Showing papers in "Physics of the Solid State in 2008"

Nanoindentation and mechanical properties of solids in submicrovolumes, thin near-surface layers, and films: A Review

Abstract: This paper presents a review devoted to the specific mechanical behavior of thin near-surface layers of various solids, materials, films, and multilayer coatings under local loading. Analysis is made of the principles, techniques, and instruments used for testing and determining mechanical properties of materials on a nanoscale, which, in recent years, have found a wide application under the general name “nanoindentation.” Information capabilities of this large and multifunctional family of methods employed for mechanical tests are discussed. Different aspects and specific features of the behavior of solids under severely constrained deformation caused by the local application of a microload to the surface of the test object are considered. Special attention is focused on the physical mechanisms of deformation and fracture under these conditions.

Effect of the size factor on the magnetic properties of manganite La 0.50 Ba 0.50 MnO 3

Abstract: Nanocrystalline manganite La0.50Ba0.50MnO3 was synthesized by an optimized sol-gel method. The initial sample was subjected to step-by-step heat treatment under air atmosphere. The ion stoichiometry, the morphology of crystallites of ceramics, and the magnetic properties were studied. It is established that the average crystallite size D increases from ∼30 nm to ∼7 μm with increasing annealing temperature. All of the samples studied are characterized by a perovskite-like cubic structure, with the unit cell parameter a increasing continuously from ∼3.787 to ∼3.904 A with the average crystallite size. The most significant lattice compression (≈3%) occurs in the sample with an average crystallite size of ∼30 nm. The increase in the average crystallite size causes a nonmonotonic increase in the Curic temperature T C from ∼264 to ∼331 K and in the spontaneous magnetic moment σ S from ∼1.52 to ∼3.31 μB/f.u. The anomalous behavior of the magnetic properties of the manganite La0.50Ba0.50MnO3 obtained is explained by the competition between two size effects, namely, the frustration of the indirect exchange interactions Mn3+-O-Mn4+ on the nanocrystallite surface and the crystal lattice compression due to the crystallite surface tension.

New method for growing silicon carbide on silicon by solid-phase epitaxy: Model and experiment

Abstract: A new method of solid-state epitaxy of silicon carbide (SiC) on silicon (Si) is proposed theoretically and realized experimentally. Films of various polytypes of SiC on Si(111) grow through a chemical reaction (at T = 1100–1400°C) between single-crystal silicon and gaseous carbon oxide CO (at p = 10–300 Pa). Some silicon atoms transform into gaseous silicon oxide SiO and escape from the system, which brings about the formation of vacancies and pores in the silicon near the interface between the silicon and the silicon carbide. These pores provide significant relaxation of the elastic stresses caused by the lattice misfit between Si and SiC. X-ray diffraction, electron diffraction, and electron microscopy studies and luminescence analysis showed that the silicon carbide layers are epitaxial, homogeneous over the thickness, and can contain various polytypes and a mixture of them, depending on the growth conditions. The typical pore size is 1 to 5 μm at film thicknesses of ∼20 to 100 nm. Thermodynamic nucleation theory is generalized to the case where a chemical reaction occurs. Kinetic and thermodynamic theories of this growth mechanism are constructed, and the time dependences of the number of new-phase nuclei, the concentrations of chemical components, and the film thickness are calculated. A model is proposed for relaxation of elastic stresses in a film favored by vacancies and pores in the substrate.

Mechanisms of charging of insulators under irradiation with medium-energy electron beams

Abstract: The electron emission and charge characteristics of a large number of massive insulators are investigated qualitatively and quantitatively. It is demonstrated that irradiation of insulators with a medium-energy continuous electron beam leads to a decrease in the equilibrium energy of incident electrons (the second critical energy) as compared to the theoretical value. The equilibrium state of charging to saturation is attained for times from several seconds to several hundred seconds depending on the irradiation current density, the electron energy, and the insulator material. The mechanisms of charging are explained in the framework of the model according to which irradiation is accompanied by the formation of a charged double layer that consists of a positively charged layer (with the thickness equal to the escape depth of secondary electrons) and a negatively charged layer (with the thickness equal to the penetration depth of primary electrons).

