Showing papers in "Physics of the Solid State in 2009"

Ab initio calculations of phonon spectra in ATiO3 perovskite crystals (A = Ca, Sr, Ba, Ra, Cd, Zn, Mg, Ge, Sn, Pb)

Abstract: First-principles calculations of phonon spectra based on the density functional theory are carried out for calcium, strontium, barium, radium, cadmium, zinc, magnesium, germanium, tin, and lead titanates with a perovskite structure. By analyzing unstable modes in the phonon spectrum, the possible types of lattice distortion are determined and the energies of the corresponding phases are calculated. From analyzing the phonon spectra, force constants, and eigenvectors of TO phonons, a conclusion is drawn concerning the nature of ferroelectric phenomena in the crystals studied. It is shown that the main factors determining the possible appearance of off-center atoms in the A position are the geometric size and electronic configuration of these atoms.

Formation of quasi-free graphene on the Ni(111) surface with intercalated Cu, Ag, and Au layers

Abstract: This paper reports on a study by angle-resolved photoelectron and low-energy electron energy loss spectroscopy of graphene monolayers, which are produced by propylene cracking on the Ni(111) surface, followed by intercalation of Cu, Ag, and Au atoms between the graphene monolayer and the substrate, for various thicknesses of deposited metal layers and annealing temperatures. It has been shown that the spectra of valence-band π states and of phonon vibrational modes measured after intercalation become similar to those characteristic of single-crystal graphite with weak interlayer coupling. Despite the strong coupling of the graphene monolayer to the substrate becoming suppressed by intercalation of Cu and Ag atoms, the π state branch does not reach at the K point of the Brillouin zone the Fermi level, with the graphene coating itself breaking up partially to form graphene domains. At the same time after intercalation of Au atoms, the electronic band structure approaches the closest to that of isolated graphene, with linear π-state dispersion near the K point of the Brillouin zone, and the point of crossing of the filled, (π), with empty, (π*), states lying in the region of the Fermi level, which makes this system a promising experimental model of the quasi-free graphene monolayer.

Specific features of the thermal, magnetic, and dielectric properties of multiferroics BiFeO3 and Bi0.95La0.05FeO3

Abstract: Thermophysical, magnetic, and dielectric properties of multiferroic BiFeO3 and Bi0.95La0.05FeO3 ceramic compounds were comprehensively studied. Anomalies of the permittivity near an antiferromagnetic phase transition related to the structural variations were detected. The temperature T N was determined from the temperature dependences of the thermal expansion coefficient, heat capacity, and differential susceptibility. It is shown that the transition point is shifted to higher temperatures as the rare-earth La ion substitutes for Bi. It is established that an insignificant substitution of lanthanum for bismuth enhances the magnetic properties of bismuth ferrite and the magnetodielectric effect.

Raman Spectroscopic Study of Structural Disordering in YVO 4 , GdVO 4 , and CaWO 4 Crystals

Abstract: The Raman spectra of single-crystal YVO4, GdVO4, and ZrSiO4 with a zircon structure, as well as CaWO4 and BaWO4 with a scheelite structure, are studied in detail over a wide temperature range 14–800 K. An inhomogeneous splitting of the A1g(ν2) vibrational lines in the Raman spectra of YVO4 and GdVO4 and the Ag(ν1) vibrational lines in the spectrum of CaWO4 is detected. It is shown that the profiles of these lines can be decomposed into two components, whose integrated intensities are redistributed with temperature and also depend on the matrix kind in which they are detected. The phenomenon observed is associated with the thermally activated processes of disorientation of the tetrahedral anions in the zircon and scheelite structures.

Electronic structure, magnetic properties, and stability of the binary and ternary carbides (Fe,Cr)3C and (Fe,Cr)7C3

Abstract: The structural, electronic, and magnetic properties of the binary and ternary carbides (Fe,Cr)3C and (Fe,Cr)7C3 have been investigated within the ab initio density functional theory. The crystal structure of the binary carbides has been optimized and the preferred positions for replacement of chromium or iron impurities in the corresponding carbides have been determined. The changes in the electronic structure and magnetic properties have been investigated, the formation energies of the ternary carbides as functions of the impurity concentrations have been calculated, and conclusions have been drawn regarding the influence of the impurity on the stability of the carbides under investigation.

