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We present Raman spectroscopy of the polycrystalline Si and Ge films deposited by molecular beam deposition on a dielectric substrate. The Raman study has been made using lasers with different wavelengths. Structural properties of the poly-films have been studied by XRD and TEM. The Raman spectra are characterized by appearance of the additional wide peaks around 500 cm$^{-1}$ and 290 cm$^{-1}$ in the main vibrational bands of TO(c-Si) and TO(c-Ge) phonons, respectively. It is shown that these peaks correspond to scattering in grain boundary area. For the poly-Si films, both a downward shift and an asymmetrical broadening of the vibrational band of TO(c-Si) near 520 cm$^{-1}$ are observed, whereas there is only a symmetric broadening in the spectra of poly-Ge. The Raman line shape has been modeled within the framework of the phonon confinement theory taking into account the sizes of coherent scattering domains obtained using XRD. The model includes a symmetrical band broadening observed in polycrystalline films. It is shown that confinement of phonon propagation might be in the poly-Si films. The phonon dispersion and the density of phonon states have been simulated using density functional theory. It has been found that phonon confinement relates to grain boundaries rather than other extended defects such as twins (multiple twins, twin boundaries), the appearance of which does not lead to significant changes in phonon dispersion and density of phonon states., Comment: 46 pages, 23 figures, 1 table

Nanoparticles of elemental Boron are generated for the first time by laser ablation of a sintered Boron target in liquid isopropanol and subsequent laser fragmentation of the suspension. For this purpose an ytterbium doped fiber laser was used at wavelength of 1060-1070 nm, pulse repetition rate of 20 kHz, and pulse duration of 200 ns. The size of Boron nanoparticles after ablation and fragmentation of the suspension is around 30 nm. Nanoparticles are made mostly of Boron and carbon, some particles have carbon shell. Allotropic composition of nanoparticles differs from that of the initial Boron target. Possible applications are discussed., Comment: 13 pages, 10 figures

The technique of laser ablation in liquids is applied to produce of Boron-containing nanoparticles from ablation of a Fe$_2$B bulk target enriched in 10B isotope. Laser ablation of the target in liquid isopropanol results in partial disproportionation to free Fe and Boron while nanoparticles of Fe$_2$B are also presented. Nanoparticles are magnetic and can be collected using a permanent magnet. Average size of nanoparticles is of 15 nm. The content of 10B in generated nanoparticles amounts to 76,9 %. Nanoparticles are biocompatible and can be used in Boron Neutron Capture Therapy.

Ge/Si(001) multilayer heterostructures containing arrays of low-temperature self-assembled Ge quantum dots and very thin Si$_x$Ge$_{1-x}$ layers of varying composition and complex geometry have been studied using Raman spectroscopy and scanning tunneling microscopy. The dependence of Raman spectra on the effective thickness of deposited Ge layers has been investigated in detail in the range from 4 to 18 \r{A}. The position and shape of both Ge and SiGe vibrational modes are of great interest since they are closely related to the strain and composition of the material that plays a role of active component in perspective optoelectronic devices based on such structures. In this work, we present an explanation for some peculiar features of Raman spectra, which makes it possible to control the quality of the grown heterostructures more effectively. A dramatic increase of intensity of both the Ge$-$Ge and Si$-$Ge bands for the structure containing Ge layers of 10 \r{A} and anomalous shift and broadening of the Si$-$Ge band for structures comprising Ge layers of 8 and 9 \r{A} thick were observed. In our model, the anomalous behavior of the Raman spectra with the change of thickness of deposited Ge is connected with the flatness of Ge layers as well as transitional SiGe domains formed via the stress-induced diffusion from {105} facets of quantum dots. The conclusions are supported by the STM studies and the numerical calculations., Comment: 17 pages, 11 figures

Forces induced by quantum fluctuations of electromagnetic field control adhesion phenomena between rough solids when the bodies are separated by distances ~10nm. However, this distance range remains largely unexplored experimentally in contrast with the shorter (van der Waals forces) or the longer (Casimir forces) separations. The reason for this is the pull-in instability of the systems with the elastic suspension that poses a formidable limitation. In this paper we propose a genuine experimental configuration that does not suffer from the short distance instability. The method is based on adhered cantilever, whose shape is sensitive to the forces acting near the adhered end. The general principle of the method, its possible realization and feasibility are extensively discussed. The dimensions of the cantilever are determined by the maximum sensitivity to the forces. If the adhesion is defined by strong capillary or chemical interactions, the method loses its sensitivity. Special discussion is presented for the determination of the minimum distance between the rough solids upon contact, and for the compensation of the residual electrostatic contribution. The proposed method can be applied to any kind of solids (metals, semiconductors, dielectrics) and to any intervening medium (gas or liquid)., Comment: 27 pages, 8 figures, to be published in Phys. Rev. Applied

