Showing papers in "Physics of the Solid State in 2014"

Synthesis of epitaxial silicon carbide films through the substitution of atoms in the silicon crystal lattice: A review

Abstract: A review of recent advances in the field of epitaxial growth of SiC films on Si by means of a new method of epitaxial substitution of film atoms for substrate atoms has been presented. The basic statements of the theory of the new method used for synthesizing SiC on Si have been considered and extensive experimental data have been reported. The elastic energy relaxation mechanism implemented during the growth of epitaxial SiC films on Si by means of the new method of substitution of atoms has been described. This method consists in substituting a part of carbon atoms for silicon matrix atoms with the formation of silicon carbide molecules. It has been found experimentally that the substitution for matrix atoms occurs gradually without destroying the crystalline structure of the matrix. The orientation of the film is determined by the “old” crystalline structure of the initial silicon matrix rather than by the silicon substrate surface only, as is the case where conventional methods are used for growing the films. The new growth method has been compared with the classical mechanisms of thin film growth. The structure and composition of the grown SiC layers have been described in detail. A new mechanism of first-order phase transformations in solids with a chemical reaction through an intermediate state promoting the formation of a new-phase nuclei has been discussed. The mechanism providing the occurrence of a wide class of heterogeneous chemical reactions between the gas phase and a solid has been elucidated using the example of the chemical interaction of the CO gas with the single-crystal Si matrix. It has been shown that this mechanism makes it possible to grow a new type of templates, i.e., substrates with buffer transition layers for growing wide-band-gap semiconductor films on silicon. A number of heteroepitaxial films of wide-band-gap semiconductors, such as SiC, AlN, GaN, and AlGaN on silicon, whose quality is sufficient for the fabrication of a wide class of micro- and optoelectronic devices, have been grown on the SiC/Si substrate grown by solid-phase epitaxy.

Electronic structure of silicon dioxide (a review)

Abstract: Silicon dioxide amorphous films are the key insulators in silicon integrated circuits. The physical properties of silicon dioxide are determined by the electronic structure of this material. The currently available information on the electronic structure of silicon dioxide has been systematized.

Synthesis, structure, and magnetic properties of iron and nickel nanoparticles encapsulated into carbon

Abstract: Nanocomposites based on iron and nickel particles encapsulated into carbon (Fe@C and Ni@C), with an average size of the metal core in the range from 5 to 20 nm and a carbon shell thickness of approximately 2 nm, have been prepared by the gas-phase synthesis method in a mixture of argon and butane. It has been found using X-ray diffraction, transmission electron microscopy, and Mossbauer spectroscopy that iron nanocomposites prepared in butane, apart from the carbon shell, contain the following phases: iron carbide (cementite), α-Fe, and γ-Fe. The phase composition of the Fe@C nanocomposite correlates with the magnetization of approximately 100 emu/g at room temperature. The replacement of butane by methane as a carbon source leads to another state of nanoparticles: no carbon coating is formed, and upon subsequent contact with air, the Fe3O4 oxide shell is formed on the surface of nanoparticles. Nickel-based nanocomposites prepared in butane, apart from pure nickel in the metal core, contain the supersaturated metastable solid solution Ni(C) and carbon coating. The Ni(C) solid solution can decompose both during the synthesis and upon the subsequent annealing. The completeness and degree of decomposition depend on the synthesis regime and the size of nickel nanoparticles: the smaller is the size of nanoparticles, the higher is the degree of decomposition into pure nickel and carbon. The magnetization of the Ni@C nanocomposites is determined by several contributions, for example, the contribution of the magnetic solid solution Ni(C) and the contribution of the nonmagnetic carbon coating; moreover, some contribution to the magnetization can be caused by the superparamagnetic behavior of nanoparticles.

Deformation resistance and fracture of iron over a wide strain rate range

Abstract: The results of measurements of the decay of an elastic precursor in iron at the distances from 0.13 to 10 mm and the spall strength of the samples with such thicknesses have been compared with similar data for the nanometer-scale samples. The decay has been described by a unique dependence whose differentiation gives the relationship between the initial plastic strain rate in the range from 103 to 109 s−1 and the compression stress in the elastic shock wave from 1.5 to 27.5 GPa. The dynamic breaking strength (spall strength) varies in this range of shock-wave load time from 1.5 to 20 GPa.

