Showing papers in "Physics of the Solid State in 2005"

Band structure and the magnetic and elastic properties of SrFeO 3 and LaFeO 3 perovskites

Abstract: The band structure and the magnetic and elastic characteristics of SrFeO3 and LaFeO3 perovskites with ferromagnetic and antiferromagnetic collinear spin configurations (of the A, C, and G types) are investigated using the ab initio pseudopotential method (the VASP program package) with the inclusion of the single-site Coulomb correlations (the LSDA + U formalism). It is shown that, in the pressure range 0–50 GPa, the most stable states are the ferromagnetic metal state for the SrFeO3 compound and the antiferromagnetic insulator state of the G type for the LaFeO3 compound.

The kinetics and energy balance of mechanochemical transformations

Abstract: The kinetic theory of strength formulated by Zhurkov is expanded to include an energy consumption analysis of the processes of deformation and fracture of solids and their compounds. The dependence of the kinetics of the structure defect formation and of chemical reactions in the processes of deformation and fracture of solids on the energy expended is found. Experiments and calculations concerning energy yields of the defect formation, deformational mixing, and mechanochemical reactions are discussed.

Atomic structure of MBE-grown GaAs nanowhiskers

Abstract: The structural properties of MBE-grown GaAs and Al0.3Ga0.7 As nanowhiskers were studied. The formation of wurtzite and 4H-polytype hexagonal structures with characteristic sizes of 100 nm or larger in these materials was demonstrated. It is concluded that the Au-Ga activation alloy symmetry influences the formation of the hexagonal structure.

Theory of the magnetoelectric effect in ferromagnetic-piezoelectric heterostructures

Abstract: A theory of the magnetoelectric effect in ferromagnetic-piezoelectric bilayer structures is considered for platelike samples. The magnetoelectric voltage coefficient is expressed through the parameters characterizing the magnetic and piezoelectric phases. It is shown that the magnetoelectric voltage coefficient considerably increases in the region of electromechanical resonance. The thickness ratio between the ferromagnetic and piezoelectric phases at which the magnetoelectric voltage coefficient is maximum is determined. The calculated magnetoelectric voltage coefficients for Permendur-PZT (lead zirconate titanate) structures are presented and compared with the experimental data.

Atomic structure and strength of inorganic glasses

Abstract: The role of the atomic structure in the fracture processes is considered using borate, silicate, and phosphate glasses as an example. Primary attention is focused on the degree of connectivity of the atomic structure. It is shown that the degree of connectivity is a major factor responsible for the structural strength of glasses under conditions excluding the influence of both accidental surface defects and the environment. The change in the Young’s modulus as a measure of elastic deformation and the change in the hardness as a characteristic of irreversible deformation are analyzed. The ultimate elastic strain experienced by a glass at the instant of fracture is examined. It is found that the ultimate elastic strain is approximately equal to 10% for glasses with a three-dimensional atomic structure and 5% for glasses with a two-dimensional (layered) or chain structure. It is assumed that this behavior of the strength as a function of the degree of connectivity of the atomic structure is associated with the degree of uniformity of the external load distribution over atomic bonds.

The intrinsic structure of spherical particles of opal

Abstract: The intrinsic structure of spherical SiO2 particles synthesized by hydrolysis of tetraethyl orthosilicate in an alcohol-water-ammonia medium was studied using transmission electron microscopy. It was established that the relatively large spherical silica particles were “tertiary” structures made up of smaller spherical particles (“ secondary” particles), which in turn consisted of even smaller primary spherical particles 5–10 nm in diameter. It was shown that, under the experimental conditions, the large SiO2 particles can contain a central core comprising primary particles surrounded by several layers of secondary particles smaller than the core diameter.

Evolution of microscopic cracks and pores in solids under loading

Abstract: Experimental data on the development and partial healing of microscopic cracks and pores in loaded crystalline materials are considered. An analysis of the data indicates that fracture development has a number of specific features depending on the state of the materials and the testing conditions and is a kinetic thermal fluctuation process occurring virtually throughout the entire time of loading.