Thermal stability of cubane C8H8

Abstract: The reasons for the anomalously high thermal stability of cubane C8H8 and the mechanisms of its decomposition are studied by numerically simulating the dynamics of this metastable cluster at T = 1050–2000 K using a tight-binding potential. The decomposition activation energy is found from the temperature dependence of the cubane lifetime obtained from the numerical experiment; this energy is fairly high, Ea = 1.9 ± 0.1 eV. The decomposition products are, as a rule, either C6H6 and C2H2 molecules or the isomer C8H8 with a lower energy.

Manifestation of the heterogeneous mechanism upon melting of low-dimensional systems

Abstract: A melting process that is always heterogeneous in semi-infinite systems having a surface has been analyzed. It has been shown in terms of the classical thermodynamics that, in real one-component systems, a liquid layer on the solid-phase surface is formed at temperatures lower than the reference melting temperature of the bulk material at which the system is completely melted. Depending on the temperature, a liquid layer of particular thickness on the surface is in equilibrium with the other crystalline phase. The heterogeneous melting is shown to influence a number of processes and mechanisms, such as the dispersion of a thin film into droplets, the mechanism of vapor-liquid-solid epitaxy, the mechanism of layer-by-layer crystal growth, and the mechanism of growth of carbon nanotubes.

Properties of nanoporous aluminum oxide with triglycine sulfate and Rochelle salt inclusions

Abstract: The possibility of producing the ferroelectric state in nanoporous aluminum oxide loaded with triglycine sulfate and Rochelle salt has been studied. Our dielectric, pyroelectric and emission measurements give grounds to suggest that this state persists in the composites under study up to temperatures exceeding the Curie point of bulk crystals by several tens of degrees.

Nonlinear Optical Properties of Gold Nanoparticles Dispersed in Different Optically Transparent Matrices

Abstract: The nonlinear optical properties of gold nanoparticles dispersed in optically transparent matrices Al2O3, ZnO, and SiO2 are investigated using the classical and off-axis techniques of the Z-scan method at a wavelength of 532 nm (radiation from a nanosecond Nd: YAG laser). The experimental data on the nonlinear refraction in composite materials are obtained. The nonlinear refractive indices and the light absorption coefficients are determined, and the real and imaginary parts of the third-order nonlinear susceptibility for the structures under investigation are calculated. It is demonstrated that, for the composite materials under consideration, the nonlinear properties of the medium under the chosen conditions of laser irradiation are predominantly determined by the Kerr effect and the contribution of this effect exceeds the contribution of the thermal lens.

Simulation of a solid-state cooler with electrocaloric elements

Abstract: The dynamics of change in temperature at the edges of a layered structure consisting of one or more electrocaloric and heat-conducting elements in response to pulses of a periodic electric field has been studied with the use of the finite-element method. The possibility of using ceramic materials and (Ba,Sr)TiO3 films as electrocaloric and heat-conducting elements of linear and radial cooling structures is considered. The difference between the temperatures at the center and periphery of the radial film microstructure with two interdigital circular electrode lines is 2.5 K. An increase in the number of lines and the electric field frequency leads to an increase of the thermal effect.

Small-angle X-ray scattering study of the structure of glassy nanoporous matrices

Abstract: The structure of high-silica glassy nanoporous matrices prepared from two-phase glasses has been investigated using the small-angle X-ray scattering technique. Parameters of materials, such as the density, porosity, specific surface area, average nanopore radius, average radius of scattering particles filling the pore space, and their fractal dimensions, have been determined. The dependence of the obtained structural parameters on the conditions of chemical treatment of glasses has been established. It has been demonstrated that the results obtained are in good agreement with porosimetry and electron microscopy data.

Ferroelectrics find their “niche” among microwave control devices

Abstract: Rapid progress made in the physics of ferroelectrics is accompanied by the appearance of new requirements for the design of microwave phase shifters that serve as base elements in phased-array antennas, which, apart from traditional applications in radiolocation, have been widely used in various telecommunication systems. Nowadays, microwave phase shifters that are intended for operating at a high level of microwave power with a limited power in control circuits and should have a low cost of mass production can be implemented only using thin-film ferroelectric microwave integrated circuits. The development of phase shifters is based on reliable models of a dielectric response of ferroelectrics to dc and high-frequency electric fields.