Using known relation between $SU(2,2|4)$ supertwistors and $SU(2)$ bosonic and fermionic oscillators we identify the physical states of quantized massless $AdS_5\times S^5$ superparticle in supertwistor formulation and discuss how they fit into the spectrum of fluctuations of IIB supergravity on $AdS_5\times S^5$ superbackground., Comment: 17 pages, v2: expanded version accepted for publication in Physics Letters B

Supertwistors relevant to $AdS_5\times S^5$ superbackground of IIB supergravity are studied in the framework of the $D=10$ massless superparticle model in the first-order formulation. Product structure of the background suggests using $D=1+4$ Lorentz-harmonic variables to express momentum components tangent to $AdS_5$ and $D=5$ harmonics to express momentum components tangent to $S^5$ that yields eight-supertwistor formulation of the superparticle's Lagrangian. We find incidence relations of the supertwistors with the $AdS_5\times S^5$ superspace coordinates and the set of the quadratic constraints they satisfy. It is shown how using the constraints for the (Lorentz-)harmonic variables it is possible to reduce eight-supertwistor formulation to the four-supertwistor one. Respective supertwistors agree with those introduced previously in other models. Advantage of the four-supertwistor formulation is the presence only of the first-class constraints that facilitates analysis of the superparticle model., Comment: 25 pages. v2: presentation refined at the end of Sec.5, typos corrected. Version accepted for publication

The method of software analysis of high-resolution TEM images using the peak pairs algorithm in combination with Raman spectroscopy was employed to study lattice deformations in Ge/Si(001) structures with low-temperature Ge quantum dots. It was found that the stresses do not spread in a thick Si layer above quantum dots, but completely relax via the formation of a thin boundary layer of mixed composition. However, intermixing of Ge and Si is absent beneath the Ge layer in samples with a Ge coverage of 10 \r{A}. Besides intermixing was not observed at all, both beneath and above the Ge layer, in samples with a Ge coverage of 6 \r{A} or less. This may be due to the predominance of Ge diffusion into the Si matrix from the {105} facets of Ge huts, not from the Ge wetting layer, at low temperatures of the Ge/Si structure deposition. The critical thickness of Si coverage at which the intense stress-induced diffusion takes place is determined to lie in the range from 5 to 8 nm., Comment: 18 pages, 9 figures; accepted to Semicond. Sci. Technol. (2020)

The first-principles calculations of the electronic structures and the interband optical conductivity (OC) spectra have been performed for the stoichiometric Fe$_{2}$MnGa alloy with L2$_{1}$ and L1$_{2}$ types of atomic ordering. The calculated optical properties of Fe$_{2}$MnGa alloy for the L2$_{1}$ and L1$_{2}$ phases are complemented by the experimental OC spectra for bulk and thin film Fe-Mn-Ga alloy samples near the stoichiometry 2:1:1 with L2$_{1}$ and L1$_{2}$ (for bulks) as well as the body-centered-cubic and face-centered-cubic (for films) structures, respectively. A reasonable agreement between experimental and calculated interband OC spectra was obtained for both phases of the alloy. The experimental data show no significant difference in the OC spectra with respect to the degrees of atomic and magnetic orders of the samples., Comment: a to J. Phys.: Condens. Matter

Effect of the temperature and magnetic field induced martensitic transformation (MT) on the electronic structure and some physical properties of bulk Fe$_{45.2}$Mn$_{25.9}$Ga$_{28.9}$ Heusler alloy has been investigated. {According to the experimental results of DSC, magnetic and transport measurements direct and reverse martensitic transformation without external magnetic field takes place within 194 $\leq T \leq$ 328 K temperature range with a hysteresis up to $\Delta T \approx$ 100 K defined as $\Delta T$ = $A_{f,s}$ - $M_{s,f}$, where $A_{f,s}$ and $M_{s,f}$ are the critical temperatures of direct and reverse martensitic transformation. External magnetic field of $\mu_{0}H$ = 5 T causes a high-temperature shift of MT temperatures.} MT from parent austenite L2$_{1}$ phase to martensitic tetragonally distorted L2$_{1}$ one (i. e. to L1$_{0}$) causes significant changes in the electronic structure of alloy, a drastic increase in alloy magnetization, a decrease in the alloy resistivity, and a reversal of the sign of the temperature coefficient of resistivity from negative to positive. At the same time experimentally determined optical properties of Fe$_{45.6}$Mn$_{25.9}$Ga$_{28.9}$ Heusler alloy in austenitic and martensitic states look visually rather similar being noticeable different in microscopic nature as can be concluded from first-principle calculations. Experimentally observed changes in the physical properties of the alloy are discussed in terms of the electronic structures of an austenite and martensite phases., Comment: Intermetallics, submitted

