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Since the 1940s, it has been known that diffusion in crystalline solids occurs due to lattice defects. The diffusion of defects can have a great impact on the processing and heat treatment of materials as the microstructural changes caused by diffusion can influence the material qualities and properties. It is, therefore, vital to be able to control the diffusion. This implies that we need a deep understanding of the interactions between impurities, matrix atoms, and intrinsic defects. The role of density functional theory (DFT) calculations in solid-state diffusion studies has become considerable. The main parameters to obtain in defect diffusion studies with DFT are formation energies, binding energies, and migration barriers. In particular, the utilization of the nudged elastic band and the dimer methods has improved the accuracy of these parameters. In systematic diffusion studies, the combination of experimentally obtained results and theoretical predictions can reveal information about the atomic diffusion processes. The combination of the theoretical predictions and the experimental results gives a unique opportunity to compare parameters found from the different methods and gain knowledge about atomic migration. In this Perspective paper, we present case studies on defect diffusion in wide bandgap semiconductors. The case studies cover examples from the three diffusion models: free diffusion, trap-limited diffusion, and reaction diffusion. We focus on the role of DFT in these studies combined with results obtained with the experimental techniques secondary ion mass spectrometry and deep-level transient spectroscopy combined with diffusion simulations. [ABSTRACT FROM AUTHOR]

When a cubic lattice is confined by a surface layer, the effective thermoelastic properties can be tailored by the prestress produced by the surface. The thermal expansion coefficient, temperature derivative of elasticity, and the equation of state (EOS) of the solid depend on the potential of each bond and the lattice structure, which can be predicted by the recently developed singum model. This paper first uses a granular lattice confined by a spherical shell to demonstrate singum modeling of the thermoelastic behavior of the cubic lattices and then extends it to atomic crystal lattices by considering the surface tension and long-range interactions. Given the elasticity and the EOS of a cubic crystal, the interatomic potential can be inversely derived. As the bond length changes with thermal expansion and pressure, the singum model predicts the temperature- and pressure-dependent elasticity. Using the orientational average, isotropic elastic constants can be obtained for polycrystals. The case study of copper (Cu) demonstrates the versatility of the model for different cubic lattices and predicts the experimental results of pressure- and temperature-dependent elasticity. The singum model is general for different lattice types and EOS forms and provides clear physical and mechanical meanings to correlate the interatomic potential, EOS, and elasticity in the closed-form formulation, which is very useful in engineering design and analysis of metal structural members in fire, geothermal, and space applications without the needs of large-scale numerical simulations. [ABSTRACT FROM AUTHOR]

Brillouin light scattering is a versatile measurement technique of the dispersion of bulk acoustic phonons in amorphous and crystalline solids. It allows contactless and non-destructive characterization of the relevant material tensors of optically transparent materials, provided that the optical refraction indices, and in addition the dielectric tensor for piezoelectric materials, are known beforehand. The complete estimation of the anisotropic material tensors is often performed using many different crystal orientations, and hence different samples of the same crystal. In this paper, we consider the problem of measuring those tensors using a single sample of an anisotropic single crystal with trigonal symmetry. A complete measurement requires sufficient experimental diversity in the phonon wavevectors when sampling the phonon velocity surfaces, hence the consideration of a large number of directions away from crystallographic axes. We estimate all six independent elastic constants of non-piezoelectric sapphire ( 3 ¯ m point group) with a single X-cut wafer and the six independent elastic constants together with the four independent piezoelectric constants of piezoelectric lithium niobate (3 m point group) with a single Y-cut wafer. The estimated tensors are in close agreement with those reported in the literature based on resonant ultrasonic techniques. [ABSTRACT FROM AUTHOR]

Crystalline Na2V6O16·3H2O nanobelts have been synthesized by refluxing V2O5 and NaF and self-weaved into a sheet of paper via a vacuum filtration process. Scanning electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy were used to characterize the morphology, structure, and chemical composition of the nanobelt papers. The dehydrated Na2V6O16 nanobelts are excellent field emission candidates, with a low turn-on field of 6.8 V/μm, a large current density up to 2.5 mA/cm2 at an electric field of 10 V/μm, and a very uniform distribution of emission sites. [ABSTRACT FROM AUTHOR]

Photochromic diarylethene is known to exhibit reversible photoisomerization under irradiation with ultraviolet (UV) and visible light. Besides reversible optical properties upon light irradiation, a variety of discontinuous morphological changes of the crystals are reported in the literature, such as sudden crystal bending, cracking, and photosalient effects, which are caused simply by UV and visible light irradiation. These morphological phenomena with discontinuities are micro-scale changes caused by photoisomerization of molecules at the nanoscale and lead to the realization of important functions for optical devices. However, the theoretical models behind these phenomena are not well understood. In this paper, we construct a mathematical model that can treat diverse phenomena in a unified model by using swallow-tail catastrophe, a higher-order catastrophe than cusp catastrophe, from the seven elementary catastrophes that can describe discontinuities in the phenomena. By introducing hyperbolic operating curves in the model, the intrinsic properties of the photochromic crystals are represented. The induced morphogenesis, such as bending, cracking, and photosalient effects, are systematically classified by the proposed catastrophe model, which even implies the possibility of unexplored operating conditions of the crystals and explains known phenomena. The proposed catastrophe-theory-based modeling provides a foundation for understanding and discovering the versatile morphogenesis in photochromic crystals. Furthermore, the proposed approach provides a basis for understanding and discovering various morphological changes in photochromic crystals and similar systems. [ABSTRACT FROM AUTHOR]

In this paper, novel nonlocal reformulations of the conventional continuum-based models for modeling the thermoelastic behavior of cubic crystals based on a recently developed lattice particle method are presented. Like molecular dynamics simulation, the lattice particle method decomposes the grain domain into discrete material particles that are regularly packed according to the underlying atomic lattice. Nonlocal interactions are introduced between material particles and top-down approaches are used to relate model parameters to the material physical constants. Three equivalency assumptions are used in the top-down approach, namely, energy equivalency for the mechanical model, heat transfer rate equivalency for the thermal model, and thermal strain equivalency for the thermal-mechanical coupling model. Different from coordinates transformation used in the conventional continuum-based models, lattice rotation is adopted in the lattice particle method to equivalently represent the material anisotropy while explicitly capturing the crystallographic orientation. Two most common Bravais cubic lattices are studied, i.e., the body-centered cubic lattice and the face-center cubic lattice. The validity and prediction accuracy of the developed models are established by comparing the predicted displacements and temperature results with solutions of conventional continuum theories using the finite element method. [ABSTRACT FROM AUTHOR]