Raman spectra of structures with CdTe-, ZnTe-, and CdSe-based quantum dots and their relation to the fabrication technology

Abstract: Quantum-dot structures based on the CdTe, ZnTe, and CdSe semiconductors are prepared by molecular-beam epitaxy, colloid chemistry methods, and ball milling, and their Raman spectra are studied. Localized longitudinal phonons are observed in all spectra. The dependence of the localized phonon frequency on the thickness of the ZnTe barrier in CdTe/ZnTe quantum-dot superlattices is used to derive the dispersion relation for longitudinal phonons in ZnTe. The Raman spectra of ensembles of colloidal quantum dots differ from the spectra of the other objects by the absence of tellurium bands and a strong intensity of the longitudinal phonon band of CdTe. It is revealed that the spectra depend on the technology employed to prepare quantum-dot structures.

Domain wall dynamics in ferromagnets

Abstract: The main results of studying the linear and nonlinear dynamics of a domain wall in ferromagnets are presented beginning with the first publications. The experimental data obtained on polycrystals, single crystals, and films differing in terms of their anisotropy, as well as on magnetic nanostructures, are compared with theoretical results obtained by various methods.

Generation of shear bands in subsurface layers of metals in sliding

Abstract: Nanostructuring of the surface of specimens during friction is investigated under the conditions of shear instability of the subsurface layers of the material due to the strong localization of deformation. It is demonstrated that the localization of deformation in the subsurface layers occurs in three stages. The structure of the localization zone is studied. The formation of a nanocrystalline material on the surface due to the shear instability is considered by analogy with the formation of the shear band. The formation of the nanocrystalline material can be responsible for the crossover from the mild wear to the adhesive wear in the absence of mechanisms providing structural adaptability.

Frequency response of the low-temperature ionic conductivity of single crystals R 1 − y M y F 3 − y ( R = La-Er; M = Ca, Sr, Ba, Cd)

Abstract: The frequency response (10−1–10−7 Hz) of the ionic conductivity σ of R 1 − y M y F3 − y single crystals (R is a rare-earth element, and M stands for an alkaline-earth element and Cd) with a tysonite structure is studied over a wide temperature range (114–410 K), which includes (for the first time for these phases) the interval below room temperature. The dependences σ(ν) obtained are discussed within the hopping relaxation model. The characteristics of the relaxation and migration processes and the carrier concentration and mobility are determined.

Adiabatic calorimetric study of the intense magnetocaloric effect and the heat capacity of (La0.4Eu0.6)0.7Pb0.3MnO3

Abstract: The temperature dependences of the intense magnetocaloric effect ΔTAD(T, H) and the heat capacity Cp(T) of the (La0.4Eu0.6)0.7Pb0.3MnO3 manganite are directly measured using adiabatic calorimetry. The experimental dependences ΔTAD(T) are in satisfactory agreement with those calculated from the data on the behavior of the magnetization. The factors responsible for the absence of an anomaly in the experimental temperature dependence of the heat capacity Cp(T) in the range of the magnetic phase transition are discussed.

Electromagnetic solitons in bundles of zigzag carbon nanotubes

Abstract: The possibility of forming solitons in zigzag carbon nanotubes is investigated using the coupled equations for the classical function of the electron distribution and the Maxwell equations for an electromagnetic field. It is demonstrated that the solitons are generated as a result of correlated changes in the classical distribution function and the electric field induced by nonequilibrium electrons of a carbon nanotube. The effective equation describing the dynamics of the electromagnetic field is derived. The existence of solitons is confirmed by the results of numerical calculations. The characteristics of solitons are investigated as a function of the diameter of zigzag carbon nanotubes.

Methods of terahertz-subterahertz BWO spectroscopy of conducting materials

Abstract: This paper considers present spectroscopic methods developed for measurement of dielectric response (conductivity and permittivity spectra) of condensed media in the terahertz-subterahertz spectral regions. The techniques based on the use of BWO spectrometers and designed for direct quantitative measurements (without invoking the Kramers-Kronig relations) in the terahertz-subterahertz frequency range (0.03–1.45 THz) of the conductivity σ(ν) and permittivity ɛ′(ν) spectra of conducting and absorbing materials are described. The techniques are based on measuring the amplitude and phase of the transmission coefficient of film samples on dielectric substrates and the reflection coefficient of a reference plane-parallel dielectric plate contacting the surface of the sample under study. The use of these techniques in measurement of the σ(ν)) and ɛ′(ν) spectra of conductors and superconductors is illustrated with examples.

