Effects of annealing temperature on the electrical property and microstructure of aluminum contact on n-type 3C–SiC

The 3C-SiC thin films used herein are grown on Si substrates by chemical vapor deposition. A1 contacts with differ- ent thickness values are deposited on the 3C-SiC/Si （100） structure by the magnetron sputtering method and are annealed at different temperatures. We focus on the effects of the annealing temperature on the ohmic contact properties and mi- crostructure of A1/3C-SiC structure. The electrical properties of A1 contacts to n-type 3C-SiC are characterized by the transmission line method. The crystal structures and chemical phases of A1 contacts are examined by X-ray diffraction, Raman spectra, and transmission electron microscopy, respectively. It is found that the A1 contacts exhibit ohmic contact behaviors when the annealing temperature is below 550 ℃, and they become Schottky contacts when the annealing tem- perature is above 650 ℃. A minimum specific contact resistance of 1.8 × 10-4 Ω cm2 is obtained when the A1 contact is annealed at 250 ℃.     

Current status of modelling the semi-insulating 4H–SiC transient photoconductivity for application to photoconductive switches

In this paper we present the current status of modelling the time evolution of the transient conductivity of photoexcited semi-insulating (SI) 4H–SiC taking into account the properties of defect centres. A comprehensive model that includes the presence of six, the most significant, point defects occurring in SI 4H–SiC crystals is presented. The defect centres are attributed to the two kinds of nitrogen-related shallow donors, a boron-related shallow acceptor, deep electron and hole traps, and the Z_1/2 recombination centre. We present the results of the state-of-the-art numerical simulations showing how the photoconductivity transients change in time and how these changes are affected by the properties of defect centres. The properties of defect centres assumed for modelling are compared with the results of experimental studies of deep-level defects in high purity (HP) SI 4H–SiC wafers performed by the high-resolution photoinduced transient spectroscopy (HRPITS). The simulated photoconductivity transients are also compared with the experimental photocurrent transients for the HP SI 4H–SiC wafers with different deep-level defects. It is shown that a high-temperature annealing producing the C-rich material enables the fast photocurrent transients to be achieved. The presented analysis can be useful for technology of SI 4H–SiC high-power photoconductive switches with suitable characteristics.     

3D 13C–13C–13C correlation NMR for de novo distance determination of solid proteins and application to a human α-defensin

The de novo structure of an antimicrobial protein, human α-defensin 1 (HNP-1), is determined by combining a 3D ¹³C–¹³C–¹³C (CCC) magic-angle spinning (MAS) correlation experiment with standard resonance assignment experiments. Using a short spin diffusion mixing time to assign intra-residue cross peaks and a long mixing time to detect inter-residue correlation peaks, we show that the 3D CCC experiment not only reduces the ambiguity of resonance assignment, but more importantly yields two orders of magnitude more long-range distances without recourse to existing crystal structures. Most of these distance constraints could not be obtained in a de novo fashion from 2D correlation spectra due to significant resonance overlap. Combining the distance constraints from the 3D CCC experiment and the chemical-shift-derived torsion angles, we obtained a de novo high-resolution NMR structure of HNP-1, with a heavy-atom RMSD of 3.4 Å from the crystal structure of the analogous HNP-3. The average energy of the minimum-energy ensemble is less than of 40 kcal/mol. Thus, the 3D CCC experiment provides a reliable means of restraining the three-dimensional structure of insoluble proteins with unknown conformations.     

Numerical approach for the evolution of spin-boson systems and its application to the Buck–Sukumar model

We study the evolution properties of spin-boson systems by a systematic numerical iteration approach, which performs well in the whole coupling regime. This approach evaluates a set of coefficients in the formal expression of the time-dependent Schr?dinger equation by expanding the initial state in Fock space. This set of coefficients is unique for the spin-boson Hamiltonian studied, allowing one to calculate the time evolution from different initial states. To complement our numerical calculations, we apply the method to the Buck–Sukumar model. We find that when the ground-state energy of the model is unbounded and no ground state exists in a certain parameter space, the time evolution of the physical quantities is naturally unstable.     

Optimal locations and orientations of piezoelectric transducers on cylindrical shell based on gramians of contributed and undesired Rayleigh–Ritz modes using genetic algorithm

In this study, the active vibration control and configurational optimization of a cylindrical shell are analyzed by using piezoelectric transducers. The piezoelectric patches are attached to the surface of the cylindrical shell. The Rayleigh–Ritz method is used for deriving dynamic modeling of cylindrical shell and piezoelectric sensors and actuators based on the Donnel–Mushtari shell theory. The major goal of this study is to find the optimal locations and orientations of piezoelectric sensors and actuators on the cylindrical shell. The optimization procedure is designed based on desired controllability and observability of each contributed and undesired mode. Further, in order to limit spillover effects, the residual modes are taken into consideration. The optimization variables are the positions and orientations of piezoelectric patches. Genetic algorithm is utilized to evaluate the optimal configurations. In this article, for improving the maximum power and capacity of actuators for amplitude depreciation of negative velocity feedback strategy, we have proposed a new control strategy, called “Saturated Negative Velocity Feedback Rule (SNVF)”. The numerical results show that the optimization procedure is effective for vibration reduction, and specifically, by locating actuators and sensors in their optimal locations and orientations, the vibrations of cylindrical shell are suppressed more quickly.     

