal stability and luminous intensity are required. Herein, we found that the simple substitution of yttrium for lanthanum in La2MgTiO6:Mn4+ (LMT:Mn4+) system could satisfy above three criteria simultaneously. The photoluminescence properties can be regulated by continuously controlling the chemical composition of La2-xYxMgTiO6:Mn4+ solid solution. The La sites are occupied by Y ions, causing a significant blue shift in the emission spectra which owing to the change of local crystal field strengthen. Meanwhile, the thermal stability and decay lifetimes are also varied due to the variation of local structure and band gap energy. The thermal stability of the material reaches 83.5% at 150 °C, which is better than the reported La2MgTiO6:Mn4+ and Y2MgTiO6:Mn4+ phosphors. The electronic luminescence (EL) of pc-LED devices using La2-xYxMgTiO6:Mn4+ red phosphor is evaluated, which matching the absorption regions of plant pigments well, reflecting the superiority of the studied phosphors in plant growth lighting areas.

Stability and gas sensing properties of Ta2X3M8(X = Pd, Pt; M = S, Se) nanoribbons: a first-principles investigation

Sukhanova, Ekaterina, V; Visotin, Maxim A.; Popov, Zakhar, I; Sorokin, Pavel B. Physical Chemistry Chemical Physics., Phys. Chem. Chem. Phys., 2020,22, 14651. https://doi.org/10.1039/D0CP01545H.

One dimensional Ta2(Pd/Pt)3(S/Se)8 nanoribbons (TPS-NR) are considered as a promising material in nanoelectronics due to their intrinsic semiconducting electronic properties. In this article, we study the stability of TPS-NR by considering their oxidation process. Our calculations showed that the Ta2(Pd/Pt)3Se8 nanoribbons are more environmentally stable than Ta2(Pd/Pt)3S8-NR. We studied the thermodynamics of the formation of monovacancies and their impact on the electronic properties of TPS-NR. Additionally, the sensing properties of environmentally stable Ta2Pd3Se8 nanoribbons were investigated. The observed changes of the electronic structure and transport properties after the adsorption of CO, NH3 and NO2 molecules reveal the mechanisms of possible application of Ta2Pd3Se8 nanoribbons as a gas sensor. The electronic transport properties of the nanoribbons exhibit a notable response to the presence of gas molecules.

Interaction between dielectric particles enhances the Q factor

Bulgakov, E. N.; Pichugin, K. N.; Sadreev, A. F. Metanano 2019. Advanced Electromagnetics DOI https://doi.org/10.7716/aem.v8i4.1292

We study behavior of resonant modes with a distance between two identical dielectric cylinders and disks. We reveal two basic scenarios of evolution of resonances with the distance between the cylinders. For larger distances and respectively weaker interaction of particles the resonances are bound around the Mie resonances and evolve by spiral way. For shorter distances and respectively stronger interaction the resonances bypass the Mie resonances. Both scenarios demonstrate considerable enhancement of the Q factor  compared to the case of isolated particle.

OPTICAL TRANSMISSION OF THE NEMATIC DOMAIN STRUCTURE ON THE POLYMER SURFACE IN A MAGNETIC FIELD

Parshin, A. M.; Barannik, A., V Liquid Crystals And Their Application. DOI: 10.18083/LCAppl.2020.2.62

The propagation of a laser beam through a layer of a hybrid ordered nematic on a polycarbonate film is studied. The nematic has a surface disclination line consisting of domains with a radial configuration of the nematic director. 

Collective resonances in hybrid photonic-plasmonic nanostructures

Ershov, Alexander E.; Bikbaev, Rashid G.; Rasskazov, Ilia L.; Gerasimov, Valeriy S.; Timofeev, Ivan V.; et al. Metanano 2019. Journal of Physics: Conference Series, Volume 1461, METANANO 2019 15-19 July 2019, St. Petersburg, Russian Federation.

We present the theoretical model to predict the spectral position of Rayleigh anomalies emerged in hybrid system consisting of periodic array of plasmonic nanodisks embeded into the middle of defect layer of 1D photonic crystal (PhC). The spectral positions of these new emerged Rayleigh anomalies agree well with the results of exact simulations with Finite-Difference Time-Domain (FDTD) method.