First-order phase transition through an intermediate state

Abstract: The theory of first-order phase transitions in systems where the direct formation of nuclei of a new phase is inhibited for any reason, for example, because of the extremely high elastic energy, has been constructed using the example of the silicon-silicon carbide phase transition due to the chemical reaction with carbon monoxide. It has been shown that, in this case, the phase transition occurs through an intermediate state, which significantly promotes the formation of new-phase nuclei. For the silicon-silicon carbide phase transition, such an intermediate state is the “pre-carbide” state of silicon saturated with dilatation dipoles, i.e., pairs formed by a carbon atom and a silicon vacancy that are strongly attracted to each other. The model dependence of the potential energy of systems with an intermediate phase on the reaction coordinates has been investigated. The kinetics of transformation of the intermediate state into a new phase has been described.

An extended critical state model: Asymmetric magnetization loops and field dependence of the critical current of superconductors

Abstract: An extended critical state model has been developed. The model has considered the equilibrium magnetization of a surface layer and the magnetization of the central region of a superconducting sample. The magnetic flux distributions in the sample have been calculated. An analytical dependence of the critical cur� rent density on the magnetic field with different behaviors in strong and weak fields has been proposed. A rela� tion of the asymmetry of the magnetization loops and the critical current density to the sample size has been established. The model is applicable to the parameterization of magnetization loops of singlecrystal and polycrystalline superconductors.

Specific features of the optical and electrical properties of polycrystalline CdTe films grown by the thermal evaporation method

Abstract: The results of studying the physical properties of thin CdTe films obtained by the thermal evaporation method have been presented. The optical constants and the band gap of the films under study have been determined (Eg = 1.46 eV). It has been established based on the investigation of optical properties and the Raman spectrum of the films that they possess high structural quality. The activation energy of the electrical conductivity of CdTe films has been determined: Ea = 0.039 eV. The measured spectral dependences of the impedance of CdTe thin films are characteristic of the inhomogeneous medium with two time constants: τgb = RgbCgb = 1/ωgb = 1.62 × 10−3 s and τg = RgCg = 1/ωg = 9.1 × 10−7 s for grain boundaries and grains, respectively.

Mechanical properties of bulk carbon nanomaterials

Abstract: Bulk carbon nanomaterials, which open prospects for the development of a new generation of supercapacitors, are actively investigated for recent years, but their mechanical properties and structure remain poorly understood. In connection with this fact, the influence of the hydrostatic and uniaxial compression on mechanical properties and structure of three bulk nanomaterials consisting of (i) bent graphene flakes, (ii) short carbon nanotubes, and (iii) fullerenes C240 are investigated by the molecular dynamics method. It is shown that the strength of the material and its stability to graphitization depend on its constituent structural units. At large degrees of deformation, the material consisting of bent graphene sheets has the highest strength, whereas at the material density lower than 2.5 g/cm3, the highest strength is observed in the nanomaterial consisting of fullerene molecules. The differences in mechanical properties of the materials under consideration are explained by their structural features.

Magnetic and Piezoelectric Properties of the Bi 1- x La x FeO 3 System near the Transition from the Polar to Antipolar Phase

Abstract: The crystal structure, piezoelectric and magnetic properties of the Bi1 − x La x FeO3 solid-solution system near the structural transition between the rhombohedral and orthorhombic phases (0.15 ≤ x ≤ 0.2) have been investigated. The regions of existence of the polar rhombohedral and orthorhombic phases have been determined, and the sequence of structural transitions as a function of the lanthanum ion concentration and temperature has been studied. The maximum piezoelectric signal is found for the solid solution with the composition x = 0.16, which has a single-phase rhombohedral structure. The relation between the type of crystal structure distortions and the increase in the magnetization upon the concentration-driven structural transition from the polar to antipolar phase has been established.