Structure and properties of Ti-B-N, Ti-Cr-B-(N), and Cr-B-(N) coatings deposited by magnetron sputtering of targets prepared by self-propagating high-temperature synthesis

Abstract: Transmission and scanning electron microscopy, x-ray phase analysis, x-ray photoelectron spectroscopy, and atomic-force microscopy were used to study the structure and surface topography of Ti-B-N, Ti-Cr-B-(N), and Cr-B-(N) thin films. Physical, mechanical, and tribological characteristics of coatings were comparatively analyzed, including determination of the hardness, elastic modulus, elastic recovery, critical load, friction coefficient, and wear rate. It was shown that Ti-B-N and Ti-Cr-B-N coatings are superior to conventional TiN-and Ti-C-N-based coatings in terms of their physicomechanical and tribological properties. Ti-B-N and Ti-Cr-B-N coatings deposited under optimum conditions were characterized, accordingly, by a hardness of 31–34 and 40–47 GPa, an average elastic modulus of 378 and 506 GPa, a friction coefficient of 0.49–0.60 and 0.45–0.52, a dry-wear rate of (3.4–4.6) × 10−7 and (6.0–6.8) × 10−7 mm3 N−1 m−1, and a largest critical load of 50 and 22 N. Features in the determination of the physicomechanical properties of films during nanoindentation and their wear properties are discussed.

Mechanical properties of thin Ag films on a silicon substrate studied using the nanoindentation technique

Abstract: The mechanical properties of thin Ag films of equal thickness containing grains of various sizes were studied. The film hardness was measured using the Oliver-Pharr techniques based on indentation work calculations or on direct measurements of the area of pyramid imprints in AFM images. In order to avoid the influence of a substrate on the measured hardness, a technique was developed to determine the true values of the film hardness. It was established that the hardness of Ag films decreases with an increase in mean grain size, whereas the elastic modulus remains almost unchanged. It was shown that the dependence of the yield stress of Ag films on grain size does not obey the classical Hall-Petch law. © 2005 Pleiades Publishing, Inc.

Optical properties of thulium orthoferrite TmFeO3

Abstract: Optical properties of the orthorhombic thulium orthoferrite TmFeO3 were studied in the spectral range from 0.64 to 5.4 eV. In the weak absorption region, below 2.2 eV, the energies of localized optical transitions in the Tm3+ and Fe3+ ions were determined. The dispersion relations of the real and imaginary parts of the principal refractive indices along three crystallographic axes were found. In the region of strong absorption, above 2.2 eV, the energies of six charge-transfer transitions were determined. The experimental data fit well to the concept of charge-transfer transitions in the FeO 6 9− octahedral complexes providing a dominant contribution to the optical properties of the orthoferrites. Optical birefringence and its temperature dependence were measured for the three principal directions of light propagation, and the anisotropic magnetic contribution to birefringence in the region of spin-orientational transitions was isolated.

Electrical breakdown of thin polymer films

Abstract: Data are presented on the dielectric strength of thin polymer films. The conclusion is drawn that the electron avalanche concept is inapplicable to the breakdown of thin films. It is proposed to consider electrical breakdown as a consequence of an abrupt local field enhancement induced by evolution of the space charge injected into the polymer from electrodes. It is shown that the lifetime of polymer films depends exponentially on electric field strength.

Pentagonal copper crystals: Various growth shapes and specific features of their internal structure

Abstract: It is shown that at least eight types of copper crystals having one or six fivefold symmetry axes can form during electrodeposition. Their structures and the possible mechanisms of their formation and growth are considered. Different-type pentagonal crystals that form during electrodeposition are assumed to have the same disclination nature.

Anomalous thermal stability of metastable C-20 fullerene

Abstract: The results of computer simulation of the dynamics of fullerene C-20 at different temperatures are presented. It is shown that, although it is metastable, this isomer is very stable with respect to the transition to a lower energy configuration and retains its chemical structure under heating to very high temperatures, T approximate to 3000 K. Its decay activation energy is found to be E-a approximate to 7 eV. Possible decay channels are studied, and the height of the minimum potential barrier to decay is determined to be U = 5.0 eV. The results obtained make it possible to understand the reasons for the anomalous stability of fullerene C-20 under normal conditions. (C) 2005 Pleiades Publishing, Inc.

Atomic-level fluctuation mechanism of the fracture of solids (computer simulation studies)

Abstract: Experimental studies of the fracture kinetics of solids have shown that the process culminating in fracture of a stressed solid comprises a sequence of elementary events in which stressed atomic bonds are ruptured by local energy fluctuations. This recognition sparked investigations into elementary fracture events and into the fluctuations themselves that are responsible for bond rupture. Currently, only computer simulation of the dynamics of atoms offers the possibility of tracing, in considerable detail, the evolution of fluctuation events characterized by very short duration (∼10−13−10−12 s). An analysis is made of the results obtained in computer simulation experiments on the temporal and spatial localization of atomic-energy fluctuations and atomic-bond strains, fluctuation migration, the mechanism of fluctuation formation, the role of anharmonicity in atomic interactions, and the magnitude of the volume activated in an elementary fracture event.

Problems in searching for new solid-state UV-and VUV-active media: The role of photodynamic processes

Abstract: The impact of photodynamic processes induced by pump radiation in crystals doped with rare-earth (RE) ions on the possibility of generating stimulated radiation in the UV and VUV spectral ranges is analyzed. It is shown that, in addition to objective factors, one of the main causes preventing the lasing effect from being obtained using the interconfigurational 4ƒn−15d-4ƒn transitions of RE ions is insufficient consideration of the photodynamic processes involved in experiments. It is proposed to correct laser test techniques and “pumpprobe” experiments aimed at investigating active media appropriate for lasing in the UV and VUV ranges. Extended criteria oriented to search for new solid-state UV and VUV active media are formulated.

Rotation of the inner shell in a C 20 @C 80 nanoparticle

Abstract: The stability of a C20@C80 nanoparticle and the rotation of its inner shell are studied theoretically within the tight-binding approximation. It is found that the C20 skeleton in the free state is described by space group D 3d ; in the case where C20 is placed into the C80(I h ) fullerene field, the space group of C20 is raised to I h due to isomerization. The total energy surface of the C20@C80 compound is scanned over two rotation angles. Based on an analysis of the surface relief and energy isoline map, orientational melting of the nanoparticle is predicted. A nanoparticle gyroscope—C20 rotating in the field of C80 at a certain relative orientation and energy supply—is also predicted to exist.

Raman Spectroscopy and Electroreflectance Studies of Self-Assembled SiGe Nanoislands Grown at Various Temperatures

Abstract: SiGe nanoislands grown in a silicon matrix at temperatures of 300 to 600°C are studied using Raman spectroscopy and electroreflectance. For islands grown at relatively low temperatures (300–500°C), phonon bands are observed to have a doublet structure. It is shown that changes in the percentage composition, size, and shape of nanoislands and, hence, in the elastic stresses (depending on the growth temperature of the structures) have a significant effect on the energies of optical electronic interband transitions in the islands. As a consequence, the resonance conditions for Raman scattering also change. It is found that interdiffusion from the silicon substrate and the cover layer (determining the mixed composition of SiGe islands) is of importance even at low growth temperatures of nanostructures (300–400°C).

Structure factors that influence the stability of plastic strain of bcc metals under tensile load

Abstract: The effect of the Peierls stress on the ultimate tensile stress and uniform strain prior to the formation of a neck during stretching of metals and alloys with bcc structure is theoretically analyzed. The analysis is based on the equation for the variation of the dislocation density with deformation; this equation determines the shape of the work-hardening curve for a bcc material and the effect of the Peierls stress on the parameters of this equation (the annihilation coefficient for screw dislocations). Using the Considere condition for plastic instability of the neck type, the ultimate tensile stress and the magnitude of uniform strain are found theoretically as a function of the Peierls stress at different temperatures below 0.15T m , where T m is the melting temperature of the bcc metal. Theoretical results are illustrated with experimental data on the temperature dependences of the annihilation coefficients for screw dislocations and of the magnitude of uniform strain in molybdenum and Armco iron.

