Elastic properties of GaxIn1−xP semiconductor

A theoretical procedure is presented for the study of elastic properties of the ternary alloy Ga_xIn_1−xP. The calculations are based on the pseudopotential formalism in which local potential coupled with the virtual crystal approximation (VCA) is applied to evaluate elastic constants c₁₁, c₁₂ and c₄₄, bulk modulus, shear modulus, Young's modulus and Poisson's ratio for the entire range of the alloy composition x of the ternary alloy Ga_xIn_1−xP. The effect of compositional disorder is included. Our results for parent compounds are compared to experimental and other theoretical calculations and showed generally good agreement. The inclusion of compositional disorder increases values of all elastic constants. During the present study it is found that elastic constant c₁₁ is largely influenced by compositional disorder.     

Ultrasonic wave propagation in IIIrd group nitrides

The ultrasonic attenuation in hexagonal structured (wurtzite) third group nitrides (GaN, AlN and InN) has been evaluated at 300 K for an ultrasonic wave propagating along the unique axis of the crystal. Higher order elastic constants of these materials are calculated using the Lennard-Jones potential for the determination of ultrasonic attenuation. The ultrasonic velocity, Debye average velocity, thermal relaxation time and acoustic coupling constant are evaluated along the z-axis of the crystal using the second order elastic constants and other related parameters. The contributions of the elastic constants, thermal conductivity, thermal energy density, ultrasonic velocity and acoustic coupling constant to the total attenuation are studied. On the basis of the ultrasonic attenuation, it can be concluded that the AlN is more ductile than either GaN or InN at 300 K. Orientation dependent characterization has been achieved by calculation of the orientation dependent ultrasonic velocity, Debye average velocity and thermal relaxation time for the materials.     

Grazing bifurcations in an elastic structure excited by harmonic impactor motions

In this article, non-smooth dynamics of an elastic structure excited by a harmonic impactor motion is studied through a combination of experimental, numerical, and analytical efforts. The test apparatus consists of a stainless steel cantilever structure with a tip mass that is impacted by a shaker. Soft impact between the impactor and the structure is considered, and bifurcations with respect to quasi-static variation of the shaker excitation frequency are examined. In the experiments, qualitative changes that can be associated with grazing and corner-collision bifurcations are observed. Aperiodic motions are also observed in the vicinity of the non-smooth bifurcation points. Assuming the system response to be dominated by the structure’s fundamental mode, a non-autonomous, single degree-of-freedom model is developed and used for local analysis and numerical simulations. The predicted grazing and corner-collision bifurcations are in agreement with the experimental results. To study the local bifurcation behavior at the corner-collision point and explore the mechanism responsible for the aperiodic motions, a derivation is carried out to construct local Poincaré maps of periodic orbits at a corner-collision point such as the one observed in the soft-impact oscillator.     

Alternating approach of implementing frequency encoded all-optical logic gates and flip-flop using semiconductor optical amplifier

In conduction of parallel logic, arithmetic and algebraic operations, optics has already proved its successful role. Since last few decades a number of established methods on optical data processing were proposed and to implement such processors different data encoding/decoding techniques have also been reported. Currently frequency encoding technique is found be a promising as well as a faithful mechanism for the conversion of all-optical processing as the frequency of light remains unaltered after refection, refraction, absorption, etc. during the transmission of light. There are already proposed some frequency encoded optical logic gates. In this communication the authors propose a new and different concept of frequency encoded optical logic gates and optical flip-flop using the non-linear function of semiconductor optical amplifier.     

A new alternative approach of all optical frequency encoded clocked S-R flip-flop exploiting the non-linear character of semiconductor optical amplifiers

In all optical networking and computing system, the role of all-optical flip-flops is very much essential. For signal synchronization with a reference clock and for storage of digital bits the flip-flop has no alternative. In this communication the authors propose a method of developing an all optical frequency encoded clocked R-S flip-flop using the non-linear character of semiconductor optical amplifiers. Frequency is the basic character of light and several encoding/decoding problems in computations and communications can be solved using the frequency encoding principle of optical data. The proposed system is all-optical and therefore it can extend a super fast speed of operation (far above THz limit).     

Spin field topological linear defects in quasicrystals with magnetic order

It is proven that magnetizable quasicrystals undergoing large deformations admit elastic ground states characterized by a net of linear topological defects for the magnetic spin field.     

Exact and approximate solutions of Riemann problems in non-linear elasticity

Eulerian shock-capturing schemes have advantages for modelling problems involving complex non-linear wave structures and large deformations in solid media. Various numerical methods now exist for solving hyperbolic conservation laws that have yet to be applied to non-linear elastic theory. In this paper one such class of solver is examined based upon characteristic tracing in conjunction with high-order monotonicity preserving weighted essentially non-oscillatory (MPWENO) reconstruction. Furthermore, a new iterative method for finding exact solutions of the Riemann problem in non-linear elasticity is presented. Access to exact solutions enables an assessment of the performance of the numerical techniques with focus on the resolution of the seven wave structure. The governing model represents a special case of a more general theory describing additional physics such as material plasticity. The numerical scheme therefore provides a firm basis for extension to simulate more complex physical phenomena. Comparison of exact and numerical solutions of one-dimensional initial values problems involving three-dimensional deformations is presented.     

Effect of interface/surface stress on the elastic wave band structure of two-dimensional phononic crystals

In the present Letter, the multiple scattering theory (MST) for calculating the elastic wave band structure of two-dimensional phononic crystals (PCs) is extended to include the interface/surface stress effect at the nanoscale. The interface/surface elasticity theory is employed to describe the nonclassical boundary conditions at the interface/surface and the elastic Mie scattering matrix embodying the interface/surface stress effect is derived. Using this extended MST, the authors investigate the interface/surface stress effect on the elastic wave band structure of two-dimensional PCs, which is demonstrated to be significant when the characteristic size reduces to nanometers.     

Linear and non-linear behavior of cavity polaritons

We report on the linear and non-linear emission of cavity-polaritons under resonant excitation. At low excitation density, in addition to polariton photoluminescence, strong Rayleigh scattering is observed. At higher excitation densities, a sudden transition to a highly emissive state is observed, accompanied by spatial patterning. We attribute such phenomena to a combination of nonlinear cavity-polariton relaxation mechanism and nonlinear response of the cavity, leading to transverse pattern formation. A careful analysis of near- and far-field emission patterns with spatial filtering as well as reflectivity shows that an inhomogeneous situation develops, the center of the excited region undergoing a strong to weak coupling transition while the periphery is still in strong coupling. Despite the complex non-linear behavior we observe no signatures of Bose–Einstein condensation or Boser action.     

Temperature dependence of elastic constants for ionic solids

In this paper the thermal variation of volume for NaCl, KCl, MgO and CaO has been investigated on the basis of Anderson model. We have evaluated the values of elastic constants C₁₁, C₄₄ and K_T at different temperature on the basis of Murnaghan and Tallon models. Tallon's model with second approximation presents slightly better agreement with experimental results which shows that the Anderson–Grüneisen parameter is directly proportional to the volume ratio. Tallon's model can be used to evaluate the values of elastic constants for solids at different temperatures.     

New materials from high pressure experiments: Challenges and opportunities

Recent improvements in high pressure techniques have led to new scientific discoveries in fields ranging from Earth and Planetary Science, to fundamental physics and biology. Here we examine the possibilities for development of new technologically-important materials based on high pressure research.     

Pressure Dependence of the Bandgap Bowing in Zinc-Blende ZnTe 1− x Se x

We report on the pressure dependence of the bandgap bowing in the ZnTe 1 m x Se x alloy, in the whole composition range. The bandgap bowing parameter is shown to increase almost linearly with pressure from 1.23 at ambient pressure to 1.6 at 7 GPa. Saturation effects observed in the pressure dependence for x =0.1 and x =0.2 are shown to be related to the direct-to-indirect crossover. Results are discussed and interpreted in the framework of structural relaxation models for gap bowing. A prediction of these models (the negative bowing of the o 15 m ;X 1 transition) is shown to be compatible with the fact that the direct-to-indirect crossover pressure increases with the Se content.     