Misfit dislocation loops in composite core-shell nanoparticles

Abstract: The critical conditions have been calculated for the generation of circular prismatic loops of misfit dislocations at the interfaces in spherically symmetric composite core-shell nanoparticles. It has been shown that the formation of these loops becomes energetically favorable if the misfit parameter exceeds a critical value, which is determined by the geometry of the system. The most preferred position of the dislocation loop is in the equatorial plane of the nanoparticle. For a given radius of the nanoparticle, there is a minimum value of the critical misfit parameter below which the generation of a misfit dislocation is energetically unfavorable for any ratio of the core and shell radii. For a misfit parameter exceeding the minimum critical value, there are two critical values of the reduced radius of the particle core in the interval between which the generation of a dislocation loop is energetically favorable. This interval increases with increasing misfit parameter for a fixed particle size and decreases with decreasing particle size for a fixed misfit parameter.

Calculation of diffusion coefficients of defects and ions in UO2

Abstract: This paper has presented molecular dynamics calculations of the diffusion coefficients of interstitials, vacancies, and vacancy complexes of oxygen and uranium in UO2, as well as the coefficients of ion diffusion provided by these defects. The interatomic potentials have been chosen by comparing the defect formation energies with data of the DFT + U calculations. The results of the calculations have been compared with experimental data on the annealing of defects and the measurements of self-diffusion coefficients of ions. The limitations of the model of point defects for the description of the self-diffusion in nominally stoichiometric UO2 have been discussed.

Edge-modified zigzag-shaped graphene nanoribbons: Structure and electronic properties

Abstract: Control of the band gap of graphene nanoribbons is an important problem for the fabrication of effective radiation detectors and transducers operating in different frequency ranges. The periodic edge-modified zigzag-shaped graphene nanoribbon (GNR) provides two additional parameters for controlling the band gap of these structures, i.e., two GNR arms. The dependence of the band gap E g on these parameters is investigated using the π-electron tight-binding method. For the considered nanoribbons, oscillations of the band gap E g as a function of the nanoribbon width are observed not only in the case of armchair-edge graphene nanoribbons (as for conventional graphene nanoribbons) but also for zigzag GNR edges. It is shown that the change in the band gap E g due to the variation in the length of one GNR arm is several times smaller than that due to the variation in the nanoribbon width, which provides the possibility for a smooth tuning of the band gap in the energy spectrum of the considered graphene nanoribbons.

Ferromagnetic magnetization switching by an electric field: A review

Abstract: Possible methods and problems of the development of magnetoresistive memory with electric field assisted writing have been considered. It has been shown that the most promising is the memory based on the compensated cut of multiferroic bismuth ferrite BiFeO3. Small values of the weak ferromagnetic moment and linear magnetoelectric effect in BiFeO3 are not obstacles to the realization of the magnetoresistive memory. Of interest is the memory switchable via piezoelectric layer induced elastic stresses, which uses the bistability of magnetization of the ferromagnetic layer.

Magnetization reversal processes in layered high-temperature superconductors with ferromagnetic impurities

Abstract: The self-consistent interaction of a vortex system of a high-temperature superconductor and ferromagnetic impurities, including single impurities and their clusters, has been considered in the model of a layered high-temperature superconductor. For different temperatures and concentrations of ferromagnetic impurities, the magnetization reversal loops have been calculated by the Monte Carlo method taking into account an ensemble of ferromagnetic particles with different orientations of their easy magnetization axes with respect to the direction of an external magnetic field and for different magnetic anisotropy energies. It has been demonstrated that there is a nonlinear interaction of the high-temperature superconductor with ferromagnetic impurities, in which the initially thermodynamically reversible character of the magnetization reversal of the ferromagnetic ensemble can become irreversible. For a periodic lattice of clusters of ferromagnetic impurities, the magnetization curves of the high-temperature superconductor have been calculated for different sizes and configurations of the clusters. It has been revealed that, when extended defects are oriented parallel to the direction of the entrance of vortices in the sample, the length of the defects does not affect the remanent magnetization. It has been shown that the inclusion of the interaction between the magnetic moments inside the impurity cluster leads to a decrease in the magnetization reversal loop, the coercivity, and, accordingly, the energy loss due to magnetization reversal.