Influence of contact potential difference and electric potential on the microhardness of metals

Abstract: The effect of the electric potential on the microhardness of aluminum, zirconium, and ferrosilicon was studied experimentally. The effect of the proper electric potential applied to a sample is compared with the effect of the potential induced by the contact potential difference upon contact with metals with a different electron work function. These two types of electrical action are revealed to be qualitatively equivalent to each other. It is established that these effects can markedly (up to 15%) change the microhardness of the metals.

Effect of the concentration of a rare-earth component on the parameters of the nanocrystalline structure in aluminum-based alloys

Abstract: The effect of the concentration of a rare-earth component on the parameters of the nanocrystalline structure formed during crystallization of an amorphous phase in the Al88Ni6Y6 and Al88Ni10Y2 alloys is studied using X-ray diffraction and transmission electron microscopy. It is shown that, as the yttrium concentration increases, the nanocrystal size increases and the content of the nanocrystalline component of the structure decreases. The precipitation of nanocrystals is accompanied by separation of the amorphous matrix into regions with different radii of the first coordination spheres due to the enrichment or depletion with the rare-earth element. The parameters of the nanocrystalline structure support the assumption of the heterogeneous nucleation of the nanocrystals.

Local states of iron ions in multiferroics Bi1−xLaxFeO3

Abstract: Mossbauer studies of perovskites Bi1−x La x FeO3 (x = 0, 0.10, 0.20, 0.61, 0.90, 1.00) were conducted at 295 and 87 K. The spatial spin-modulated structure (SSMS) observed in perovskites BiFeO3 and Bi0.9La0.1FeO3 leads to a specific distribution of hyperfine fields P(B) with two peaks. Substitution of La for Bi (x = 0.2) destructs the SSMS. The concentration dependences of the hyperfine field (B), isomer shift (ɛ) and quadrupole shift (δ) were measured. The iron ions are in the trivalent state. The local magnetic moments μ(Fe) of the Fe3+ ions are determined.

Ab initio calculations of the vibrational spectra of AgInSe2 and AgInTe2

Abstract: The phonon spectra and densities of states of AgInSe2 and AgInTe2 semiconductor crystals with a chalcopyrite structure have been calculated from first principles by the linear response method The frequencies calculated at the center of the Brillouin zone are in agreement with the experimental data obtained using IR and Raman spectroscopy According to the atomic contributions to the vibrational modes, the spectra of the AgInSe2 and AgInTe2 crystals exhibit three groups of bands: the vibrations in the low- and medium-frequency ranges are mixed in character with approximately identical contributions of all sublattices, and the bands at higher frequencies are associated with the contributions of Ag, C VI and In, C VI (C VI = Se, Te) atoms The position of these bands allows us to make the inference that, in the crystals under investigation, the In-C VI bonding is stronger than the Ag-C VI bonding

Magnetic properties of single crystals of ludwigites Cu2MBO5 (M = Fe3+, Ga3+)

Abstract: High-quality single crystals of ludwigites Cu2 MBO5 (M = Fe3+, Ga3+) have been grown, and the magnetic, resonance, and Mossbauer studies have been performed. It is established that the Cu2FeBO5 and Cu2GaBO5 compounds are antiferromagnets with Neel temperatures of 32 and 3.4 K, respectively. A model of the magnetic structure of the compounds is proposed. It is shown that the magnetic properties of the ludwigites are substantially dependent on the degree of ion distribution over crystallographic positions.

Fine sructure of Na5Lu9F32 nanocrystallites formed at the initial stages of crystallization

Abstract: X-ray diffraction investigations have demonstrated that the initial stages of the formation and growth of Na5Lu9F32 nanocrystallites from a powder mixture of lutetium fluoride, ammonium fluoride, and sodium fluoride during the solid-phase synthesis are accompanied by complex structural transformations, among which a cyclic transformation of the type single-phase → two-phase → single-phase state can be distinguished. It is shown that the two-phase state consists of two isomorphous phases with a small misfit of the lattice parameters. The isomorphous phases have common extended interphase boundaries, along which a continuous change of the lattice parameters occurs as one phase transforms into the other. A long-term annealing of the two-phase state leads to the final single-phase state, which is characterized by the growth of nanocrystallites to the scale of microcrystallites.

Magnetic and electrical properties of cation-substituted sulfides MexMn1 − xS (Me = Co, Gd)

Abstract: The temperature dependences of the specific magnetization σ and the electrical resistivity ρ of MexMn1 − xS single crystals (Me= Co, Gd; x= 0.05) have been studied in the temperature range 80 K < T < 1000 K. The samples under study have revealed the presence of a spontaneous magnetic moment below the Neel temperature (TN) and ferromagnetic clusters in Gd0.05Mn0.95S in the temperature range 146 K < T < 680 K. Substitution of gadolinium for manganese initiates a transition from p-type to n-type conduction. The change in the conduction type is accompanied by an increase in the electrical resistivity at 300 K by approximately one order of magnitude and, accordingly, by a decrease in the activation energy. The magnetic and electrical properties of the crystals under study have been interpreted in terms of the cluster model with temperature-dependent ferromagnetic exchange and an electron localized in this cluster.

Crystal structure of nanostructured PbS films at temperatures of 293-423 K

Abstract: The crystal structure of lead sulfide films prepared by the hydrochemical deposition has been studied by X-ray diffraction The thickness of the films synthesized is ∼100 nm, the size of coherent scattering regions is ∼70 nm, and the value of microstrains is ∼020% It is established, for the first time, that the as-synthesized PbS films and the same films annealed in the temperature range 293–423 K have a cubic crystal structure (space group Fm3m) different from the B1-type structure In the crystal lattice of the structure revealed, sulfur atoms are located not only in the 4(b) positions but also in the 8(c) positions The occupancies of the 4(b) and 8(c) positions by the S atoms are ∼084 and 008, respectively

Electrocaloric response of a ferroelectric capacitor to a periodic electric field

Abstract: The electrocaloric response of a ferroelectric capacitor to a periodic electric field has been analyzed in terms of the nonstationary heat conduction equation. A linear physical model is considered for an electrocaloric element in which one end (x = 0) is thermally insulated and a constant temperature T0 is maintained at the boundary x = l. The effect of a periodic electric field on the capacitor gives rise to temperature oscillations about a decreasing average value that reaches saturation. A relatively simple analytic expression is derived for the temperature distribution along the electrocaloric element and the heat flux density under stationary conditions. The calculations are carried out using the results obtained from measurements performed for a PMN-PT relaxor ferroelectric in the temperature range of the phase transition. A temperature gradient and a heat flux of ∼150 W/cm2 are observed in an electric field of 2.4 V/μm at a frequency of 10 Hz.

Nature of electronic states and optical functions of sodium oxyanionic compounds

Abstract: The band structure, the density of states, the partial electron densities, and optical functions (such as permittivity, refraction index, reflection and absorption coefficients) of sodium nitrite, nitrate, carbonate, chlorate, sulfite, perchlorate, and sulfate are calculated in a local approximation of the density-functional theory using the Troullier-Martins pseudopotentials in the basis of numerical pseudoatomic orbitals. The nature of the upper valence bands and the lower empty bands is established. It is shown that the specific features of the optical functions at energies of up to 8 eV and at E> 8 eV are due to the excitation of electrons into a localized anionic conduction band and into the bands of anion-cation states, respectively. The results are compared to experimental photoelectron spectra and reflection and absorption spectra.

Structure and properties of the intercalation compound CuxTiSe2

Abstract: Compounds in the pseudobinary Cu-TiSe2 intercalation system are directly synthesized from elements. The phase diagram of the system is investigated, the solubility limit of copper is measured, and the structure of the material is determined. In the copper concentration range up to 60 mol %, single crystals are grown and the temperature dependence of the electrical resistance is measured. It is demonstrated that, in the concentration range under investigation, the intercalation of the system with copper gives rise to a set of phenomena observed upon intercalation of alkali metals.

Multiphonon absorption in a MgO single crystal in the terahertz range

Abstract: The reflection and transmission spectra of a MgO single crystal at frequencies of 10–1000 cm−1 in the temperature range 10–300 K have been measured using submillimeter and infrared Fourier spectros-copy. The evolution of anharmonic absorption with variations in temperature has been investigated in the frequency range below the frequency of the transverse optical phonon. The parameters of the dispersion model that adequately describes the lattice absorption of terahertz radiation in single-crystal magnesium oxide have been determined.