Preparation and characterization of CuI/PVA–PEDOT:PSS core–shell for photovoltaic application

In the present paper nano polymer composite of CuI/PVA blended with PEDOT:PSS has been prepared by growing CuI nano particles inside aqueous solution of PVA. The XRD characterization illustrated the growth of CuI nano crystals of 22–33 nm. The optical absorption showed direct transition with an energy band gap equals to 1.18 eV and 1.3 eV for colloidal and thin solid films respectively. The PL investigation illustrates a quenching with increasing PEDDOT:PSS concentration. The results are interpreted according to energy confinement enhanced by plasmon–exciton interaction of CuI–PVA/PEDOT:PSS core–shell. The frequency dependence of conductivity suggested hopping conduction where the bulk conductivity is thermally activated with an activation energy in the range varies from 0.07 to 0.46 eV by increasing PEDOT:PSS concentration. The cyclic voltammetry measurements have been performed to ascertain the position of both HOMO and LUMO levels which illustrated a movement of HOMO level toward vacuum level, with a decrease in the chemical band gap from 1.72 to 1.3 eV with increasing PEDOT:PSS concentration.     

Comparative PEEM and AES study of surface morphology and composition of Cu films on Si and 3C–SiC substrates

We have conducted photoemission electron microscope (PEEM) and Auger electron spectroscopy (AES) studies on the Cu(30 nm)/3C–SiC(1 0 0) and Cu(30 nm)/Si(1 0 0) samples annealed successively up to 850 °C. With PEEM, lateral diffusion of Cu atoms on the 3C–SiC substrate was observed at 400 °C while no lateral diffusion was seen for the Cu/Si(1 0 0) samples up to 850 °C. The 30 nm Cu thin film on 3C–SiC began to agglomerate at 550 °C, similar to the case for the Cu/Si(1 0 0) system. No further spread of the lateral diffusion region was found in subsequent annealing up to 850 °C for Cu/3C–SiC while separated regular-sized dot structures were found at 850 °C for Cu/Si(1 0 0). AES studies of Cu/Si(1 0 0) system showed partial interface reaction during Cu deposition onto the Si(1 0 0) substrate and oxidation of the resultant Cu₃Si to form SiO₂ on the specimen surface at room temperature in air. Surface segregation of Si and C was observed after annealing at 300 °C for Cu/Si(1 0 0) and at 850 °C for the Cu/3C–SiC system. We have successfully elucidated the observed phenomena by combining PEEM and AES considering diffusion of the constituent elements and/or reaction at interfaces.     

Theory of multinonlinear media and its application to the soliton processes in ferrite–ferroelectric structures

A theory is developed to describe the wave processes that occur in waveguide media having several types of nonlinearity, specifically, multinonlinear media. It is shown that the nonlinear Schrödinger equation can be used to describe the general wave process that occurs in such media. The competition between the electric wave nonlinearity and the magnetic wave nonlinearity in a layered multinonlinear ferrite–ferroelectric structure is found to change a total repulsive nonlinearity into a total attractive nonlinearity.     

Mechanisms of the oxidation and combustion of normal paraffin hydrocarbons: Transition from C1–C10 to C11–C16

Recently, detailed kinetic mechanisms of the oxidation and combustion of higher hydrocarbons, composed of hundreds of components and thousands of elementary reactions, have been proposed. Despite the undoubtful advantages of such detailed mechanisms, their application to simulations of turbulent combustion and gas dynamic phenomena is difficult because of their complexity. At the same time, to some extent limited, they cannot be considered exhaustive. This work applies previously proposed algorithm for constructing an optimal mechanism of the high- and low-temperature oxidation and combustion of normal paraffin hydrocarbons, which takes into account the main processes determining the reaction rate and the formation of key intermediates and final products. The mechanism has the status of a nonempirical detailed mechanism, since all the constituent elementary reactions have a kinetic substantiation. The mechanism has two specific features: (1) it does not include reactions of so-called double oxygen addition (first to the peroxide radical, and then to its isomeric form), i.e., the first addition turns out to be sufficient; (2) it does not include isomeric compounds and their derivatives as intermediates, since this oxidation pathway is slower than the oxidation of molecules and radicals with normal structure. Application of the algorithm makes it possible to compile a compact mechanism, which is important for modeling chemical processes involving paraffin hydrocarbons C _n with large n. Previously, based on this algorithm, compact mechanisms of the oxidation and combustion of propane, n-butane, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane have been constructed. In this work, we constructed a nonempirical detailed mechanism of the oxidation and combustion of hydrocarbons from n-undecane to n-hexadecane. The most important feature of the new mechanism is its staged nature, which manifests itself through the emergence of cool and blue flames during low-temperature autoignition. The calculation results are compared with experimental data.     

Synchrotron-radiation photoemission study of the ultrathin Ba/3C–SiC(111) interface

Electronic structure of the Ba/3C–SiC(111) interface has been detailed studied in situ in an ultrahigh vacuum using synchrotron radiation photoemission spectroscopy with photon energies in the range of 100–450 eV. The 3C–SiC(111) samples were grown by a new method of epitaxy of low-defect unstressed nanoscaled silicon carbide films on silicon substrates. Valence band photoemission and both the Si 2p, C 1s core level spectra have been investigated as a function of Ba submonolayer coverage. Under Ba adsorption two induced surface bands are found at binding energies of 2 eV and 6 eV. It is obtained that Ba/3C–SiC(111) interface can be characterized as metallic-like. Modification of both the Si 2p and C 1s surface-related components were ascertained and shown to be provided by redistribution effect of electron density between Ba adatoms and both the Si surface and C interface atoms.     

Symmetry and Fourier analysis of the ab initio-determined torsional variation of structural and Hessian-related quantities for application to vibration–torsion–rotation interactions in CH3OH

The aim of the present paper is to investigate the use of quantum chemistry calculations to obtain the torsional dependence of various structural and vibrational-force-field-related quantities that could help in estimating the vibration–torsion–rotation interaction terms needed to treat perturbations observed in the spectra of methanol-like molecules. We begin by using the Gaussian suite of programs to determine the steepest-descent path from a stationary point at the top of the internal rotation potential barrier in methanol to the equilibrium structure at the bottom of the barrier. This procedure requires determining the gradient ?V of the potential (as calculated in mass-weighted Cartesian coordinates) along the internal rotation path. In addition, we use the Gaussian suite to calculate the Hessian ??V along this path and to generate from these second derivatives the 3N ? 7 small-amplitude vibrational frequencies and the 3N Cartesian vibrational displacements for each of these vibrations. We then symmetrize the internal coordinates used in presenting the structures, gradients, Hessians and vibrational displacements along the path to take into account the periodic variation of the behavior of the three methyl hydrogen atoms H_i as they pass in turn through the C_s-plane of the HOC frame. The symmetrized linear combinations of the CH_i stretches, of the OCH_i bends, and of the HOCH_i dihedral angles of the methyl group depend on the internal rotation angle γ and they are determined by considering coordinate transformations from the G₆ permutation-inversion group appropriate for internally rotating methanol. This symmetrization procedure permits us to explore the feasibility of expressing the structures, gradients, Hessians, and vibrational displacement vectors along the internal rotation path as short Fourier series in γ, which is one of the main goals of this paper. In summary, we find that the symmetrized structures, gradients, and Hessians, as well as nine of the 11 projected vibrational frequencies and the vibrational displacement vectors for the three vibrations occurring primarily in the HOC frame can be expressed by short Fourier series expansions to their precision in the Gaussian output, and that these series involve only sin 3nγ or only cos 3nγ terms, as required by G₆ symmetry considerations. A preliminary discussion is given of why short Fourier expansions fail for the projected frequencies of the two methyl asymmetric stretches, and for the vibrational displacement vectors of the methyl group vibrational modes. Looking more closely at the symmetrized and projected 3N × 3N Hessian, we find algebraically that only elements in the (3N ? 7) × (3N ? 7) small-amplitude-vibrational block of the Hessian are useful for spectroscopic problems. Non-zero elements in the rest of the 3N × 3N symmetrized and projected Hessian cannot be converted into quantities needed for perturbation studies.     

Spectral and informational analysis of seismicity: An application to the 1996–2012 seismicity of the Northern Caucasus–Azerbaijan part of the greater Caucasus–Kopet Dag region

The time series analysis of seismic sequences needs proper methodologies that allow us to capture the main features of the time dynamics of earthquakes. Among these features, the identification of periodicities along with the quantification of their intensity represents an important task, concerning the detection of regular dynamical behaviours, with clear implications for earthquake prediction. In the present study, we applied three different methods to investigate the time dynamics of the seismic activity of the Northern Caucasus–Azerbaijan part of the Greater Caucasus–Kopet Dag region. We analysed the monthly number of earthquakes which occurred between 1996 and 2012 by means of: (i) the robust estimation of the periodogram, (ii) the singular spectrum analysis (SSA), and (iii) the Fisher–Shannon method. Two main significant periodicities are detected: 102 months and 20 months. The first corresponds actually to the long-term variation of the monthly seismic activity of the area, while the second represents the more intense cyclic component. Periodicities of 7 and 30 months are also identified, but with a lower intensity than the 20-month periodicity. The Fisher–Shannon method has revealed that the long-term variation of the series is also characterized by higher organization and lower degree of disorder. The present study shows how the application of methods from statistical mechanics could contribute to unveil dynamical features in seismicity.     

Increase of photoluminescence blinking frequency of 3C–SiC nanocrystals with excitation power

Super-resolution optical fluctuation imaging is dependent on the blinking frequency of fluorophores.Consequently,improvement of the photoluminescence(PL)blink frequency is important.This is achieved for 3C–SiC nanocrystals(NCs)by simply increasing the excitation power.Using an excitation of 488 nm with powers of 5μW to 50μW,individual 3C–SiC NC always exhibits PL blinking with a short on-state sojourn time(0.1 s).A fast Fourier transform method is exploited to determine the PL switching frequency.It is found that the frequency of the bright state increases from 2 Hz to 20 Hz as the excitation power increases from 5μW to 50μW,which is explained by the Auger photonionization model.     

Simple approach to approximate predictions of the vapor–liquid equilibrium curve near the critical point and its application to Lennard-Jones fluids

We propose a simple analytical form of the vapor–liquid equilibrium curve near the critical point for Lennard-Jones fluids. Coexistence densities curves and vapor pressure have been determined using the Van der Waals and Dieterici equation of state. In described method the Bernoulli differential equations, critical exponent theory and some type of Maxwell?s criterion have been used. Presented approach has not yet been used to determine analytical form of phase curves as done in this Letter. Lennard-Jones fluids have been considered for analysis. Comparison with experimental data is done. The accuracy of the method is described.     

Growth of piezoelectric quartz crystals of large dimension on seeds parallel to the (10–11) face and lengthened along the Y-axis

Abstract

We have grown crystals on seeds parallel to the side of the negative rhombohedron by the hydrothermal method. Their dimension was equal to the projection one along the Y-axis to the plane (01–11) and approximately exceeded the size of the crystal along the X-axis by one order. In Russia these crystals are referred to as - ry′. The maximal sizes of ry′ -cryszals are 410mm and 120mm along the Y- and X-axes, respectively. It is established that each crystal increases its dimension along the X-axis due to the formation of the sides of the sharpest rhombohedron (20–21). These faces form the pyramids of growth by which it is possible to get the plates from an average part of each crystal. Their dimension exceeds the initial one by 15-20mm along the X-axis. We have studied the structural and morphological properties of ry′ -crystals grown at different rates at the temperature of crystallization of 330-410°C and a pressure up to 150 MPa.     

Bloch–Siegert shift in application to the astrophysical determination of the fundamental constants variation

We have evaluated the Bloch–Siegert shift for the different values of magnetic field?s strengths defined at astrophysical conditions, i.e. when the stars with the strong surface magnetic fields are taken as a powerful pumping source of radiation. It is found that the additional shift of resonant frequency should be taken into account in the search for the time variation of the fundamental constants. The main conclusion is that the influence of the electromagnetic field should be considered carefully in each special case of the corresponding frequency determination.     

Ferromagnetism induced by double impurities Mn and Cr in 3C–SiC

Using first principles calculations based on KKR Korringa Kohn Rostoker method related to the Coherent Potential Approximation (CPA), we have investigated the magnetic properties of silicon-carbide doped by transition metals (TM). In addition, for some concentrations of the TM elements (Mn, Cr), we have found that the ferromagnetic states appear in the 3C–Si_1?x?yMnxCr_yC system. Moreover, the total magnetic moment of 3C–Si_1?xTM_xC increases with increasing TM (TM?=?Mn or Cr) concentrations. As well, our results indicate that the Curie temperature of 3C–Si_1?xTM_xC and 3C–Si_1?x?yMnxCr_yC increases with increasing transition metal concentration. Finally, we have found that the double exchange interaction between Cr and Mn is the mechanism responsible for the ferromagnetism in 3C–Si_1?xTM_xC and 3C–Si_1?x?yMn_xCr_yC systems.     

Calculations of potential of mean force: application to ion-pairs and host–guest systems

ABSTRACT

We report the calculation of the potential of mean force (PMF) of different types of associations by various techniques: two no biased methods (thermodynamic integration and finite difference thermodynamic integration), a constraint biased technique (adaptive biasing force) and an umbrella biased method (umbrella sampling). We apply these methodologies to the association between two methane molecules in water, to the formation of an ion pair in water and to the formation of an insertion complex between a macrocycle and a cation. The different PMFs are compared to each other on the basis of the depth of the free energy minimum and on the location of different specific points.