Single-domain magnetic nanoparticles in an alternating magnetic field as mediators of local deformation of the surrounding macromolecules

Abstract: The forces, deformations, and stresses generated in macromolecules attached to single-domain magnetic nanoparticles under the influence of a low-frequency (nonheating) magnetic field have been analyzed analytically and numerically. It has been shown that, in bioactive macromolecules, an alternating magnetic field with an induction of 0.1–1.0 T and a circular frequency of ≲104 s−1 can induce forces up to several hundred piconewtons, absolute deformations up to a few tens of nanometers, as well as compressive and shear stresses exceeding 107 Pa. These mechanical stimuli are sufficient for a significant change of interatomic distances in active centers, conformation of macromolecules, and even a breaking of some bonds, which makes it possible to develop a new technological platform for targeted delivery of drugs, remote control of their activity, and cancer-cell destruction.

Maximum yield strength under quasi-static and high-rate plastic deformation of metals

Abstract: The dependence of the yield strength of metals on the grain size and initial dislocation density in a wide range of strain rates has been analyzed within a unified approach. It has been shown that the barrier stress and characteristic time of plastic relaxation completely determine the shear strength of metals for all strain rates. The existence of alternative (to dislocation glide) mechanisms of plastic deformation in the material, limits the increase in the yield strength with increasing strain rate and leads to the appearance of a maximum in the dependence of the yield strength on the grain size. It has been found that, at extremely high strain rates, the maximum yield strength corresponds to grain sizes of the order of several hundred nanometers. This has been explained by the dislocation starvation effect of the material.

Structural and magnetic inhomogeneities, phase transitions, 55 Mn nuclear magnetic resonance, and magnetoresistive properties of La 0.6 − x Nd x Sr 0.3 Mn 1.1 O 3-δ ceramics

Abstract: The structure, lattice imperfection, and properties of ceramic samples La0.6 − x Nd x Sr0.3Mn1.1O3-δ (x = 0–0.4) have been investigated using the X-ray diffraction, resistive, magnetic (χac, 55Mn NMR), magnetoresistive and microscopic methods. It has been shown that there is a satisfactory agreement between the concentration decrease in the lattice parameters a of the rhombohedral (x = 0, 0.1, 0.2) and cubic (x = 0.3, 0.4) perovskite structures and the average ionic radii $$\bar R$$ for the lattice containing anion vacancies, cation vacancies, and nanostructured clusters with Mn2+ ions in A-positions. With an increase in the neodymium concentration x, the vacancy-type imperfection increases, the cluster-type imperfection decreases, the temperatures of metal-semiconductor phase transition T ms and ferromagnetic-paramagnetic phase transition T C decrease, and the content of the ferromagnetic phase decreases. The anomalous hysteresis is associated with the appearance of unidirectional exchange anisotropy induced in a clustered perovskite structure consisting of a ferromagnetic matrix and a planar antiferromagnetic cluster coherently coupled with it. An analysis of the asymmetrically broadened 55Mn NMR spectra has revealed a high-frequency electronic double exchange (Mn3+-O2−-Mn4+) ↔ (Mn4+-O2−-Mn3+) and an inhomogeneity of the magnetic and charge states of manganese due to the heterogeneous environment of the manganese ions by other ions and defects. The observed changes in the resonant frequency and width of the resonance curve are caused by changes in the ratio Mn3+/Mn4+ and magnetic inhomogeneity. An increase in the neodymium concentration x leads to a decrease in the ferromagnetic phase content determined from the dependences 4πNχac(T) and the 55Mn NMR curves. The phase diagram characterizes an interrelation between the composition, the imperfection of the structure, and the transport, magnetic, and magnetoresistive properties of lanthanum neodymium manganite perovskites. It has been found that there is a correlation between the imperfection, magnetic inhomogeneity, coercive force, and magnetoresistance effect exhibited by the perovskite structure.

Converse flexoelectric effect in the SrTiO 3 single crystal

Abstract: The converse flexoelectric effect in the SrTiO3 single crystal as a response of inhomogeneous strain (bending strain) to an applied electric field has been studied. The temperature dependence of the effect in the temperature range of 77–450 K has been obtained.

Temperature dependence of elastic moduli of submicrocrystalline copper

Abstract: The possibility of applying the two-component model of single-phase hybrid materials to the explanation of the anomalous temperature dependence of elastic moduli of copper with a submicrocrystalline structure has been discussed. An analysis has been performed based on new experimental data. A twin mechanism of changes in the crystallite orientation has been proposed. These changes cause the anomalous behavior of the elastic moduli of copper during deformation and subsequent heatings.