An alternative optical method of determining the unknown microwave frequency by the use of electro-optic materials and semiconductor optical amplifier

There are found different established methods for measuring the frequency of an unknown microwave signal. Several resonating and electronic methods are found where the frequency of a microwave is measured with good performances, where each method has its own advantage. Here in this communication the authors propose a new concept of measuring the frequency of unknown microwave with the joint uses of reflecting semiconductor optical amplifier (RSOA) and electro-optic Pockel cell. To measure the frequency a known microwave source of variable and calibrated frequency is also required. Then with the help of a RSOA and electro-optic material one can find the unknown microwave frequency more accurately than that of any conventional mechanism. This method can extend a high degree of accuracy as optics is used to measure the unknown microwave frequency. As optical signal has million time greater frequency than that of microwave signal therefore a high degree of accuracy of frequency measurement is achieved. The electro-optic material takes the role of phase modulation for splitting an optical wave into several frequencies.     

Optical Properties of High Pressure Phases in ZnTe 1− x Se x

We report on the optical properties of high pressure semiconducting phases in ZnTe 1 m x Se x . In the Te rich side, the cinnabar phase is observed in the upstroke between typically 9.5 and 12.5 GPa with a pressure interval of existence that decreases with increasing the Se content. In most studied samples, the indirect absorption edge could be determined, with values of the bandgap increasing with the Se content and ranging from 1.2 to 1.7 eV. In the downstroke, the cinnabar phase is observed in the whole composition range but its bandgap can not be unambiguously determined in the Se-rich side, as it coexists with rocksalt or zincblende phases. The indirect semiconducting rocksalt phase is observed in the Se-rich side, with an indirect bandgap of the order of 0.7 eV. Within the experimental errors, the bandgaps of both the cinnabar and NaCl phases are pressure insensitive, in agreement with first-principles pseudopotential band structure calculations, that predict very low pressure coefficients for both indirect transitions.     

First principles calculations of mechanical properties of cubic 5d transition metal monocarbides

The electronic and elastic properties of cubic 5d transition metal monocarbides in rocksalt, cesium chloride, and zinc blende structures have been studied by first principles calculations. The calculations show that the incompressibility for ReC in cesium chloride structure is even higher than that of diamond under pressure (above 89 GPa). The transformation pressure from zinc blende structure to rocksalt structure takes place at about 47 GPa for PtC. HfC-NaCl, ReC-CsCl, and HfC-ZnS have the smallest metallicity, leading to higher hardness. A valence electron number of 8/cell may be a stable valence shell configuration for 5d transition metal monocarbides in rocksalt and zinc blende structures.     

Nonlocal elasticity theory for thermal buckling of nanoplates lying on Winkler–Pasternak elastic substrate medium

In the present work, thermal buckling of single-layered graphene sheets lying on an elastic medium is analyzed. For this purpose, the sinusoidal shear deformation plate theory in tandem with the nonlocal continuum theory, which takes the small scale effects into account, is employed. The non-linear stability equations, which contain the reaction of Winkler–Pasternak elastic substrate medium, are derived and then solved analytically for a plate with various boundary conditions and based on various plate theories. Closed form solutions are formulated for three types of thermal loadings as uniform, linear and nonlinear temperature rise through the thickness of the plate. A number of examples are presented to illustrate the numerical results concerned with the buckling temperature response of nanoplates resting on two-parameter elastic foundations. The influences played by transversal shear deformation, plate aspect ratio, side-to-thickness ratio, nonlocal parameter, and elastic foundation parameters are all investigated.     

Structural and elastic properties of antiperovskites XNBa3 (X=As, Sb) under pressure effect

First-principle calculations of structural, elastic and high pressure properties of antiperovskites XNBa₃ (X=As, Sb) are performed, using the full-potential linear muffin-tin orbital (FP-LMTO) method. The local density approximation (LDA) is used for the exchange-correlation (XC) potential. Results are given for lattice constant, bulk modulus and its pressure derivatives. We have determined the elastic constants C₁₁, C₁₂ and C₄₄ and their pressure dependence. We derived shear moduli, Young's modulus, Poisson's ratio and Lamé's constants for ideal polycrystalline XNBa₃ aggregates. By analyzing the ratio of the bulk to shear moduli, we conclude that XNBa₃ compounds are brittle in nature. We estimated the Debye temperature of XNBa₃ from the average sound velocity. This is the first quantitative theoretical prediction of the elastic properties of AsNBa₃ and SbNBa₃ compounds, and it still awaits experimental confirmation.