High-temperature heat capacity of the Al63Cu25Fe12 quasicrystal

Abstract: The mechanisms of energy absorption by metallic alloys with long-range aperiodic lattice order and electronic properties of marginal metals are studied. The heat capacity and linear expansion coefficient of the Al63Cu25Fe12 icosahedral phase in the temperature range 300–1000 K are measured for the first time. Disagreement between the measured heat capacity and predictions made from the Debye model is found and analyzed. It is shown that the excess heat capacity observed at the temperatures of the experiment is fitted by Einstein’s function in the approximation T ΘE.

Size effect of the thermal expansion of nanostructural copper oxide

Abstract: The temperature dependence of the thermal expansion ΔL/L of samples of high-density nanostructural CuO ceramics with a crystallite size of 20, 70, and 90 nm was measured. The nanoceramics were obtained from coarse-grained CuO powders under converging spherical shock waves. It is found that, at temperatures T > 50 K, the thermal expansion coefficient α(T) of the nanoceramic samples increases with decreasing crystallite sizes and exceeds the value of α(T) of the CuO single crystal by a factor of 3.5 to 4.5. At temperatures T < 50 K, regions with zero and negative values of α(T) were revealed. The possible reasons for the increase in the thermal expansion coefficient of nanoceramics based on 3d-metal oxides are discussed.

Specific features of luminescence properties of nanostructured aluminum oxide

Abstract: Aluminum oxide nanopowders are prepared by the gas phase method and characterized according to the particle sizes and the phase composition. Samples of the nanostructured ceramic material are produced by pressing and annealing in air. The photoluminescence and cathodoluminescence spectra of the Al2O3 nanostructured ceramic material and α-Al2O3 anion-defect single crystals are investigated under comparable conditions. The luminescence bands of centers formed by oxygen vacancies are revealed in the spectra of two types of samples. The nanostructured ceramic material is characterized by the appearance of a new luminescence band at 3.4 eV and a decrease in the luminescence decay time. The inference is made that the characteristic features of the luminescence of the nanostructured ceramic material can be associated with the presence of non-equilibrium phases and the specific features of relaxation processes.

Location of the energy levels of the rare-earth ions in BaF2 and CdF2

Abstract: The location of the energy levels of rare-earth (RE) elements in the energy band diagram of BaF2 and CdF2 crystals is determined. The role of RE 3+ and RE 2+ ions in the capture of charge carriers, luminescence, and the formation of radiation defects is evaluated. It is shown that the substantial difference in the luminescence properties of BaF2: RE and CdF2: RE is associated with the location of the excited energy levels in the band diagram of the crystals.

Magnetic properties and structural parameters of nanosized oxide ferrimagnet powders produced by mechanochemical synthesis from salt solutions

Abstract: The phase composition, the structural parameters, and the basic magnetic characteristics of a number of nanosized powders of simple spinel ferrites produced by the method of mechanochemical reactions from salt systems were studied. The influence of size and surface effects on the saturation magnetization and magnetic anisotropy was determined. The applicability of the shell model to the description of the magnetic properties of nanosized spinel ferrites is discussed.

Phenomenological Description of Phase Transitions in Thin BaTiO 3 Films

Abstract: Phase transitions in thin epitaxial films of BaTiO3 are described phenomenologically in terms of Landau potentials with sixth-and eighth-order terms. It is established that the phase diagram depends on the electrostrictive constant Q12. The phase diagrams calculated for different values of Q12 available in the literature differ qualitatively. The dependence of the misfit strain of a film on the film tetragonality at room temperature is found, which makes it possible to determine the thermodynamic path in the phase diagram for a specific film. The dependences of the spontaneous polarization and dielectric constant of a film on the misfit strain at room temperature are constructed.

The role of deep traps in luminescence of anion-defective α-Al2O3: C crystals

Abstract: This paper presents the results of a study of deep traps responsible for thermally stimulated luminescence in single crystals of the anion-defective corundum α-Al2O3: C observed at temperatures of about 703, 778, 830, and 903 K. The effect of the filling of these traps by carriers of both signs on the sensitivity to radiation of the main thermoluminescence peak at 450 K, optically stimulated luminescence, and phototransferred thermoluminescence has been investigated. The results obtained are treated in terms of a model involving interactive competition of traps of different depth.