New crystals for Raman lasers

Abstract: Steady-state and transient stimulated Raman scattering (SRS) in crystals is analyzed. The basic laws of an increase in the SRS gain in crystals are revealed, and methods for searching and creating new SRS laser materials are developed. New crystals for picosecond and nanosecond SRS lasers are proposed, fabricated, and characterized. These materials have the highest SRS cross section, a low SRS threshold, and a wide spectral range of operation.

Crystal fields of hexameric rare-earth clusters in fluorites

Abstract: In solid solutions of alkaline-and rare-earth fluorides with a fluorite structure, ions of most elements of the rare-earth (RE) row form hexameric clusters that assimilate the minor component of the solid solutions (fluorine) and build it into the cubic fluorite lattice without changing its shape. An analysis of the EPR spectra of paramagnetic RE ions (Er3+, Tm3+, Yb3+) in clusters of diamagnetic ions (Lu3+, Y3+) confirms their hexagonal structure, which was established when studying the superstructures of the compounds under study. In such a cluster, a RE ion is in a nearly tetragonal crystal field, with the parameters of this field differing radically from those of single cubic and tetragonal RE centers in crystals with a fluorite structure. In particular, this field causes high (close to limiting) values of the g∥ factors of the ground states of the paramagnetic RE ions. Computer simulation is used to determine the atomic structure of a hexameric cluster in MF2 crystals (M = Ca, Sr, Ba). The crystal field and energy spectrum of Er3+, Tm3+, and Yb3+ ions in such clusters are calculated, and the spectroscopic parameters of the ground states of these ions are determined. The calculations confirm the earlier assumption that the unusual EPR spectra of nonstoichiometric fluorite phases are related to RE ions in hexameric clusters.

Critical properties of the three-dimensional frustrated Ising model on a cubic lattice

Abstract: The critical properties of the three-dimensional fully frustrated Ising model on a cubic lattice are investigated by the Monte Carlo method. The critical exponents α (heat capacity), γ (susceptibility), β (magnetization), and ν (correlation length), as well as the Fisher exponent η, are calculated in the framework of the finite-size scaling theory. It is demonstrated that the three-dimensional frustrated Ising model on a cubic lattice forms a new universality class of the critical behavior.

Structural Mechanisms of Fracture of Nanocrystalline Materials

Abstract: Structural mechanisms and features of brittle and quasi-brittle fracture of nanocrystalline materials are theoretically analyzed. The role of size effects and internal stresses caused by a nonequilibrium structure during brittle trans-and intercrystallite fracture is studied. The dependence of the nanocrystalline material durability on the working stress and grain size is calculated. The conditions for certain mechanisms of plastic deformation to be operative in nanocrystalline materials are analyzed. The influence of the grain-boundary and dislocation mechanisms of plastic deformation on the conditions of nanocrack formation is studied. The dependence of the fracture toughness of nanomaterials on structure parameters is calculated.

Electrical breakdown in solid dielectrics

Abstract: The mechanism of electrical breakdown in solid dielectrics is analyzed using the results of our investigations performed in this direction over a period of several decades. It is shown that the electrical breakdown in solid dielectrics involves interrelated prebreakdown processes, such as high-voltage polarization, defect formation, electron impact excitation and electron impact ionization of luminescence centers and ions in the host crystal lattice, etc. The electrical breakdown is initiated by electric-field and thermal generation of defects in the crystal. In turn, the generation of defects leads to the formation of defect regions and channels that provide an assisted transfer of charge carriers. Electron currents flow (and electrons are accelerated by the electric field to energies sufficient to induce impact ionization) in these regions of the crystal with a lattice distorted by defects. In this respect, the known approaches to the elaboration of the breakdown theory for alkali halide and other dielectric crystals on the basis of analyzing the motion and acceleration of electrons in an ideal crystal structure have appeared to be incorrect.