This paper demonstrates a novel Eulerian computational framework for modeling anisotropic elastoplastic deformations of organic crystalline energetic materials (EM) under shocks. While Eulerian formulations are advantageous for handling large deformations, constitutive laws in such formulations have been limited to isotropic elastoplastic models, which may not fully capture the shock response of crystalline EM. The present Eulerian framework for high-strain rates, large deformation material dynamics of EM incorporates anisotropic isochoric elasticity via a hypo-elastic constitutive law and visco-plastic single-crystal models. The calculations are validated against atomistic calculations and experimental data and benchmarked against Lagrangian (finite element) crystal plasticity computations for shock-propagation in a monoclinic organic crystal, octahydro-1,3,5,7-tetranitro-1,3,5,7 tetrazocine (β-HMX). The Cauchy stress components and the resolved shear stresses calculated using the present Eulerian approach are shown to be in good agreement with the Lagrangian computations for different crystal orientations. The Eulerian framework is then used for computations of shock-induced inert void collapse in β-HMX to study the effects of crystal orientations on hotspot formation under different loading intensities. The computations show that the hotspot temperature distributions and the collapse profiles are sensitive to the crystal orientations at lower impact velocities (viz., 500 m/s); when the impact velocity is increased to 1000 m/s, the collapse is predominantly hydrodynamic and the role of anisotropy is modest. The present methodology will be useful to simulate energy localization in shocked porous energetic material microstructures and other situations where large deformations of single and polycrystals govern the thermomechanical response. [ABSTRACT FROM AUTHOR]

The background energy spectrum of lutetium oxyorthosilicate/lutetium yttrium oxyorthosilicate scintillation crystals used in positron emission imaging systems is a research topic of great interest owing to the possibility of using its structure to perform detector calibration and quality control without the need of using external radiation sources. In this paper, the background energy spectra for crystals of different sizes and geometries, including cuboids, frusta, cylinders, tetrahedra, and spheres, have been computed. This was done using a previously developed analytical model that requires the photon absorption probabilities for each shape, obtained through simulations using the PENELOPE Monte Carlo code. It has been found that crystals of different geometries, but with the same volume-to-surface area ratio, give rise to very similar background energy spectra, simplifying its computation. In addition, simple ray tracing simulations were performed, and a robust geometrical model was devised to investigate the geometrical grounds of this practical and compelling result. [ABSTRACT FROM AUTHOR]

We have been observing the kinetic buildup of electrical conduction current upon x-ray irradiation in ZnSe single crystals. The objective is to understand the behavior of ZnSe crystals under x-ray irradiation, toward their potential use in x-ray detectors that do not require cooling and can operate above room temperature. The paper focuses on the factors that affect the behavior of traps at room temperature. A model is developed to understand the origin of the behavior of the conduction current under x-ray irradiation, taking into account the contributions from three types of traps for electrons (shallow, phosphorescent, and deep) and two luminescence centers (luminescence bands of 630 and 970 nm). Principally, the model is applied to the fitting of the experimental curves. Consequently, the work puts in evidence the role of electron accumulation on traps on the x-ray conduction current buildup on ZnSe crystals. We have obtained experimental dependencies of x-ray conduction current buildup current at 85 K, which indicates the presence of delay in comparison with x-ray luminescence buildup. This paper will demonstrate the effectiveness of our theoretical method in the framework of the multi-center model for wide-gap semiconductors. [ABSTRACT FROM AUTHOR]

Ferroelectric tin hypothiodiphosphate (Sn 2 P 2 S 6) crystals are well-known for their significant piezoelectric, electro-optic, and nonlinear optical properties. These crystals have usually been grown by a vapor transport technique. We report in this paper on the first study of photorefractive nonlinearity in Sn 2 P 2 S 6 crystals grown by the Bridgman method. Pronounced photorefraction is observed in the near-infrared region of the spectrum even with no preliminary optical sensitizing. [ABSTRACT FROM AUTHOR]

This paper is devoted to the study of the ductile-brittle behavior of a central nanocrack (1 ¯ 10) [ 110 ] (crack plane/crack front) under biaxial loading via free 3D molecular dynamics (MD) simulations, as well as the comparison of MD results with continuum predictions concerning T-stress. The so called T-stress is a constant stress component acting along the crack plane, which should be considered (together with the stress intensity factor K) in the assessment of brittle-ductile behavior, namely, in the case of the short cracks. Previous 2D atomistic simulations under plane strain conditions indicated that the level of T-stress (controlled by the biaxiality ratio σB/σA from the external loading) affects dislocation emission from the crack and can cause the ductile-brittle transition. The plane strain simulations using the periodic or translational boundary conditions in the bcc lattice have certain limitations: they enable the in-plane dislocation emission (Burgers vector lies in the observation plane), but they do not allow the complete dislocation emission on the all slip systems favored by the shear stress. As presented, our new free 3D atomistic simulations (without periodic or symmetry conditions) enable the activity of the all favored slip systems. Thus, they offer a more realistic insight into the microscopic processes generated by the crack itself in dependence on the T-stress level. [ABSTRACT FROM AUTHOR]

This paper investigates the dynamic and vibration energy harvesting behaviors of defect phononic crystal beams in thermal environments. Variations in the first bandgap and the defect band for flexural waves are discussed with different geometric parameters of the defect cell and structural temperatures by using the supercell technique for infinite defect phononic crystal beams. Transmission characters of defect models with a finite length are studied, considering the influence of thermal loads, and the performance of the defect based energy harvesting is discussed as well. Predicted results indicate that temperature increments reduce the frequency ranges of bandgap and defect band due to the softening effect of thermal load on the phononic crystal beam, and the performance in defect based energy harvesting is obviously depressed with temperature rise. Defect cells with a higher content of the soft component are beneficial for generating defect modes which have relatively stable frequencies in thermal environments. However, higher levels of open-circuit voltage and output power of harvested energy are more likely to be generated with defect cells which have a higher content of the hard component. [ABSTRACT FROM AUTHOR]

