Showing papers in "Physica B-condensed Matter in 2012"

Cu-doped ZnO nanoparticles: Synthesis, structural and electrical properties

Abstract: Pure and Cu doped ZnO nanopowders (5, 10, 15, 20, 25 and 30 at% Cu) have been synthesized using co-precipitation method. Transmission Electron Microscopic analysis has shown the morphology of ZnO nanopowders to be quasi-spherical. Powder X-ray Diffraction studies have revealed the systematic doping of Cu into the ZnO lattice up to 10% Cu, though the peaks corresponding to CuO in 10% Cu are negligibly very small. Beyond this level, there was segregation of a secondary phase corresponding to the formation of CuO. Fourier Transform Infrared spectra have shown a broad absorption band at ∼490 cm −1 for all the samples, which corresponds to the stretching vibration of Zn–O bond. DC electrical resistivity has been found to decrease with increasing Cu content. The activation energy has also been observed to decrease with copper doping i.e. from ