Analysis of geometric and strain effects in homo-Diels-Alder reactions

Molecular mechanics calculations have been carried out to quantify the key geometric and strain effects which are likely to control the homo-Diels-Alder reactivity of 1,4-dienes. The criteria considered include C1..C5 and C2..C4 distances in the diene, twist angle of the two pi units, and the magnitude of strain increase as a result of cycloaddition. By first considering these factors in a number of non-conjugated dienes with known reactivity, the ranges of values within which the reaction is favoured are proposed. Calculations are also reported on several substrates which have not been investigated so far. Promising systems for experimental study are suggested which, in addition to being intrinsically interesting, would place the present proposals on a firm basis.

Uniaxial Strain Effects on Optical Properties of c-plane Wurtzite GaN

We investigate the uniaxial strain effect in the c-plane on optical properties of wurtzite GaN based on k center dot p theory, the spin-orbit interactions are also taken into account. The energy dispersions show that the uniaxial strain in the c-plane gives an anisotropic energy splitting in the k(x) - k(y) plane, which can reduce the density of states. The uniaxial strain also results in giant in-plane optical polarization anisotropy, hence causes the threshold carrier density reduced. We clarify the relations between the uniaxial strain and the optical polarization properties. As a result, it is suggested that the compressive uniaxial strain perpendicular to the laser cavity direction in the c-plane is one of the preferable approaches for the effcient improvement of GaN-based laser performance.

Strain Effects on the Optical Polarization Properties of R-Plane Wurtzite GaN

Using the effective-mass Hamiltonian for an arbitrary direction wurtzite semiconductor on the basis of k.p theory, we investigate the strain effects on the transition energies and optical properties in the R-plane ([1012]-oriented plane) GaN. The results show that (1) the transition energies decrease with the biaxial strains changing from -0.5 to 0.5%; and (2) giant optical anisotropy appears in the R-plane which is significantly affected by the biaxial strains. We clarify the relation between the strains and the polarization properties. Finally, we discuss the application of these properties to the R-plane GaN based devices. (c) 2009 The Japan Society of Applied Physics

Strain effects on optical polarisation properties in (11(2)over-bar2) plane GaN films

We present the theoretical results of the electronic band structure of wurtzite GaN films under biaxial strains in the (11 (2) over bar2)-plane The calculations are performed by the kappa p perturbation theory approach through using the effective-mass Hamiltonian for an arbitrary direction The results show that the transition energies decrease with the biaxial strains changing from -0 5% to 0 5% For films of (11 (2) over bar2)-plane, the strains are expected to be anisotropic in the growth plane Such anisotropic strains give rise to valence band mixing which results in dramatic change in optical polarisation property The strain can also result in optical polarisation switching phenomena Finally, we discuss the applications of these properties to the (11 (2) over bar2) plane GaN based light emitting diode and lase diode

Using pMOS Pass-Gates to Boost SRAM Performance by Exploiting Strain Effects in Sub-20-nm FinFET Technologies

Strained fin is one of the techniques used to improve the devices as their size keeps reducing in new nanoscale nodes. In this paper, we use a predictive technology of 14 nm where pMOS mobility is significantly improved when those devices are built on top of long, uncut fins, while nMOS devices present the opposite behavior due to the combination of strains. We explore the possibility of boosting circuit performance in repetitive structures where long uncut fins can be exploited to increase fin strain impact. In particular, pMOS pass-gates are used in 6T complementary SRAM cells (CSRAM) with reinforced pull-ups. Those cells are simulated under process variability and compared to the regular SRAM. We show that when layout dependent effects are considered the CSRAM design provides 10% to 40% faster access time while keeping the same area, power, and stability than a regular 6T SRAM cell. The conclusions also apply to 8T SRAM cells. The CSRAM cell also presents increased reliability in technologies whose nMOS devices have more mismatch than pMOS transistors.

Versatile single-layer sodium phosphidostannate(II): Strain-tunable electronic structure, excellent mechanical flexibility, and an ideal gap for photovoltaics

Density functional theory (DFT) calculations were performed to study the structural, mechanical, electrical, optical properties, and strain effects in single-layer sodium phosphidostannate(II) (NaSnP). We find the exfoliation of single-layer NaSnP from bulk form is highly feasible because the cleavage energy is comparable to graphite and MoS2. In addition, the breaking strain of the NaSnP monolayer is comparable to other widely studied 2D materials, indicating excellent mechanical flexibility of 2D NaSnP. Using the hybrid functional method, the calculated band gap of single-layer NaSnP is close to the ideal band gap of solar cell materials (1.5 eV), demonstrating great potential in future photovoltaic application. Furthermore, strain effect study shows that a moderate compression (2%) can trigger indirect-to-direct gap transition, which would enhance the ability of light absorption for the NaSnP monolayer. With sufficient compression (8%), the single-layer NaSnP can be tuned from semiconductor to metal, suggesting great applications in nanoelectronic devices based on strain engineering techniques.

Structures, Stabilities and Strain in C60 and C70 Derivatives Fullerenes, Nanotubes and Carbon Nanostructures

The stabilities of a number of small adducts as well as larger hydrides of C-60 and C-70 are reported using semiempirical MO methods. The data are shown to be consistent with the nature of bond alternation in the parent fullerenes and strain effects in the cage systems.

Effects of kinetin and nitrogen on growth rates, pigment and protein contents in wild and phycoerythrin-deficient strains of Hypnea musciformis (Rhodophyta)

Hypnea musciformis (Wulfen in Jacqu.) J.V. Lamour. is the main source for carrageenan production in Brazil and strains with selected characteristics could improve the production of raw material. The effects of kinetin on growth rates, morphology, protein content, and concentrations of pigments (chlorophyll a, phycoerythrin, phycocyanin, and allophycocyanin) were assessed in the wild strain (brown phenotype) and in the phycoerythrin-deficient strain (green phenotype) of H. musciformis. Concentrations of kinetin ranging from 0 to 50 mu M were tested in ASP 12-NTA synthetic medium with 10 mu M nitrate (N-limited) and 100 mu M nitrate (N-saturated). In N-limited condition, kinetin stimulated growth rates of the phycoerythrin-deficient strain and formation of lateral branches in both colour strains. Kinetin stimulated protein biosynthesis in both strains. However, differences between both nitrogen conditions were significant only in the phycoerythrin-deficient strain. In the wild strain, effects of kinetin on concentrations of phycobiliproteins were not significant in both nitrogen conditions, except for chlorophyll content. However, the phycoerythrin-deficient strain showed an opposite response, and kinetin stimulated the phycobiliprotein biosynthesis, with the highest concentrations of phycoerythrin in N-saturated medium, while the highest concentrations of allophycocyanin and phycocyanin were observed in N-limited medium. These results indicate that the effects of kinetin on growth, morphology, protein and phycobiliprotein contents are influenced by nitrogen availability, and the main nitrogen storage pools in phycoerythrin-deficient strain of H. musciformis submitted to N-limited conditions were phycocyanin and allophycocianin, the biosynthesis of which was enhanced by kinetin.

Effects of hypertension on maternal adaptations to pregnancy: experimental study on spontaneously hypertensive rats

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Simultaneous measurement of strain and temperature using the first- and second-order diffraction wavelengths of Bragg gratings

A method of discriminating between temperature and strain effects in fibre sensing using a conventionally written, in-fibre Bragg grating is presented. The technique uses wavelength information from the first and second diffraction orders of the grating element to determine the wavelength dependent strain and temperature coefficients, from which independent temperature and strain measurements can be made. The authors present results that validate this matrix inversion technique and quantify the strain and temperature errors which can arise for a given uncertainty in the measurement of the reflected wavelength.

Simultaneous measurement of strain and temperature using the first- and second-order diffraction wavelengths of Bragg gratings

A method of discriminating between temperature and strain effects in fibre sensing using a conventionally written, in-fibre Bragg grating is presented. The technique uses wavelength information from the first and second diffraction orders of the grating element to determine the wavelength dependent strain and temperature coefficients, from which independent temperature and strain measurements can be made. The authors present results that validate this matrix inversion technique and quantify the strain and temperature errors which can arise for a given uncertainty in the measurement of the reflected wavelength.

Local probing of ferroelectric and ferroelastic switching through stress-mediated piezoelectric spectroscopy

Strain effects have a significant role in mediating classic ferroelectric behavior such as polarization switching and domain wall dynamics. These effects are of critical relevance if the ferroelectric order parameter is coupled to strain and is therefore, also ferroelastic. Here, switching spectroscopy piezoresponse force microscopy (SS-PFM) is combined with control of applied tip pressure to exert direct control over the ferroelastic and ferroelectric switching events, a modality otherwise unattainable in traditional PFM. As a proof of concept, stress-mediated SS-PFM is applied toward the study of polarization switching events in a lead zirconate titanate thin film, with a composition near the morphotropic phase boundary with co-existing rhombohedral and tetragonal phases. Under increasing applied pressure, shape modification of local hysteresis loops is observed, consistent with a reduction in the ferroelastic domain variants under increased pressure. These experimental results are further validated by phase field simulations. The technique can be expanded to explore more complex electromechanical responses under applied local pressure, such as probing ferroelectric and ferroelastic piezoelectric nonlinearity as a function of applied pressure, and electro-chemo-mechanical response through electrochemical strain microscopy.

The effect of additional Si and SiGe layers on the confinement potential of GeMn diluted ferromagnetic semiconductor

In this work we analyze the spin-polarized charge density distribution in the GeMn diluted ferromagnetic semiconductors (DFS). The calculations are performed within a self-consistent k.p method, in which the exchange correlation effects in the local density approximation, as well as the strain effects due to the lattice mismatch, are taken into account. Our findings show that the extra confinement potential provided by the barriers and the variation of the Mn content in the DFS are responsible for a separation between the different spin charge densities, giving rise to higher mobility spin-polarized currents or high ferromagnetism transition temperatures systems. (c) 2008 Elsevier B.V. All rights reserved.

Luminescence studies on nitride quaternary alloys double quantum wells

We present theoretical photoluminescence (PL) spectra of undoped and p-doped Al(x)In(1-xy)Ga(y)N/Al(X)In(1) (X) (Y)Ga(Y)N double quantum wells (DQWs). The calculations were performed within the k.p method by means of solving a full eight-band Kane Hamiltonian together with the Poisson equation in a plane wave representation, including exchange-correlation effects within the local density approximation. Strain effects due to the lattice mismatch are also taken into account. We show the calculated PL spectra, analyzing the blue and red-shifts in energy as one varies the spike and the well widths, as well as the acceptor doping concentration. We found a transition between a regime of isolated quantum wells and that of interacting DQWs. Since there are few studies of optical properties of quantum wells based on nitride quaternary alloys, the results reported here will provide guidelines for the interpretation of forthcoming experiments. (C) 2008 Elsevier B.V. All rights reserved.

Modeling the growth of LT and TL-oriented fatigue cracks in longitudinally and transversely pre-strained Al 2524-T3 alloy

The aluminum alloy 2524 (Al-Cu-Mg) was developed during the 90s mainly to be employed in aircraft fuselage panels, replacing the standard Al 2024. In the present analysis the fatigue crack growth (FCG) behavior of 2524-T3 was investigated, regarding the influence of three parameters: load ratio, pre strain and crack plane orientation of the material. The pre strain of aluminum alloys is usually performed in order to obtain a more homogeneous precipitates distribution, accompanied by an increase in the yield strength. In this work, it was evaluated the resistance of Al 2524-T3 sheet samples to the fatigue crack growth, having L-T and T-L crack orientations. FCG tests were performed under constant amplitude loading at three distinct positive load ratios. The three material conditions were tested: as received(AR), pre strained longitudinally (SL) and transversally (ST) in relation to rolling direction. In order to describe FCG behavior, two-parameter kinetic equations were compared: a Paris-type potential model and a new exponential equation introduced in a previous work conducted by our research group. It was observed that the exponential model, which takes into account the deviations from linearity presented by da/dN versus AK data, describes more adequately the FCG behavior of Al 224-T3 in relation to load ratio, pre strain effects and crack plane orientation. © 2011 Published by Elsevier Ltd.