Magnetic and transport properties of near-stoichiometric metastable FexMnyGaz alloys (46

One-step laser generation of Au elongated nanoparticles (NPs) and their successive fragmentation and agglomeration are experimentally studied for the first time. In the present work, laser-assisted formation of Au elongated nanoparticles by ablation of a solid Au (99.99%) target in water was done using a ytterbium-doped fiber laser sources with pulse duration of 200 ns and pulse energy of 1mJ. Extinction spectrum correlating with TEM shows the appearance of absorption signal in red region and near IR-spectrum that corresponds to longitudinal plasmon resonance of electrons in elongated Au NPs. In addition, generated elongated Au nanoparticles were exposed to pulsed laser beam with various pulse energy and laser exposure time. It was found that at early stages of irradiation NPs agglomerate as the NPs chains with size of order of 1 micrometer long. Further laser exposure results in fragmentation of these chains. Possible processes of laser-assisted formation of elongated Au NPs in aqueous solutions of calcium chloride and their subsequent interaction with pulsed laser irradiation are discussed., Comment: Reported at ANGEL2018 conference, Lyon, France

Optical bistability of exciton polaritons in semiconductor microcavities is a promising platform for digital optical devices. Steady states of coherently driven polaritons can be toggled in tens of picoseconds by a short external pulse of appropriate amplitude and phase. We have analyzed the switching behavior of polaritons depending on the pulse amplitude, phase, and duration. The switches are found to change dramatically when the inverse pulse duration becomes comparable to the frequency detuning between the driving field and polariton resonance. If the detuning is large compared to the polariton linewidth, the system becomes extremely sensitive to initial conditions and thus can serve as a fast random-number generator., Comment: 6 pages, 7 figures

Elongated gold nanoparticles obtained by laser ablation in liquid demonstrate ferromagnetic behavior in external magnetic field., Comment: Reported at ANGEL2018 conference, Lyon, France

Starting with the first-order formulation of the massless superparticle model on the $AdS_5\times S^5$ superbackground and presenting the momentum components tangent to $AdS_5$ and $S^5$ subspaces as bilinear combinations of the constrained $SU(2)$-Majorana spinors allows to bring the superparticle's Lagrangian to the form quadratic in supertwistors. The $SU(2,2|4)$ supertwistors are assembled into a pair of $SU(2)$ doublets, one of which has even $SU(2,2)$ and odd $SU(4)$ components, while the other has odd $SU(2,2)$ and even $SU(4)$ components. They are subject to the first-class constraints that generate the $psu(2|2)\oplus u(1)$ gauge algebra. This justifies previously proposed group-theoretic definition of the $AdS_5\times S^5$ supertwistors and allows to derive the incidence relations with the $(10|32)$ supercoordinates of the $AdS_5\times S^5$ superspace. Whenever superparticle moves within the $AdS_5$ subspace of the $AdS_5\times S^5$ space-time, twistor formulation of its Lagrangian involves just one $SU(2)$ doublet of $SU(2,2|4)$ supertwistors with even $SU(2,2)$ and odd $SU(4)$ components. If in addition particle's 5-momentum is null, four first-class constraints which are the $su(2)\oplus u(1)$ generators single out upon quantization the states of $D=5$ $N=8$ gauged supergravity multiplet in the superambitwistor formulation., Comment: 24 pages, LaTeX; v4: in Sec.2.3 corrected wrong expressions for variations of supertwistors and Lagrange multipliers under gauge symmetries

Lack of fast and strong actuators to drive microsystems is well recognized. Electrochemical actuators are considered attractive for many applications but they have long response time (minutes) due to slow gas termination. Here an electrochemical actuator is presented for which the response time can be as short as 1 ms. The alternating polarity water electrolysis is used to drive the device. In this process only nanobubbles are formed. The gas in nanobubbles can be terminated fast due to surface assisted reaction between hydrogen and oxygen that happens at room temperature. The working chamber of the actuator contains concentric titanium electrodes; it has a diameter of 500 um and a height of 8 um. The chamber is sealed by a polydimethylsiloxane (PDMS) membrane of 30 um thick. The device is characterized by an interferometer and a fast camera. Cyclic operation at frequency up to 667 Hz with a stroke of about 30% of the chamber volume is demonstrated. The cycles repeat themselves with high precision providing the volume strokes in picoliter range. Controlled explosions in the chamber can push the membrane up to 90 um., Comment: author's version, 8 pages, 9 figures (to appear in Sensors and Actuators B: Chemical)