Dynamic collective displacements of atoms in metals and their role in the vacancy mechanism of diffusion

Abstract: The phenomenon of dynamic collective displacements of atoms in face-centered cubic crystals has been revealed using molecular dynamics method. This phenomenon plays an important role in the vacancy mechanism of diffusion. The vacancy mechanism is provided by the collision of two regions of collective atomic displacements that move a migrating atom and a vacancy toward each other. The collective thermal atomic displacements from crystal lattice sites occur as a result of the nonuniform momentum distribution of atoms according to the Maxwellian distribution. Owing to their statistical nature, the degree of correlation of the atomic displacements depends neither on the temperature nor on the interatomic interaction potential.

An isotropic metamaterial formed with ferroelectric ceramic spherical inclusions

Abstract: A metamaterial that is formed by two types of spherical particles of different diameters has been considered. The particles are prepared from a ferroelectric ceramic material with a high permittivity (ɛ ≥ 400), so that the electromagnetic field inside the particles obeys the electrodynamic laws and the external field can be considered in the electro- or magnetostatic approximation. The dependences of the properties of the structure under investigation on the parameters of the ferroelectric ceramic material and on the sizes of the particles are analyzed. The results of the experimental investigation of the ferroelectric ceramic resonators are reported.

Growth and luminescence properties of single-crystalline films of RAlO3 (R = Lu, Lu-Y, Y, Tb) perovskite

Abstract: The conditions and the mechanisms of crystallization by liquid-phase epitaxy of undoped and Ce3+-doped RAlO3 (R = Lu, Lu-Y, Y, Tb) single-crystalline films on YAlO3 single-crystalline substrates at a considerable mismatch between the lattice constants of the single-crystalline films and the substrate are analyzed. The maximum values of this mismatch at which the stable growth of the single-crystalline films is observed are determined. It is demonstrated that transition layers are present between the substrates and the grown single-crystalline films in which the difference between the lattice parameters of the single-crystalline film and the substrate is diminished. The optical and luminescence characteristics of the undoped and Ce3+-doped RAlO3 (R = Y, Y-Lu, Lu, Tb) single-crystalline films, as well as the scintillation characteristics of the (Lu-Y)AlO3: Ce single-crystalline films under excitation by alpha-particles of the 239Pu source (5.15 MeV), are investigated.

Pressure-Induced Phase Transition in the Cubic ScF 3 Crystal

Abstract: Pressure-induced phase transitions in the ScF3 crystal were studied using synchrotron radiation diffraction, polarization microscopy, and Raman spectroscopy. The phase existing in the range 0.6–3.0 GPa is optically anisotropic; its structure is described by space group R 3 c (Z = 2), and the transition is due to rotation of ScF6 octahedra around a threefold axis. The pressure dependence of the structural parameters and angle of rotation are determined. The number of Raman spectral lines corresponds to that expected for this structure; above the phase transition point, a recovery of soft modes takes place. At a pressure of 3.0 GPa, a transition occurs to a new phase, which remains metastable as the pressure decreases. The results are interpreted using an ab initio method based on the Gordon-Kim approach.

Electrically controlled BST-Mg ceramic components for applications in accelerator technology

Abstract: This paper reports on the results of an experimental investigation into the dielectric properties of a new composite ceramic material BSM, whose tunability increases with increasing concentration of the composite magnesium-containing additive. The main applications of the ferroelectric ceramic material in the accelerator technology are considered.

Electronic and elastic properties of the superconducting nanolaminate Ti2InC

Abstract: The electronic properties and elastic parameters of the superconducting nanolaminate Ti2InC are analyzed using the ab initio full-potential linearized augmented-plane-wave (FLAPW) method with the generalized gradient approximation (GGA) of the local spin density. The equilibrium parameters of the crystal lattice, the band structure, the total and partial densities of states, and the Fermi surface are determined within a unified approach. The independent elastic constants, the bulk modulus, and the shear modulus are calculated, and the elastic parameters are numerically estimated for the first time for polycrystalline Ti2InC.