Size dependence of the activation energy of diffusion in multilayer Cu-Ni films

Abstract: The results of studying the effect of the characteristic size on the rate of diffusion processes in nanometer Cu-Ni film systems have been reported. The film system has been prepared by sequential vacuum deposition of the components, and the activation energy of diffusion has been determined from a change in the electrical resistance of the film system in a heating-cooling cycle. It has been shown that the activation energy of grain-boundary diffusion decreases with decreasing characteristic size of the system and amounts to 0.25 eV for the film system with a characteristic size of 5 nm, which corresponds to an increase in the grain-boundary diffusion coefficient by 10 orders of magnitude with respect to massive samples.

Simulation of metastable CL-20 cluster structures

Abstract: Ensembles of C6H6N12O12 (CL-20) clusters with different types of intercluster bonds have been studied theoretically. The stability of such cluster has been investigated and the heights of potential barriers preventing their decomposition or isomerization have been determined by means of quantum-mechanical calculations based on the density functional theory and nonorthogonal tight-binding model. From the analysis of molecular dynamics data and potential energy hypersurface of these metastable configurations, it has been established that dimers and tetramers of CL-20 clusters are characterized by sufficiently high kinetic stability, which suggests the theoretical possibility of creation of high-energy covalent crystals on their basis.

Martensitic transformations and magnetic properties of nonstoichiometric alloys of the Ni-Mn-In system

Abstract: Martensitic transformations and magnetic properties of Ni89-xMnxIn11 (42 ≤ x ≤ 44) alloys have been investigated. Critical temperatures of magnetic and structural phase transitions in the studied alloy system have been determined. It has been shown that the martensitic transformation induced by the magnetic field is observed in all alloys. Temperature dependences of the spontaneous magnetization of austenite and martensite as well as the magnitude of the critical field, in which martensitic transformation occurs, have been determined.

Mechanical properties and structural features of nanocrystalline titanium produced by cryorolling

Abstract: A broad spectrum of physicomechanical properties of the VT1-0 nanocrystalline titanium produced by cryomechanical fragmentation of the grain structure using rolling at a temperature close to liquid-nitrogen temperature has been studied. It has been found that the mechanism of grain refinement is associated with grain fragmentation by twins. Exactly the twin nature of internal interfaces (crystallite boundaries) provides the thermal and structural stability of nanocrystalline titanium produced by cryomechanical grain fragmentation in the temperature range to ∼500 K. It has been assumed that the observed decrease in the titanium density due to cryorolling is associated with a number of factors (high density of introduced dislocations, nanopore formation, and changes in titanium lattice parameters).

Thermal expansion of nanostructured PbS films and anharmonicity of atomic vibrations

Abstract: The dependences of the coherent scattering region size and thermal expansion coefficient α of a PbS nanofilm on the annealing temperature in the range of 293–473 K and on the duration of annealing at a constant temperature of 423 K have been measured. It has been found that the thermal expansion coefficient α of the PbS nanofilm is almost twice as much as the coefficient α of coarse-grained lead sulfide. It has been shown that the large difference in the coefficients α is associated with the small size of particles in the film, which leads to an increase in the anharmonicity of atomic vibrations. The contribution from the small size of particles to the thermal expansion coefficient of the PbS nanofilm has been evaluated theoretically.

Effect of temperature and magnetic field on the evolution of a vortex structure of the granular YBa 2 Cu 3 O 7 - δ high-temperature superconductor

Abstract: The temperature dependences of the electrical resistivity ρ(T) H = const have been measured in external magnetic fields H ext (0 ≤ H ext ≤ 1420 Oe) at temperatures ranging from 70 to 273 K for samples of the granular YBa2Cu3O7 − δ high-temperature superconductor (HTSC). Cooling of the samples to the minimum temperature T min (70 K) has been performed in external magnetic fields (FC mode) and in the absence of a magnetic field (ZFC mode). Moreover, the dependences ρ(T) H = 0 for samples cooled in the FC mode have been measured in a zero field. The curves ρ(T)H = const have been converted into isotherms of the magnetore-sistance ρ(H ext) T = const. A comparative analysis of the specific features in the behavior of the curves ρ(H) T = const for samples with different “magnetic prehistories” has made it possible to elucidate the nature and mechanisms of the influence of the particular scenario of the magnetic treatment of granular HTSCs on the behavior of their galvanomagnetic properties. The temperature dependences of the critical magnetic fields of superconducting grains (H c1g , H c2g ) and Josephson weak links (H c2J ) have been determined, and the H-T phase diagrams of granular YBa2Cu3O7 − δ HTSCs have been recovered.