Effect of preliminary strain hardening on the flow stress of titanium and a titanium alloy during shock compression

Abstract: The effect of preliminary strain hardening of VT1-0 titanium and a Ti-6 wt % Al-4 wt % V alloy on their mechanical properties under quasi-static and high-rate (τ;105 s−1) loading is studied. Preliminary hardening is accomplished using equal-channel angular pressing (which results in a significant decrease in the grain size and a twofold increase in the quasi-static yield strength) and shock waves. High-rate deformation is attained via shock-wave loading of samples. The experimental results show that structural defects weaken the dependence of the yield strength on the strain rate. The difference in the rate dependences can be so high that the effect of these defects on the flow stress can change sign when going from quasi-static to high-rate loading.

Dynamic systems of concentric ring magnetic domains in a highly anisotropic garnet ferrite film in magnetic fields at infrasonic frequencies

Abstract: Dynamic domain structures in a garnet ferrite film with perpendicular anisotropy are investigated in magnetic fields at infrasonic frequencies for the first time. It is revealed that the multidomain film exists in the anger state and that stable dynamic structures with unusual properties are formed in ac magnetic fields at frequencies of 2–3 Hz.

Spectral kinetics of Ce3+ ions in double-fluoride crystals with a scheelite structure

Abstract: The influence of the cation composition on the spectral kinetics of Ce3+ ions in double-fluoride crystals with a scheelite structure is studied. The importance of the photodynamic processes induced in these crystals by the exciting radiation is demonstrated. The difference in luminescence quantum efficiency between Ce3+ ions in LiYF4 and LiLuF4 crystals is found to be due to the different lifetimes of color centers produced in the samples by the exciting radiation and to the different efficiency of the free-carrier recombination at cerium impurity centers. It is shown that Yb3+ ions can increase the carrier recombination rate in the crystals.

Dependence of the Magnetization Relaxation Time of Single-Domain Ferromagnetic Particles on Damping in the Brown Model

Abstract: Analytical expressions for the magnetization relaxation time τ of single-domain ferromagnetic particles with cubic or uniaxial anisotropy in a static transverse magnetic field are derived. The derivation is based on calculating the escape rate of a Brownian particle from a potential well; this technique is applicable at any damping and is generalized to the case of magnetic relaxation of superparamagnetic particles. The validity of the expressions obtained for τ is checked against a numerical solution of the Landau-Lifshitz-Gilbert equation over the whole range of damping (very low, intermediate, and high damping and the crossover region between low and intermediate damping).

Electrical conductivity and superconductivity of ordered indium-opal nanocomposites

Abstract: The electrical conductivity is measured experimentally and the parameters of the superconducting transition are determined in a regular spatial network of multiply connected submicron-sized indium grains embedded in voids of an ordered opal dielectric matrix. The In-opal nanocomposite was prepared by pressure injection of the molten metal into voids of opal samples. Arrays of In grains of different sizes were produced by properly varying the characteristic geometric sizes of the opal voids, which offered the possibility of observing quantitative and qualitative changes in the temperature dependence of electrical resistance and studying the size effects on the critical temperature and critical magnetic field in the In-opal nanocomposites. It was found that, as the coherence length becomes comparable to the size of the superconducting grains, the parameters of the superconducting transition in the nanocomposite increase sharply.

Diagnostics and forecasting of breakage of large-scale objects

Abstract: Using a kinetic approach to the breakage of solids, a two-stage model of breakage is constructed. The model is invariant for objects of various scale. A physical approach to forecasting the final stage of macroscopic breakage is developed. The applicability of the methods devised is tested on laboratory samples, industrial constructions, and large-scale objects.

Electronic excitations in BeAl 2 O 4 , Be 2 SiO 4 , and Be 3 Al 2 Si 6 O 18 crystals

Abstract: Low-temperature (T = 7 K) time-resolved selectively photoexcited luminescence spectra (2–6 eV) and luminescence excitation spectra (8–35 eV) of wide-bandgap chrysoberyl BeAl2O4, phenacite Be2SiO4, and beryl Be3Al2Si6O18 crystals have been studied using time-resolved VUV spectroscopy. Both the intrinsic luminescence of the crystals and the luminescence associated with structural defects were assigned. Energy transfer to impurity luminescence centers in alexandrite and emerald was investigated. Luminescence characteristics of stable crystal lattice defects were probed by 3.6-MeV accelerated helium ion beams.