Electrocaloric effect and anomalous conductivity of the ferroelectric NH4HSO4

Abstract: An intensive electrocaloric effect in the uniaxial ferroelectric NH4HSO4 near the second-order phase transition (T1 ≈ 271 K) is studied. A high electrical conductivity of the NH4HSO4 crystal is revealed, and its temperature dependence is investigated. The results obtained are analyzed together with the data available for the ferroelectrics, which hold promise for the use as solid-state cooling agents.

Atomistic simulation of plasticity and fracture of nanocrystalline copper under high-rate tension

Abstract: A molecular dynamics simulation of the plastic deformation and the onset of fracture of nanocrystalline metals is performed using the example of copper. Successive stages of the response of the microstructure of a metal to deformation are considered, namely, grain boundary sliding, the nucleation and gliding of dislocations, and the formation and growth of microdamage nuclei. The influence of the grain size of a nanocrystal on its plasticity and strength is studied.

Investigation of the influence of quenched nonmagnetic impurities on phase transitions in the three-dimensional Potts model

Abstract: The influence of quenched nonmagnetic impurities on phase transitions in the three-dimensional Potts model with the number of spin states q = 3 is investigated using the Wolff single-cluster algorithm of the Monte Carlo method. The systems with linear sizes L = 20–44 at the spin concentrations p = 1.0, 0.9, 0.8, and 0.7 are analyzed. It is demonstrated with the use of the method of fourth-order Binder cumulants that the second-order phase transition occurs in the model under consideration at the spin concentrations p = 0.9, 0.8, and 0.7 and that the first-order phase transition is observed in the pure model (p = 1.0). The static critical exponents α (heat capacity), γ (susceptibility), β (magnetization), and ν (correlation length) are calculated in the framework of the finite-size scaling theory. The problem regarding the universality classes of the critical behavior of weakly diluted systems is discussed.

Influence of silver on the Sm 3+ luminescence in ``Aerosil'' silica glasses

Abstract: The introduction of silver into the samarium-containing silica glasses prepared by the original solgel method leads to the formation of complex optical centers involving samarium ions and simple and/or complex silver ions. These centers are characterized by the effective sensitization of Sm3+ luminescence by Ag+, (Ag2)+, and (Ag+)2 ions according to the exchange mechanism for, at least, Sm3+-Ag+ centers. The formation of Sm-Ag centers is accompanied by an increase in the concentration of nonbridging oxygen ions, which prevent the reduction of silver ions by hydrogen. Silver nanoparticles formed in small amounts upon this reduction are effective quenchers of luminescence from the corresponding excited states of Sm3+ ions.

Specific features of the determination of the mechanical characteristics of thin films by the nanoindentation technique

Abstract: The hardness and the elastic modulus of Cu thin films on Si, Ti, Cu, and Al substrates are investigated. It is demonstrated that the use of the Oliver-Pharr method in combination with the technique for evaluating the true hardness makes it possible to determine uniquely the hardness of Cu thin films at different ratios between the hardnesses of the film and the substrate. The elastic modulus of thin films can be correctly measured by the Oliver-Pharr method only in the case where the film and the substrate exhibit identical elastic properties. In order to determine the elastic moduli of films with the use of the parameter P/S 2, the film and the substrate should have close values of both the hardness and the elastic modulus.

Nanoscopic size effects on martensitic transformations in shape memory alloys

Abstract: The effect of the grain size and transverse film size in nano-and micrometer ranges on the parameters of martensitic transitions in shape memory alloys is theoretically considered in the framework of the theory of diffuse martensitic transitions. A quantitative analysis of the size effects is performed including not only the thermodynamic aspect of the martensitic transformation but also its kinetic aspect, which is particularly sensitive to structural and size factors. This complex approach makes it possible to explain the following three basic facts associated with the influence of a decreased grain size or transverse film size on the parameters of the martensitic transition in shape memory alloys: a decrease in the critical (characteristic) transition temperature, an increase in the transition temperature smearing, and the existence of a critical grain size or film thickness below which the martensitic transformation in alloys is blocked.