This paper determines all the 13 elastic constants Cij of monoclinic β-Ga2O3, which has never been achieved since the discovery of this crystal about 100 years ago. We used resonant ultrasound spectroscopy with laser-Doppler interferometry, where the resonant-mode identification was unambiguously made by comparing measured and calculated displacement distributions on the vibrating specimen surface. Using more than 110 resonance frequencies, we inversely determined the Cij: C11 = 242.8 ± 2.9, C22 = 343.8 ± 3.8, C33 = 347.4 ± 2.5, C44 = 47.8 ± 0.2, C55 = 88.6 ± 0.5, C66 = 104.0 ± 0.5, C12 = 128 ± 0.1, C13 = 160 ± 1.5, C23 = 70.9 ± 2.1, C15 = –1.62 ± 0.05, C25 = 0.36 ± 0.01, C35 = 0.97 ± 0.03, and C46 = 5.59 ± 0.69 GPa. We also performed a density-functional-theory calculation and found that the local density approximation yields both the lattice parameters and Cij closer to the measurements than the generalized gradient approximation. Strong elastic-stiffness anisotropy is found in the diagonal elastic constants: C11 is smaller than C22 and C33 by ∼30%, and the difference between C44 and C66 exceeds 50%. Our measurements also reveal anomalous Poisson's ratios: ν21 and ν31 exceed 0.5, and ν23 and ν32 are almost zero. We explain these unusual elastic properties with the truss-like deformation of the tetrahedra-octahedra network and confirm this view with the ab- initio calculation. [ABSTRACT FROM AUTHOR]

In millimeter wave frequency range, hexagonal ferrites with high uniaxial anisotropic magnetic fields are used as absorbers. These ferrites include M-type barium ferrite (BaFe12O19) and strontium ferrite (SrFe12O19), which have natural ferromagnetic resonant frequency range from 40 GHz to 60 GHz. However, the higher frequency range lacks suitable materials that support the higher frequency ferromagnetic resonance. A series of gallium-substituted ε-iron oxides (ε-GaxFe2-xO3) are synthesized, which have ferromagnetic resonant frequencies appearing over the frequency range of 30 GHz to 150 GHz. The ε-GaxFe2-xO3 is synthesized by the sol-gel method. The particle sizes are observed to be smaller than 100 nm. In this paper, in-waveguide transmission and reflection method and the free space magneto-optical approach have been employed to study these newly developed ε-GaxFe2-xO3 particles in millimeter waves. These techniques enable to obtain precise transmission spectra to determine the dielectric and magnetic properties of both isotropic and anisotropic ferrites in the microwave and millimeter wave frequency range from single set of direct measurements. The complex dielectric permittivity and magnetic permeability spectra of ε-GaxFe2-xO3 are shown in this paper. Strong ferromagnetic resonances at different frequencies determined by the x parameter are found. [ABSTRACT FROM AUTHOR]

Recent research has highlighted the potential to achieve high-thermal-conductivity polymers by aligning their molecular chains. Combined with other merits, such as low-cost, corrosion resistance, and light weight, such polymers are attractive for heat transfer applications. Due to their quasi-one-dimensional structural nature, the understanding on the thermal transport in those ultradrawn semicrystalline polymer fibers or films is still lacking. In this paper, we built the ideal repeating units of semicrystalline polyethylene and studied their dependence of thermal conductivity on different crystallinity and interlamellar topology using the molecular dynamics simulations. We found that the conventional models, such as the Choy-Young's model, the series model, and Takayanagi's model, cannot accurately predict the thermal conductivity of the quasi-one-dimen- sional semicrystalline polyethylene. A modified Takayanagi's model was proposed to explain the dependence of thermal conductivity on the bridge number at intermediate and high crystallinity. We also analyzed the heat transfer pathways and demonstrated the substantial role of interlamellar bridges in the thermal transport in the semicrystalline polyethylene. Our work could contribute to the understanding of the structure-property relationship in semicrystalline polymers and shed some light on the development of plastic heat sinks and thermal management in flexible electronics. [ABSTRACT FROM AUTHOR]

This paper presents research data on cathodoluminescence, photoluminescence, and Cherenkov radiation at 200–800 nm excited in crystals with different refractive indices by a subnanosecond runaway electron beam and by KrCl excilamp radiation with a peak wavelength of 222 nm. The data include spectral and amplitude-time characteristics measured with a resolution of up to ~100 ps for natural and synthetic diamonds of type IIa, sapphire, CsI, ZnS, CaF2, ZrO2, Ga2O3, CaCO3, CdS, and ZnSe. The research suggests that cathodoluminescence and photoluminescence should be accounted for in Cherenkov-type detectors of runaway electrons. The results can be useful for detecting high-energy electrons in tokamaks. [ABSTRACT FROM AUTHOR]

In this paper, we report on a simple, feasible method to fabricate microchannels in diamond. Polycrystalline diamond microchannels were produced by fabricating trenches in a Si wafer and subsequently depositing a thin layer of diamond onto this substrate using the hot filament vapor deposition technique. Fabrication of trenches in the Si substrate at different depths was carried out by standard photolithography, and the subsequent deposition of the diamond layer was performed by the hot filament chemical vapor deposition technique. The growth mechanism of diamond that leads to the formation of closed diamond microchannels is discussed in detail based on the Knudsen number and growth chemistry of diamond. Variations in the crystallite size, crystalline quality, and thickness of the diamond layer along the trench depths were systematically analyzed using cross-sectional scanning electron microscopy and Raman spectroscopy. Defect density and formation of non-diamond forms of carbon in the diamond layer were found to increase with the trench depth, which sets a limit of 5–45 μm trench depth (or an aspect ratio of 1–9) for the fabrication of diamond microchannels using this method under the present conditions. [ABSTRACT FROM AUTHOR]

Previous studies on the interaction of collinear acoustic waves have been devoted to waves propagating along pure modes directions of cubic crystals. In this paper, we show that the calculations can be readily extended to all crystal point groups. Nonlinearity parameters characterizing the nonlinear interactions are defined here. The effective third order elastic constants involved in the parameters can be calculated by using the method presented in this paper. Our results are very useful for the study of elastic nonlinearity of crystals with any given symmetry. [ABSTRACT FROM AUTHOR]

Disordered many-particle hyperuniform systems are exotic amorphous states of matter that lie between crystals and liquids. Hyperuniform systems have attracted recent attention because they are endowed with novel transport and optical properties. Recently, the hyperuniformity concept has been generalized to characterize two-phase media, scalar fields, and random vector fields. In this paper, we devise methods to explicitly construct hyperuniform scalar fields. Specifically, we analyze spatial patterns generated from Gaussian random fields, which have been used to model the microwave background radiation and heterogeneous materials, the Cahn-Hilliard equation for spinodal decomposition, and Swift-Hohenberg equations that have been used to model emergent pattern formation, including Rayleigh-Bénard convection. We show that the Gaussian random scalar fields can be constructed to be hyperuniform. We also numerically study the time evolution of spinodal decomposition patterns and demonstrate that they are hyperuniform in the scaling regime. Moreover, we find that labyrinth-like patterns generated by the Swift-Hohenberg equation are effectively hyperuniform. We show that thresholding (level-cutting) a hyperuniform Gaussian random field to produce a two-phase random medium tends to destroy the hyperuniformity of the progenitor scalar field. We then propose guidelines to achieve effectively hyperuniform two-phase media derived from thresholded non-Gaussian fields. Our investigation paves the way for new research directions to characterize the large-structure spatial patterns that arise in physics, chemistry, biology, and ecology. Moreover, our theoretical results are expected to guide experimentalists to synthesize new classes of hyperuniform materials with novel physical properties via coarsening processes and using state-of-the-art techniques, such as stereolithography and 3D printing. [ABSTRACT FROM AUTHOR]

Practical use of Resistive Random Access Memory (ReRAM) depends on thorough understanding of the resistive switching (RS) mechanism in transition metal oxides. Although most of ReRAM samples have polycrystalline structures, both experimental studies and theoretical calculations do not often consider the effects that grain boundaries have on the RS mechanism. This paper discusses what determines resistance values in a Pt/polycrystalline NiO/Pt ReRAM structures by using both experiments and first-principles calculations. Electrical measurements suggest that the RS is caused in the grain boundaries of NiO films. First-principles calculations indicate that slight displacements of atoms with a small energy change of 0.04 eV per atom on the surfaces exposed in the grain boundaries can drastically change conductivities. We propose the tiling model, in which grain surfaces are composed by insulating and conductive micro surface structures, and the surface resistances are determined by the tiling patterns. [ABSTRACT FROM AUTHOR]

The paper discusses the behavior of the dynamical lattice thermal conductivity κ(Ω) of bulk semiconductor crystals. The calculation approach is based on solving Boltzmann-Peierls phonon transport equation in the frequency domain after excitation by a dynamical temperature gradient, within the framework of the single relaxation time approximation and using modified Debye-Callaway model. Our model allows us to obtain a compact expression for κ(Ω) that captures the leading behavior of the dynamical thermal conduction by phonons. This expression fulfils the causality requirement and leads to a convolution type relationship between the heat flux density current and the temperature gradient in the real space-time domain in agreement with Gurtin-Pipkin theory. The dynamical behavior of κ(Ω) is studied by changing temperature as well as different intrinsic and extrinsic parameters. Our calculations show the cut-off frequency of κ(Ω) to be sensitive to the changes of some of these parameters. The paper investigates also the applicability of Shastry's sum rule (SSR) in the frame work of Boltzmann theory. It is shown that within the frame work of Callaway approximated form of the collision operator and time independent Callaway parameter, the SSR breaks down and is only valid when resistive processes dominate normal processes, for which case, we derive an alternative expression to the classical limit of the expectation of the thermal operator introduced in Shastry's formalism. [ABSTRACT FROM AUTHOR]

In this paper we study the bandgap properties of two-dimensional phononic crystals with cross-like holes using the finite element method. The influence of the geometry parameters of the holes on the bandgaps is discussed. In contrast to a system of square holes, which does not exhibits bandgaps if the symmetry of the holes is the same as that of the lattice, systems of cross-like holes show large bandgaps at lower frequencies. The bandgaps are significantly dependent upon the geometry (including the size, shape, and rotation) of the cross-like holes. The vibration modes of the bandgap edges are computed and analyzed in order to clarify the mechanism of the generation of the lowest bandgap. It is found that the generation of the lowest bangdap is a result of the local resonance of the periodically arranged lumps connected with narrow connectors. Spring-mass models are developed in order to predict the frequencies of the lower bandgap edges. The study in this paper is relevant to the optimal design of the bandgaps in light porous materials. [ABSTRACT FROM AUTHOR]

Evanescent modes in complete sonic crystals (SCs) and SC with point defects are reported both theoretically and experimentally in this paper. Plane wave expansion (PWE) and in general, ω(k) methods have been used to calculate band structures showing gaps that have been interpreted as ranges of frequencies where no real k exists. In this work, we extend PWE to solve the complex k(ω) problem applied to SC, introducing the supercell approximation for studying one vacancy. Explicit matrix formulation of the equations is given. This k(ω) method enables the calculation of complex band structures, as well as enabling an analysis of the propagating modes related with real values of the function k(ω), and the evanescent modes related with imaginary values of k(ω). This paper shows theoretical results and experimental evidences of the evanescent behavior of modes inside the SC band gap. Experimental data and numerical results using the finite elements method are in very good agreement with the predictions obtained using the k(ω) method. [ABSTRACT FROM AUTHOR]

This paper is devoted to the phenomenon of resonant transmission in three-dimensional electromagnetic band gap structures (EBGs), achieved by introducing a defect into an EBG structure with the corresponding resonance frequency of the defect lying within the initial EBG region. It is shown that the resonance frequency of an EBG air cavity (acceptor defect) can either increase or decrease with increasing cavity dimensions. In particular, this paper demonstrates both experimentally and theoretically that the resonance frequency of a woodpile air cavity, formed by the partial removal of a woodpile bar, increases with increasing cavity size; it increases continuously or saturates depending on the field polarization with respect to the cavity orientation. The results obtained in this paper can be used for designing EBG devices based on resonant transmission. [ABSTRACT FROM AUTHOR]

Tunable photonic crystals that are based on acousto-optic effects were proposed in this paper. Theoretical analysis and numerical simulations were carried out for the tunable photonic crystals. According to the acousto-optic effect, acoustic waves propagating in elastic media induce periodic dielectric structures in the media, and thus the media can be used as photonic crystals. Such photonic crystals possess a prominent characteristic that the photonic band gaps can be changed real-timely by adjusting the acoustic intensity and frequency. The work done in this paper opens up a possibility to achieve tunable photonic crystals on the basis of acousto-optic effects. [ABSTRACT FROM AUTHOR]

This paper describes a theoretical approach to calculate the permeability tensor of polycrystalline ferrites in various magnetization states, from the completely demagnetized state to saturation. Demagnetizing dynamic fields related to the magnetic domain and grain shapes are introduced in the initial equations to describe the interactions between domains in a grain and between grains. In this paper, we focus on the calculation of the initial permeability of polycrystalline ferrites. Results obtained from the model are compared in a completely demagnetized state to those given by the well-known Schloemann model and to experiments. In all the models given in the literature, an unusually high value of the damping factor must be introduced into the calculations to correctly describe the low-field loss region. Values of the damping factor comparable to those calculated from the measured resonance linewidth of ferrites permit the prediction of the spreading of the magnetic losses as a function of frequency experimentally observed at low bias fields and especially in the demagnetized state. [ABSTRACT FROM AUTHOR]

A mechanism to design compact high-Q filters is proposed in this paper, based on the properties of a defect mode in a one-dimensional (1D) photonic crystal composed of alternating layers of negative-permeability material and negative-permittivity material. The eigenfrequency equation for the defect mode is derived by means of the transfer-matrix method, and then, the dependence of the eigenfrequency on the thicknesses of the two host layers and the defect layer is calculated. In contrast to the case for a filter based on a conventional 1D photonic crystal (with positive refractive indices), we find that the quality factor Q of the filter involved in this paper can be boosted noticeably while the volume of the filter decreases. [ABSTRACT FROM AUTHOR]

Ge–Sb–Te alloys are widely used for data recording based on the rapid and reversible amorphous-to-crystalline phase transformation that is accompanied by increases in the optical reflectivity and the electrical conductivity. However, uncertainties about the optical band gaps and electronic transport properties of these phases have persisted because of inappropriate interpretation of reported data and the lack of definitive analytical studies. In this paper we characterize the most widely used composition, Ge2Sb2Te5, in its amorphous, face-centered-cubic, and hexagonal phases, and explain the origins of inconsistent or unphysical results in previous reports. The optical absorption in all of these phases follows the relationship αhν∝(hν-Egopt)2, which corresponds to the optical transitions in most amorphous semiconductors as proposed by Tauc, Grigorovici, and Vancu [Tauc et al., Phys. Status Solidi 15, 627 (1966)], and to those in indirect-gap crystalline semiconductors. The optical band gaps of the amorphous, face-centered-cubic, and hexagonal phases are 0.7, 0.5, and 0.5 eV, respectively. The subgap absorption in the amorphous phase shows an exponential decay with an Urbach slope of 81 meV. We measured the photoconductivity of amorphous Ge2Sb2Te5 and determined a mobility-lifetime product of 3×10-9 cm2/V. The spectral photoconductivity shows a threshold at about 0.7 eV, in agreement with our analysis of the optical band gap. The face-centered-cubic and hexagonal phases are highly conductive and do not show freeze-out; even at 5 K the density of free carriers remains at 1019–1020 cm-3, so these are degenerate semiconductors in which the Fermi level resides inside a band. In the hexagonal phase, the effect of free electrons on the Hall coefficient is significant at high temperatures. When the Hall data are fitted using the two-carrier analysis, the hole mobility is found to decrease slowly with temperature, as expected. The considerations discussed in this paper can be readily applied to study related chalcogenide materials. [ABSTRACT FROM AUTHOR]

This paper derives exact expressions of the isothermal third order elastic constants (TOE) in crystalline solids in terms of the kinetic and potential energies of the system. These expressions reveal that the TOE constants consist of a Born component and a relaxation component. The Born component is simply the third derivative of the system's potential energy with respect to the deformation, while the relaxation component is related to the non-uniform rearrangements of the atoms when the system is subject to a macroscopic deformation. Further, based on the general expressions derived here, a direct (fluctuation) method of computing the isothermal TOE constants is developed. Numerical examples of using this fluctuation method are given to compute the TOE constants of single crystal iron. [ABSTRACT FROM AUTHOR]

Results of broadband dielectric investigations of 0.94PbMg1/3Ta2/3O3-0.06PbTiO3 (0.94PMT-0.06PT or PMT-PT) crystals in wide temperature range from 100 K to 950 K are presented. Below 300 K the dielectric properties of crystals are governed by polar nanoregions dynamics. However, at higher temperatures (above 600 K) the electrical conductivity effects also become important. The electrical conductivity occurs presumably due the hopping of oxygen vacancies and demonstrates change in the activation energy close to 750 K. The change in the activation energy can be explained by increase of concentration of single ionized vacancies. No anomaly in the temperature dependence of the static dielectric permittivity was observed in wide temperature range from 213 K to 950 K and the dependence was successfully described by spherical random bonds random fields theory. Moreover, two different contributions were successfully separated in the distributions of relaxation times in PMT-PT crystals. These contributions were attributed to different polar nanoregions dynamics. Moreover, the different freezing temperatures values were obtained for most probable and longest relaxation times, which was explained with an idea of continuous distribution of relaxation times broadening on cooling in relaxors. The critical remarks about the Vogel-Fulcher law application for relaxors are addressed in the paper. [ABSTRACT FROM AUTHOR]

Non-Contact Acousto-Thermal Signature (NCATS) analysis uses conversion of acoustic energy to heat to characterize evolving damage in materials. In the past, the observed temperature changes were interpreted using phenomenological approaches. This paper presents details of the mechanisms and the theoretical models to predict the temperature change due to conversion of acoustic energy to heat. NCATS experimental measurements performed using 20 kHz high amplitude acoustic waves on as received and fatigued polycrystalline Ti-6Al-4V are compared with theoretical calculations based on the mechanisms of transverse thermal currents, inter-crystalline thermal currents, and dislocation density changes. In the as received samples, the transverse thermal currents contribution has been found to be negligible compared with inter-crystalline thermal currents contribution. The experimentally measured maximum temperature change in the as received sample has been found to be 0.5 °C, and the theoretical prediction based on inter-crystalline thermal currents is 0.08 °C. In the fatigue damaged samples, the maximum temperature change increases with increasing damage that can be attributed to the increasing dislocation density. The theoretical prediction of the maximum temperature attained by a sample that is near failure based on dislocation contribution is 2.0 °C, while the experimental measurements have been found to be 0.95 °C. The differences between the theoretical and the experimental measurements are discussed in the context of the uncertainties in several physical parameters used in the theoretical calculations. [ABSTRACT FROM AUTHOR]

Single crystals based on solid solutions of lead-magnesium-niobate (PMN) and lead titanate (PT) have emerged as highly promising multifunctional systems combining piezoelectric, pyroelectric, and electro-optic properties that surpass by far those of the best known lead-zirkonium-titanate ceramics. In this paper we present new findings on how the phase transition temperature and the dielectric and ferroelectric properties can be tuned depending on crystal composition, orientation, and thermoelectrical treatment. Mn-doped and pure 0.72PbMg1/3Nb2/3O3-0.28PbTiO3 (0.72PMN-0.28PT) single crystals with <111> and <001> orientations were investigated. A special attention was devoted to field cooling (FC), i.e., cooling under electric field from different temperatures. The results illustrate different findings that were not reported before: the Curie temperature, i.e., ferroelectric-paraelectric transition temperature, is enhanced after field cooling of the Mn-doped, <001>-oriented crystal while such a shift is not observed in the <111>-oriented and the non-doped crystals. In addition, substantial polarization suppression occurs in the Mn-doped crystals upon FC from high temperature regardless of orientation. Based on piezoforce microscopy of the domain structure that shows suppression of domain growth following field cooling from 200 °C, we propose a mechanism for polarization suppression based on domain pinning by charged defects. The practical importance of our results lies in showing the opportunity offered by a proper choice of crystal composition and poling conditions for tuning the functional properties of PMN-PT single crystals for a specific application. This should contribute to the understanding of their properties towards advanced sensor and transducers devices. [ABSTRACT FROM AUTHOR]

This paper demonstrates a new power cycle for direct conversion of mechanical energy into electrical energy under a thermal bias. The cycle consisted sequentially of (i) an electric poling process under zero stress, (ii) an isoelectric process consisting of applying a uniaxial compressive stress σH followed by (iii) an electric de-poling process under constant compressive stress, and finally (iv) an isoelectric process consisting of removing the compressive stress. The new cycle was performed at constant bias-temperature Tb. It was demonstrated on [001]-poled 0.72PbMg1/3Nb2/3O3-0.28PbTiO3 single crystals. The power density increased with increasing cycle frequency and compressive stress for frequency up to 1 Hz. Maximum energy and power densities of 44 J/l/cycle and 44 W/l were achieved at 1 Hz for bias-temperature Tb of 80 °C and electric field cycled between 0.2 and 0.8 MV/m with compressive stress σH = 25.13 MPa. This was attributed to a tetragonal-monoclinic-orthorhombic phase transition sequence. The material efficiency reached up to 87% and exceeded that of a similar thermomechanical power cycle performed on pyroelectric material. Finally, a physical model predicting the power density was derived and yielded accurate predictions of experimental data for all bias-temperatures considered and cycle frequency up to 1 Hz. [ABSTRACT FROM AUTHOR]

Thanks to the effective electrostatic potential generated by the ordered atomic structure, bent crystals can efficiently deflect ultra relativistic charged beams by means of planar and axial channeling phenomena as well as of the recently discovered volume reflection effect. Most of the experimental knowledge about these phenomena has been gathered with Si crystals, but it has been recently demonstrated that the steering performance can be improved by using high quality Ge materials which have a larger atomic number. In this paper, we investigate channeling and volume reflection of 400 GeV protons from (110) lattice planes in highly bent Ge strips crystals. Both production and characterization of the strips are presented. Herein, the experimental results on deflection are compared with theoretical predictions, with previous published data and with the expected performances of Si crystals in similar experimental conditions. [ABSTRACT FROM AUTHOR]

This paper reports on the effects of Li+, Na+, Mg2+, Ca2+, Sr2+, and Ba2+ co-doping on the scintillation properties of LaBr3:5%Ce3+. Pulse-height spectra of various gamma and X-ray sources with energies from 8 keV to 1.33 MeV were measured from which the values of light yield and energy resolution were derived. Sr2+ and Ca2+ co-doped crystals showed excellent energy resolution as compared to standard LaBr3:Ce. The proportionality of the scintillation response to gamma and X-rays of Ca2+, Sr2+, and Ba2+ co-doped samples also considerably improves. The effects of the co-dopants on emission spectra, decay time, and temperature stability of the light yield were studied. Multiple thermoluminescence glow peaks, decrease of the light yield at temperatures below 295 K, and additional long scintillation decay components were observed and related to charge carrier traps appearing in LaBr3:Ce3+ with Ca2+, Sr2+, and Ba2+ co-doping. [ABSTRACT FROM AUTHOR]

In this paper, we report on the theoretical investigation of the propagation characteristics of Lamb wave in a phononic crystal structure constituted by a square array of cylindrical stubs deposited on both sides of a thin homogeneous plate. The dispersion relations, the power transmission spectra, and the displacement fields of the eigenmodes are studied by using the finite-element method. We investigate the evolution of band gaps in the double-sided phononic plate with stub height on both sides arranged from an asymmetrical distribution to a symmetrical distribution gradually. Numerical results show that as the double stubs in a unit cell arranged more symmetrically on both sides, band width shifts, new band gaps appear, and the bands become flat due to localized resonant modes which couple with plate modes. Specially, more band gaps and flat bands can be found in the symmetrical system as a result of local resonances of the stubs which interact in a stronger way with the plate modes. Moreover, the symmetrical double-sided plate exhibits lower and smaller band gap than that of the asymmetrical plate. These propagation properties of elastic or acoustic waves in the double-sided plate can potentially be utilized to generate filters, slow the group velocity, low-frequency sound insulation, and design acoustic sensors. [ABSTRACT FROM AUTHOR]

In this paper, we propose a method for tailoring the absorption in a photonic crystal membrane. For that purpose, we first applied time domain coupled mode theory to such a subwavelength membrane and demonstrated that 100% resonant absorption can be reached even for a symmetric membrane, if degenerate modes are involved. Design rules were then derived from this model in order to tune the absorption. Subsequently, finite difference time domain simulations were used as a proof of concept and carried out on a low absorbing material (extinction coefficient = 10-2) with a high refractive index corresponding to the optical indices of amorphous silicon at around 720 nm. In doing so, 85% resonant absorption was obtained, which is significantly higher than the commonly reported 50% maximum value. Those results were finally analyzed and confronted to theory so as to extend our method to other materials, configurations and applications. [ABSTRACT FROM AUTHOR]

Lead based relaxor single crystals have raised a considerable interest due to their outstanding piezoelectric properties. These piezoelectric properties are temperature dependent and are known to decrease when the temperature is decreased. The aim of this paper is to study the variation of the dielectric constant, the elastic compliance, the coupling factor, the piezoelectric charge coefficient, and the mechanical quality factor for rhombohedral single crystals of (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) poled along [001] in the 5 K-300 K temperature range. In particular, it has been shown that a relaxation phenomenon occurs around 100 K leading to a rapid decrease of the studied piezoelectric, mechanical, and dielectric constants just after a plateau. As a consequence, the similar behaviour of the dielectric constant, piezoelectric charge coefficient, and elastic compliance versus temperature leads to a temperature independent behaviour of the coupling factor above 100 K. This relaxation is also responsible for the minimum observed on the mechanical quality factor. By comparing the measured dielectric constant with the calculated one using the Landau-Ginsburg-Devonshire theory, this relaxation has been attributed to electrical charges freezing within the domain as the temperature is decreased. [ABSTRACT FROM AUTHOR]

This paper reports the temperature dependent cross-plane thermal conductivity of pure silica zeolite (PSZ) MFI and MEL thin films measured using the 3ω method between 30 and 315 K. PSZ MFI thin films were b-oriented, fully crystalline, and had a 33% microporosity. PSZ MEL thin films consisted of MEL nanoparticles embedded in a nonuniform and porous silica matrix. They featured porosity, relative crystallinity, and particle size ranging from 40% to 59%, 23% to 47%, and 55 to 80 nm, respectively. Despite their crystallinity, MFI films had smaller thermal conductivity than that of amorphous silica due to strong phonon scattering by micropores. In addition, the effects of increased relative crystallinity and particle size on thermal conductivity of MEL thin films were compensated by the simultaneous increase in porosity. Finally, thermal conductivity of MFI zeolite was predicted and discussed using the Callaway model based on the Debye approximation. [ABSTRACT FROM AUTHOR]

This paper investigates the morphological evolution of hydrogenated amorphous silicon layers obtained by plasma enhanced chemical vapor deposition at different H dilutions in the regime close to the formation of the nanocrystalline phase. The role of hydrogen in the transition from the amorphous to the crystalline phase is investigated by accurate structural and chemical characterization, from the early stages of nucleation, where the nuclei present size slightly larger than the critical nucleus, i.e., about 0.8 nm in radius, up to the formation of crystalline grains larger than 30 nm in radius. A correlation between the structural characteristics of such crystalline phase and the bonding mechanism of Si with H through multiple hydrides, such as Si-H2 and Si-H3 is found, particularly the tri-hydrides are found to be directly correlated to the shape and the size of the nanocrystallites present in the films. The multiple hydrides are found to play a role also in the electrical characteristics of p-i-n a-Si:H solar cells whose intrinsic layer is realized in the above H dilution conditions. An explanation of the experimental data in terms of the different bonding mechanism of H in the Si matrix is provided. [ABSTRACT FROM AUTHOR]

The refractive index of sodium iodide, an important scintillator material that is widely used for radiation detection, is based on a single measurement made by Spangenberg at one wavelength using the index-matching liquid immersion method (Z. Kristallogr. 57, 494 (1923)). In the present paper, we present new results for the refractive index of sodium iodide as measured by the minimum deviation technique at six wavelengths between 436 nm (n = 1.839 ± 0.002) and 633 nm (n = 1.786 ± 0.002). These six measurements can be fit to a Sellmeier model, resulting in a χ2 of 1.02, indicating a good fit to the data. In addition, we report on ellipsometry measurements, which suggest that the near-surface region of the air sensitive NaI crystal seriously degrades, even in a moisture-free environment, resulting in a significantly lower value of the refractive index near the surface. First-principles theoretical calculations of the NaI refractive index that agree with the measured values within 0.025-0.045 are also presented and discussed. [ABSTRACT FROM AUTHOR]

Superior piezoelectric 36-shear mode of [011] poled Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) in zxt+/-45o cut direction is the first practical piezoelectric 36-shear mode reported [P. Han, 'Piezoelectric crystal elements of shear mode and process for preparation,' U.S. patent 7,908,722 B2 (2004)]. The investigation on the piezoelectric 36-shear mode of PIN-PMN-PT crystal was carried out and summarized in this paper. The [011] poled 24%Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) crystal was chosen for the study owing to its large coercive electrical field and high de-poling temperature. Following an introduction to the concept of the piezoelectric 36-shear mode and targeting to explore the maximum d36, we have performed the theoretical calculation on d36 by coordinate transformation of a third rank tensor. To verify the results of the calculation, the d36 was measured by two ways: strain measurements (piezoelectric converse effect) and charge measurements (piezoelectric direct effect) on the same composition crystal. It has been distinguished and determined that the maximum d36 piezoelectric coefficients occurred in the zxt + 45o cut direction for negative and in the zxt-45o cut direction for positive, respectively. The electromechanical coupling factor k36 was also studied. The occurrence of the maximum d36 up to 2400 pC/N and the large electromechanical coupling factor k36 as high as 87.8% were theoretically and experimentally confirmed on zxt ± 45o cut, [011] poled 24%PIN-PMN-PT piezoelectric crystals. The 36-shear mode PIN-PMN-PT crystal enables the piezoelectric performance under high electrical field driving. [ABSTRACT FROM AUTHOR]

Axial magnetostriction in giant magnetostrictive materials with specific crystal orientation is highly anisotropic when changing magnetization direction. Based on a single-{111}-twin crystal growth model, this paper presents a domain rotation simulation of anisotropic magnetostrictions in <110> oriented crystals Tb0.36Dy0.64(Fe0.85Co0.15)2 by considering the demagnetization effect. Simulation results show that the axial magnetostrictions under different external magnetic field directions, which correspond to specific magnetic moment 'jump' and 'rotation' processes, are in good agreement with experimental data. The demagnetization effect, arising from non-coaxial magnetization, hinders moment alignments especially at low external fields and reduces the initial magnetostriction. Change of the saturation magnetostriction is also presented by multi-domain calculations through revealing the detailed magnetization processes. Such domain rotation simulations might provide a better understanding of the magnetostriction dependence on magnetization direction, i.e., the anisotropic magnetostrictive responses. [ABSTRACT FROM AUTHOR]

Vacuum annealing is a widely used method to increase the electric conductivity of SrTiO3 single crystals. The induced oxygen vacancies act as intrinsic donors and lead to n-type conductivity. Apart from the changed electronic structure, however, also crystal structure modifications arise from this treatment. Hence, electronic properties are determined by the interplay between point defects and line defects. The present paper provides a survey of the real structure of commercially available SrTiO3 single crystals and the changes induced by reducing vacuum heat-treatment. Therefore, all investigations were performed ex situ, i.e., after the annealing process. Used characterization methods include atomic force microscopy, transmission electron microscopy, spectroscopic ellipsometry, infrared spectroscopy, and photoluminescence spectroscopy. Besides the expected variation of bulk properties, especially surface modifications have been detected. The intrinsic number of near-surface dislocations in the samples was reduced by vacuum annealing. X-ray photoelectron spectroscopy proves the existence of a layer of adsorbed molecules, which influences the SrTiO3 work function. Also, the interaction between adsorbates and surface point defects as well as laser annealing due to local oxygen absorption are discussed. [ABSTRACT FROM AUTHOR]