Thermodynamic Properties of Vanadium Oxypentafluoride (IV) (NH4)(3)VOF5

Bogdanov, E. V.; Pogoreltsev, E. I.; Kartashev, A. V.; Gorev, M. V.; Molokeev, M. S.; et al. Physics Of The Solid State. DOI: https://doi.org/10.1134/S1063783420070057

The (NH4)3VOF5 crystals have been synthesized and their homogeneity and single-phase structure has been established by the X-ray diffraction, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy studies. The investigations of the temperature dependences of specific heat, entropy, strain, and pressure susceptibility show the occurrence of three phase transitions caused by the structural transformations in the (NH4)3VOF5 crystals. The Tp phase diagram shows the temperature limits of stability of the crystalline phases implemented in (NH4)3VOF5. The optical and dielectric studies disclose the ferroelastic nature of the phase transitions. An analysis of the experimental data together with the data on the isostructural (NH4)3VO2F4 crystal makes it possible to distinguish the physical properties of oxyfluorides containing vanadium of different valences (IV and V).

Magnetization Anisotropy in the Textured Bi-2223 HTS in Strong Magnetic Fields

Gokhfeld, D. M.; Balaev, D. A. Physics Of The Solid State. DOI https://doi.org/10.1134/S1063783420070069

The origin of the low magnetization anisotropy of the textured bulk samples consisting of highly anisotropic (Bi,Pb)2Sr2Ca2Cu3Ox (Bi-2223) high-temperature superconductor crystallites has been investigated. It has been established that the observed anisotropy is determined by the disordering of Bi-2223 crystallites in the sample. The measured anisotropy of the textured sample makes it possible to determine the magnetic angle characterizing the ordering of crystallites.

Model of the Behavior of a Granular HTS in an External Magnetic Field: Temperature Evolution of the Magnetoresistance Hysteresis

Semenov, S. V.; Balaev, D. A. Physics Of The Solid State. https://doi.org/10.1134/S1063783420070239

A model for describing the magnetoresistance behavior in a granular high-temperature superconductor (HTS) that has been developed in the last decade explains a fairly extraordinary form of the hysteretic R(H) dependences at T = const and their hysteretic features, including the local maximum, the negative magnetoresistance region, and the local minimum. In the framework of this model, the effective field Beff in the intergrain medium has been considered, which represents a superposition of the external field and the field induced by the magnetic moments of HTS grains. This field can be written in the form Beff(H) = H + 4παM(H), where M(H) is the experimental field dependence of the magnetization and α is the parameter of crowding of the magnetic induction lines in the intergrain medium. Therefore, the magnetoresistance is a function of not simply an external field, but also the “internal” effective field R(H) = f(Beff(H)). The magnetoresistance of the granular YBa2Cu3O7 – δ HTS has been investigated in a wide temperature range. The experimental hysteretic R(H) dependences obtained in the high -temperature range (77–90 K) are well explained using the developed model and the parameter α is 20–25. However, at a temperature of 4.2 K, no local extrema are observed, although the expression for Beff(H) predicts them and the parameter α somewhat increases (~30–35) at this temperature. An additional factor that must be taken into account in this model can be the redistribution of the microscopic current trajectories, which also affects the dissipation in the intergrain medium. At low temperatures under the strong magnetic flux compression (α ~ 30–35), the microscopic trajectories of the current Im can change and tunneling through the neighboring grain is preferred, but the angle between Im and Beff will be noticeably smaller than 90°, although the external (and effective) field direction is perpendicular to the macroscopic current direction.

Features of Relaxation of the Remanent Magnetization of Antiferromagnetic Nanoparticles by the Example of Ferrihydrite

Balaev, D. A.; Krasikov, A. A.; Balaev, A. D.; Stolyar, S. V.; Ladygina, V. P.; et al. Physics Of The Solid State. https://doi.org/10.1134/S1063783420070033

The relaxation of the remanent magnetization of antiferromagnetically ordered ferrihydrite nanoparticles at the exchange bias effect implemented in these systems has been investigated. The magnetization relaxation depends logarithmically on time, which is typical of the thermally activated hoppings of particle magnetic moments through the potential barriers caused by the magnetic anisotropy. The barrier energy obtained by processing of the remanent magnetization relaxation data under the field cooling conditions significantly exceeds the barrier energy under standard (zero field cooling) conditions. The observed difference points out the possibility of using the remanent magnetization relaxation to analyze the mechanisms responsible for the exchange bias effect in antiferromagnetic nanoparticles and measure the parameters of the exchange coupling of magnetic subsystems in such objects.