Antiferromagnetic resonance and magnetic anisotropy in single crystals of the YFe3(BO3)4-GdFe3(BO3)4 system

Abstract: The antiferromagnetic resonance in single crystals of the YFe3(BO3)4-GdFe3(BO3)4 system is studied in the frequency range 25–140 GHz and the temperature range 4.2–50.0 K. It is established that the YFe3(BO3)4 crystal containing only the magnetic subsystem of Fe3+ ions is an antiferromagnet with an easy anisotropy plane. The temperature dependences of the gaps in the antiferromagnetic resonance spectra of GdFe3(BO3)4 and YxGd1 − xFe3(BO3)4 are used to calculate the contributions of the Fe3+ and Gd3+ subsystems to the magnetic anisotropy of these crystals. The contributions are found to be close in magnitude and have opposite signs. This leads to a relatively weak uniaxial anisotropy field in the crystals under investigation. Since the exchange interaction between the Gd3+ and Fe3+ ions magnetizes the magnetic subsystem of gadolinium, both subsystems start to contribute simultaneously at the Neel temperature of the iron subsystem.

Submicrosecond strength of the D16T aluminum alloy at room and elevated temperatures

Abstract: The dynamic yield stress and the dynamic strength were measured for D16T aluminum alloy samples loaded by plane shock waves of submicrosecond duration. The temperature was varied from 20 to 470°C. It is established that the dynamic yield stress of the alloy decreases upon heating due to annealing. The dynamic yield stress of annealed samples increases with temperature, a fact that was observed earlier for aluminum. The dynamic strength of the alloy in the initial state decreases significantly upon heating, while the strength of the annealed material varies only slightly with temperature.

Intrinsic luminescence of rare-earth oxyorthosilicates

Abstract: The spectra and decay kinetics of luminescence and the excitation and reflection spectra of the luminescence of orthosilicates A2SiO5 (A = Y, Lu, Sc, Gd), both nominally pure and doped with cerium, are measured using time-resolved VUV spectroscopy in the ranges of energies ħν = 1.5–16 eV and temperatures T = 8–3000 K. The band structure of the crystals is calculated in the local electron density approximation (LDA). The origin of the intrinsic luminescence in the crystals studied is established, and the assumption regarding the existence of self-trapped excitons and their structure is made.

Mechanism and nature of phase transitions in the (NH4)3MoO3F3 oxyfluoride

Abstract: The temperature dependences of the heat capacity, the unit cell parameter, and the permittivity for the (NH4)3MoO3F3 cryolite (space group Fm\( \overline 3 \)m) are investigated. It is revealed that the compound undergoes ferroelectric and ferroelastic structural phase transitions at temperatures of 297 and 205 K, respectively. The mechanism of structural distortions is discussed in terms of the entropy parameters, pressure-temperature phase diagrams, and electron density maps for critical atoms. An analysis is made of the influence of the cation size and shape on the phase transitions in oxyfluorides of the general formula A2A′MO3 (A,A′ = NH4, K; M = Mo, W).

Intensity of the f-f transitions of Nd 3+ , Er 3+ , and Tm 3+ rare-earth ions in calcium niobium gallium garnet crystals

Abstract: This paper reports on the results of the investigation into the intensities of the f-f transitions of Nd3+, Er3+, and Tm3+ ions in calcium niobium gallium garnet (CNGG) crystals. The values of the oscillator strengths and line strengths obtained for hypersensitive transitions and the intensity parameters Ω t of the rare-earth ions in the CNGG crystals are compared with the corresponding quantities for crystals of other garnets and some oxide and fluoride crystals. The assumption is made that an increase in the oscillator strengths and line strengths for the hypersensitive transitions and the intensity parameters Ω2 of the Nd, Er, and Tm ions in the CNGG crystals as compared to those for crystals of other garnets is associated with the specific features revealed in the crystal structure of the calcium niobium gallium garnet, in particular, with the lowering of the symmetry of the positions occupied by rare-earth ions in the crystal structure.

Colossal magnetoresistance of the inhomogeneous ferromagnetic semiconductor HgCr 2 Se 4

Abstract: A new method is described for attaining high magnetoresistance in inhomogeneous magnetic materials, which makes use of the formation of a depleted layer and a contact potential difference at the interface separating two semiconductors with different Fermi levels and the magnetic-field-induced variation in the contact potential difference and thickness of the interface layer The proposed model of the magnetoresistive structure is realized on the basis of the HgCr2Se4 magnetic semiconductor Layers of n-HgCr2Se4 up to a few tens of microns thick were prepared on the surface of p-HgCr2Se4 bulk single crystals by the diffusion technique Application of a magnetic field stimulated in the structures a strong (by a factor of more than 200) increase of the current flowing through the n-layer