Magnetic quartz crystal microbalance is a new instrument that allows determination of magneto-acoustic parameters of thin magnetic films. The analysis in this paper is based on simple Butterworth-van-Dyke oscillator model. To demonstrate the use of the new instrument, CuNi magnetic alloy has been electroplated on a gold electrode of a quartz crystal microbalance substrate. The magneto-acoustic admittance and magneto-acoustic capacitance of the alloy film were measured in the static magnetic field between 0 and 10.00 kG, at room temperature. [ABSTRACT FROM AUTHOR]

For the implementation of optoelectronic devices on silicon, which could be realized by a combination of Si and direct-bandgap III/V semiconductors, a defect free nucleation layer of GaP on Si is essential. This paper summarizes the results of structural investigations carried out by transmission electron microscopy on defects, which can be observed in GaP films grown by metal organic vapor phase epitaxy on exactly oriented (001) Si substrates. Under optimized growth conditions the anti phase domains (APDs), which arise in the III/V semiconductor at the monoatomic steps on the silicon surface, show a specific typical shape. They self-annihilate on {112} planes in the GaP and can be observed in [110] cross-section, looking perpendicular to the steps on the Si surface. In contrast to that, the anti phase boundaries (APBs) lie on {110} GaP planes in the [-110] direction, parallel to the steps on the Si surface. From convergent beam electron diffraction one can show, that the GaP has Ga-polarity in the [110] direction, viewing perpendicular to the steps on the Si-surface. With the knowledge of the polarity and the shape of the APDs, we suggest a model for chemical composition of their boundaries. According to this model the APBs, which lie on {110} and {112} planes, consist of an equal amount of Ga-Ga and P-P bonds. Furthermore, when stacking faults and twins are observed, they only occur in the Ga-polar [110] GaP direction, and consequently lie on {111}A planes. With the knowledge of the structure of the defects that arise at the GaP/Si interface we suggest growth conditions and an optimum Si surface structure, which guarantee a defect-free GaP overgrowth layer after several 10 nms of III/V material, even on exact Si substrates. [ABSTRACT FROM AUTHOR]

In this paper, the dielectric constants of colloidal nanocrystals are investigated by scanning capacitance microscopy (SCM) and scanning microwave microscopy (SMM). Whereas SMM provides frequencies from 1 up to 18 GHz, conventional SCM is restricted to values below 20 kHz. With both techniques, C(V) and dC(V)/dV curves are acquired on various nanocrystalline films (Fe3O4, CoFe2O4, PbS) with monolayer thickness and on uncovered reference areas on the same samples. As we find, the dielectric constants of these nanocrystals are significantly larger as those of the bulk materials. A strong decrease of the permittivity with increasing frequency is also found. [ABSTRACT FROM AUTHOR]

Assuming an idealized piezoelectric bulk acoustic wave resonator, one typically calculates the velocity of the fundamental bulk acoustic mode as the measured frequency times twice the thickness of the piezoelectric film. In c-axis 6mm hexagonal crystals of (e.g., ZnO or AlN), both the longitudinal and thickness shear modes are peizoelectrically active using thickness excitation and lateral-field excitation, respectively. Without a loss of generality, we concentrate our study on ZnO films. The theoretical velocity of the pure thickness shear mode in sputtered ZnO, based strictly on reported material properties, is calculated to be approximately 2580 m/s. However, a variety of acoustic velocities for the thickness shear mode in ZnO have been reported in the literature ranging from about 3100-3500 m/s. These reported values represent a 20%-36% increase in acoustic velocity relative to the theoretical values. In the literature, this deviation is typically attributed to ZnO film inconsistencies and other phenomena which can be difficult to quantify. We propose that the reported inconsistencies may be attributed to a hybrid acoustic mode comprised of a coupling of shear and longitudinal particle displacements. In this paper, we present a theoretical description of a hybrid mode in ZnO solidly mounted resonator (SMR) devices. We begin first with an experimental verification of a mode with a changing velocity in a ZnO SMR with the only variable being the ZnO thickness. Using the acoustic velocity through the thickness as an effective velocity with which to reference the mode, we find the effective acoustic velocity to range from 3100-3900 m/s, with increasing ZnO thickness. We then start from the first principles of piezoelectric acoustic wave propagation and derive three coupled partial differential equations describing a hybrid mode comprised of the coupling between longitudinal and shear particle displacement and the corresponding piezoelectrically generated potential in the ZnO film. The qualitative findings described by the derived equations are then further investigated with finite element simulation (COMSOL MULTIPHYSICS®). We simulate the performance of our experimental devices using the COMSOL platform, examine the eigenfrequencies of the structure, and find a hybrid mode which is trapped both vertically and laterally in the ZnO film. Calculating the effective velocity of the simulated modes, we find the simulated effective velocities to be within 1.5% of our measured results. Finally, we compare simulation results to experimentally measured results of a previously observed hybrid mode and achieve a 0.2% agreement. [ABSTRACT FROM AUTHOR]

In this paper we report on the experimental demonstration of using relativistic electron diffraction and an radiofrequency deflecting cavity to capture in a single shot the entire time-history of the ultrafast laser-induced heating and melting of a single crystal gold sample. By recording the time variation in the Bragg peaks on the streak image of a 16 ps long electron beam it is possible to reconstruct with 400 fs temporal resolution the evolution of the sample structure induced by a 35 mJ/cm2 400 nm laser pump pulse. [ABSTRACT FROM AUTHOR]

In this paper, using molecular dynamic simulation and ab initio calculations, a novel pseudoelasticity is uncovered in a variety of bcc single crystalline nanowires. Specifically, an initial wire with a <100> axis and {100} surfaces has been transformed to a new configuration with a <110> axis and {111} lateral surfaces under uniaxial tensile loading. The loaded <110> wire spontaneously reorients back to the original one upon unloading, giving rise to about 41% recoverable strains. The primary deformation mechanisms associated with the reversible lattice reorientation are twinning and detwinning, i.e., forward and backward twin boundary migration on adjacent {112} slip planes. We reveal that the physics underlying the novel behavior in these bcc nanowires is the high propensity for twinning and detwinning, which is characterized as the small ratio of twin boundary migration energy to twin boundary formation energy. Furthermore, the relatively weaker temperature dependence of shape memory effects and larger applicable size range observed in these bcc nanowires render themselves more promising for the future nanotechnology applications. [ABSTRACT FROM AUTHOR]