Dielectric and Electrical Properties of Polymorphic Bismuth Pyrostannate Bi 2 Sn 2 O 7

Abstract: The Bi2Sn2O7 compound existing simultaneously in two polymorphic modifications, namely, orthorhombic and cubic, has been synthesized for the first time by solid-phase synthesis. The dielectric and electrical properties of the compound have been studied in the temperature range 100 K < T < 500 K. Anomalies in the temperature dependences of the electrical resistivity and the permittivity (imaginary and real parts) have been found at both low and high temperatures. These features are explained in terms of the model of martensitic phase transitions.

Raman spectroscopy study of lattice dynamics of macro-, micro-, and nanostructured barium titanates

Abstract: The temperature dependences of the components A 1(2TO) and E(1TO) of the soft ferroelectric mode during phase transitions in single crystals, ceramics, polycrystalline and epitaxial thin films of barium titanate, as well as a superlattice consisting of alternating layers of barium and strontium titanates, have been studied using the Raman spectroscopy method. Abrupt changes in soft mode frequencies have been observed in the single crystal during phase transitions between tetragonal, orthorhombic, and rhombohedral phases. Smoothing of the temperature dependences of soft modes and the coexistence of phases have been observed in ceramics and polycrystalline films. In the epitaxial film, the sequence of structural transformations fundamentally differs from that observed in the single crystal; in the superlattice, the ferroelectric phase is stable to 550 K.

Nonlinear Magnetoelectric Effect in Composite Multiferroics

Abstract: The theoretical and experimental studies of the nonlinear magnetoelectric effect in composite multiferroics in the low-frequency spectral region and in the electromechanical resonance region have been performed. It has been shown that such structures demonstrate a nonlinear magnetoelectric effect, which is quadratic in ac magnetic field strength at weak magnetic fields. In the region of the electromechanical resonance, the resonance excitation of an electric field occurs by means of ac magnetic field at a frequency lower than the resonance frequency by a factor of two. In the low-frequency spectral region, there is a difference of amplitude values of two neighboring voltage maxima due to the superposition of signals from the linear and nonlinear effects, and the difference is proportional to the dc magnetic field strength in weak fields. The results of the experimental study of the two-layer permendur-lead zirconate titanate structure are presented.

Structural position and charge state of nickel in SrTiO 3

Abstract: The properties of nickeldoped strontium titanate are studied using Xray diffraction and XAFS spectroscopy. It is shown that, independently of preparation conditions, the most stable phases in the samples are singlephase SrTi 1- xNixO3 solid solution and NiTiO3 which can coexist. According to the EXAFS data, in the singlephase SrTi 0.97Ni0.03O3 sample the nickel atoms substitute the titanium atoms and are oncenter ones. In this case, no distortions of the oxygen octahedron which would appear in the presence of oxygen vacancies in the nickel environment were detected. An analysis of the XANES spectra shows that the nickel charge state in NiTiO3 is 2+, whereas in the SrTi 1- xNixO3 solid solution it is close to 4+. It is shown that the strongest light absorption in doped samples is associated with the presence of tetravalent Ni in the SrTi1 ⎯ xNixO3 solid solution. This doping seems to be the most promising for solar energy converters based on the bulk photovoltaic effect.

Family of paramagnetic centers of Ce 3+ ions in yttrium aluminum garnet

Abstract: The electron paramagnetic resonance (EPR) spectra of Ce3+ ions in single crystals of yttrium aluminum garnet have been investigated. It has been found that, in addition to the usually observed EPR signals of Ce3+ ions located in the regular environment at dodecahedral sites of the crystal lattice, the spectra contain a group of less intense anisotropic lines with g-factors close to the corresponding parameters of Ce3+ ions in the regular environment. It has been concluded that the observed satellite lines belong to the family of Ce3+ ions in the immediate vicinity of which there are permutation defects that lead to a change in the strength and symmetry of the crystal field in the vicinity of the paramagnetic center.