Ferromagnetic resonance in iron tubes deposited on a copper grid

Stolyar, S., V; Yaroslavtsev, R. N.; Chekanova, L. A.; Rautskii, M., V; Bayukov, O. A.; et al. Journal Of Magnetism And Magnetic Materials. https://doi.org/10.1016/j.jmmm.2020.166979

In the work, a composite material, which is an iron coating on a copper microgrid with a mesh size of 50 μm, is investigated. Iron coatings were synthesized by electroless deposition using arabinogalactan as a reducing agent. Samples were investigated using transmission electron microscopy, Mossbauer spectroscopy and ferromagnetic resonance methods. Magnetic anisotropy in the grid plane was studied by measuring the FMR spectra. The results of the study are discussed by modeling the composite as a wireframe system of magnetic tubes.

Near -infrared photoluminescence and phosphorescence properties of Cr 3+-Doped garnet -type Y 3 Sc 2 Ga 3 O 12

XianYanga, Chen, Weibin; Wang, Dongsheng; Chai, Xirong; Xie, Gening, Maxim S. Molokeev. Journal Of Luminescence. https://doi.org/10.1016/j.jlumin.2020.117392

Garnet-type Y3Sc2Ga3O12:Cr3+ phosphor has been synthesized by a solid-state reaction. XRD result revealed the successful phase formation and Cr3+ doping in Y3Sc2Ga3O12. The excitation spectrum at about 260, 450, and 630 nm corresponded to three spin-allowed Cr3+ d-d intra-transitions of 4A2-4T1 (4P), 4A2-4T1 (4F), and 4A2-4T2 (4F), respectively, among which the near infrared (NIR) emission peak at around 740 nm is identified. Moreover, the phosphorescence emission property, thermoluminescence and related luminescence mechanisms are discussed as well. The fabricated NIR phosphor-converted light-emitting diodes (pc-LEDs) suggest its potential of Y3Sc2Ga3O12:Cr3+ phosphor for non-visible light source application.

Tailoring the preferable orientation relationship and shape of alpha-FeSi(2)nanocrystals on Si(001): the impact of gold and the Si/Fe flux ratio, and the origin of alpha/Si boundaries

Ivan A. Tarasov, Tatiana E. Smolyarova, Ivan V. Nemtsev, Ivan A. Yakovlev, Mikhail N. Volochaev, Leonid A. Solovyov, Sergey N. Varnakov and Sergey G. Ovchinnikov. DOI: 10.1039/D0CE00399A (Paper) CrystEngComm, 2020, 22, 3943-3955

The growth of α-FeSi2 nanocrystal ensembles on gold-activated and gold-free Si(001) surfaces at different Si/Fe flux ratios via molecular beam epitaxy is reported. The study reveals that the utilisation of gold as a catalyst regulates the preferable orientation relationship (OR) of the nanocrystals to silicon and their morphology at a given Si/Fe flux ratio. α-FeSi2 free-standing crystals with continuously tuned sizes from 30 nm up to several micrometres can be grown with an α(001)//Si(001) basic OR under gold-assisted conditions and an α(111)//Si(001) OR under gold-free growth conditions on a Si(001) surface. The preferred morphology of nanocrystals with a particular OR can be altered through changes to the Si/Fe flux ratio. Herein, the microstructure and basic OR between the silicide nanocrystals and the silicon substrate, and the formation of nanocrystal facets were analysed in detail with the help of microscopic techniques and simulation methods based on the analysis of near coincidence site (NCS) distributions at silicide/silicon interfaces. On the basis of the simulations used, we managed to reveal the nature of the interfaces observed for the main types of α-FeSi2 nanocrystals grown. Three types of interfaces typical for nanoplates with an α(001)//Si(001) basic OR, which are (i) stepped, (ii) stressed, and (iii) flat, are explained based on the tendency for the NCS density to increase at the interface. The results presented reveal the potential for the bottom-up fabrication of α-FeSi2 nanocrystals with tuned physical properties as potentially important contact materials and as building blocks for future nanoelectronic devices.

Anisotropy of piezocaloric effect at ferroelectric phase transitions in ammonium hydrogen sulphate

Mikhaleva, Ekaterina A.; Gorev, Mikhail V.; Molokeev, Maxim S.; Kartashev, Andrey V.; Flerov, Igor N. Journal Of Alloys And Compounds. https://doi.org/10.1016/j.jallcom.2020.155085

The role of anisotropy of the thermal expansion in formation of piezocaloric effect (PCE) near ferroelectric phase transitions in NH4HSO4 was studied. Strong difference in linear baric coefficients and as a result in intensive and extensive PCE associated with the different crystallographic axes was found. PCE giving the main contribution to the barocaloric effect were determined at both phase transitions. Rather strong effect of the lattice dilatation on the tuning of PCE was observed. Comparative analysis of PCE at the phase transitions in different materials showed that NH4HSO4 can be considered as a promising solid-state refrigerant. A hypothetical cooling cycle based on alternate using uniaxial pressure along two axes was considered.

Atomic deciphering of cation exchange mechanism in upconversion nanoparticles

Guan, Ming; Molokeev, Maxim S.; Zhou, Jiajia Journal Of Luminescence. https://doi.org/10.1016/j.jlumin.2020.117289

Transition metal ion doping in upconversion nanoparticles (UCNPs) provides an effective way to enhance the luminescence for their wide array of applications. However, the doping sites of transition metal ions have not been comprehensively explored, and commonly assumed that transition metal ions replace the trivalent lanthanides within the lattice. Here we report that cation exchange of transition metal (Mn2+) in β-NaYF4:Yb3+/Er3+ UCNPs occurs through alkaline metal (Na+) replacement, via 2Na+ ↔ Mn2+ + Vacancy reaction. This process distorts the LnF9 polyhedrons and tailors the surrounding environment around the trivalent lanthanides, thereby improving the upconversion intensity from active lanthanides. Further confirmed by core-shell design and spectroscopic study, we prove that the transition–alkaline metal exchange enables both the emission enhancement and transition probability variation of activators.

Evaporative Cooling and Self‐Thermalization in an Open System of Interacting Fermions

Kolovsky, Andrey R.; Shepelyansky, Dima L. Annalen Der Physik. https://doi.org/10.1002/andp.201900231

Depletion dynamics of an open system of weakly interacting fermions with two‐body random interactions is studied. In this model, fermions are escaping from the high‐energy one‐particle orbitals, that mimics the evaporation process used in laboratory experiments with neutral atoms to cool them to ultra‐low temperatures. It is shown that due to self‐thermalization the system instantaneously adjusts to the new temperature which decreases with the course of time.

Experimental Study of the Thermal Conductivity of Single-Walled Carbon Nanotube-Based Thin Films

Tambasov, I. A.; Voronin, A. S.; Evsevskaya, N. P.; Kuznetsov, Yu M.; Luk'yanenko, A., V; E. V. Tambasova, M. O. Gornakov, M. V. Dorokhin & Yu. Yu. Loginov Physics Of The Solid State. DOI https://doi.org/10.1134/S1063783420060311

The single-walled carbon nanotube-based thin films with a thickness from 11 ± 3 to 157 ± 18 nm have been formed using vacuum filtration. The thermal conductivity of the thin films as a function of thickness and temperature up to 450 K has been studied by the 3ω technique. It has been found that, in the region of 49 nm, the supplied heat from a gold strip started propagating with the high efficiency to the thin film plane. The thermal conductivity of the thin films with a thickness of 49 ± 8 nm was measured using the 3ω technique for bulk samples. It has been found that the thermal conductivity of the single-walled carbon nanotube-based thin films strongly depends on their thickness and temperature. The thermal conductivity sharply (by a factor of ~60) increases with an increase in thickness from 11 ± 3 to 65 ± 4 nm. In addition, it has been observed that the thermal conductivity of the thin film with a thickness of 157 ± 18 nm rapidly decreases from 211 ± 11 to 27.5 ± 1.4 W m–1 K–1 at 300 and 450 K, respectively.

Mode coupling in arrays of Al nanoparticles

Ershov, Alexander E.; Gerasimov, Valeriy S.; Bikbaev, Rashid G.; Polyutov, Sergey P.; Karpov, Sergey, V Journal Of Quantitative Spectroscopy & Radiative Transfer. https://doi.org/10.1016/j.jqsrt.2020.106961

The mechanisms of coupling between the lattice modes of a two-dimensional (2D) array consisting of Al nanoparticles and the localized modes of individual Al nanoparticles have been studied in detail. The results were obtained employing the finite-difference time-domain method (FDTD) and the generalized Mie theory. It was shown that interactions of single particles with 2D lattice modes significantly change the extinction spectra depending on the particle radius and the lattice period. The Rayleigh anomalies of higher orders contribute to formation of hybrid modes resulting in increase of the extinction efficiency in short wavelength range of the spectrum. It was shown that high intensity magnetic modes are excited in aluminum nanoparticles arrays. The patterns of spatial electromagnetic field distribution at the frequencies of hybrid modes have been studied. We note that comprehensive understanding the mode coupling mechanisms in arrays paves the way for engineering different types of modern photonic devices with controllable optical properties.

Prediction of orientation relationships and interface structures between alpha-, beta-, gamma-FeSi2 and Si phases

Visotin, Maxim A.; Tarasov, I. A.; Fedorov, A. S.; Varnakov, S. N.; Ovchinnikov, S. G. Acta Crystallographica Section B-structural Science Crystal Engineering And Materials. https://doi.org/10.1107/S2052520620005727

A pure crystallogeometrical approach is proposed for predicting orientation relationships, habit planes and atomic structures of the interfaces between phases, which is applicable to systems of low-symmetry phases and epitaxial thin film growth. The suggested models are verified with the example of epitaxial growth of α-, γ- and β-FeSi2 silicide thin films on silicon substrates. The density of near-coincidence sites is shown to have a decisive role in the determination of epitaxial thin film orientation and explains the superior quality of β-FeSi2 thin grown on Si(111) over Si(001) substrates despite larger lattice misfits. Ideal conjunctions for interfaces between the silicide phases are predicted and this allows for utilization of a thin buffer α-FeSi2 layer for oriented growth of β-FeSi2 nanostructures on Si(001). The thermal expansion coefficients are obtained within quasi-harmonic approximation from the DFT calculations to study the influence of temperature on the lattice strains in the derived interfaces. Faster decrease of misfits at the α-FeSi2(001)||Si(001) interface compared to γ-FeSi2(001)||Si(001) elucidates the origins of temperature-driven change of the phase growing on silicon substrates. The proposed approach guides from bulk phase unit cells to the construction of the interface atomic structures and appears to be a powerful tool for the prediction of interfaces between arbitrary phases for subsequent theoretical investigation and epitaxial film synthesis.

A Highly Selective Stripline Lowpass Filter with More Than 100-dB Wide Stopband Attenuation

Belyaev, B. A.; Serzhantov, A. M.; Leksikov, An. A.; Bal'va, Ya. F.; Grushevskii, E. O.; et al. Technical Physics Letters. https://doi.org/10.1134/S1063785020040173

A new miniature design of a highly selective lowpass filter based on a suspended substrate with a two-sided stripline pattern has been developed. The filter frequency response slope (cutoff attenuation rate) and stopband attenuation depth are determined by transmission zeros, the number of which is equal to the filter order. An experimental prototype of a fifth-order lowpass filter on 0.5-mm-thick alumina substrate with dielectric permittivity ε = 9.8 has been synthesized with the aid of numerical electrodynamic analysis of a three-dimensional model. The cutoff frequency of the filter passband at a –1-dB level is   fc = 1.75 GHz. The stopband width at a –100-dB attenuation level reaches 4.4fc.

Calorimetric, dilatometric and DTA under pressure studies of the phase transitions in elpasolite (NH4)(2)KZrF7

Gorev, Mikhail, V; Kartashev, Andrey, V; Bogdanov, Evgeniy, V; Flerov, Igor N.; Laptash, Natalia M. Journal Of Fluorine Chemistry. https://doi.org/10.1016/j.jfluchem.2020.109523

Heat capacity, thermal expansion, and sensitivity to the hydrostatic pressure of (NH4)2KZrF7 elpasolite are studied in a wide temperature range. The changes in deformation and entropy during successive phase transitions are determined: Δ(ΔV/V) = 3·10−4; ΔS = 8 J/mol K The temperatures and entropies of phase transitions turned out to be slightly sensitive to pressure changes. An analysis of the entropy of phase transformations was performed in the framework of the model of the cubic phase structure Fm-3 m. In the low temperature phase, an anomalous behavior of thermodynamic properties, which is not characteristic of phase transitions, was observed, accompanied by a significant change in the crystal lattice entropy.

Quantum entanglement and the Born-Markov approximation for an open quantum system

Kolovsky, Andrey R. Physical Review E. DOI:https://doi.org/10.1103/PhysRevE.101.062116

We revisit the Born-Markov approximation for an open quantum system by considering a microscopic model of the bath, namely, the Bose-Hubbard chain in the parameter region where it is chaotic in the sense of quantum chaos. It is shown that strong ergodic properties of the bath justify all approximations required for deriving the Markovian master equation from the first principles.

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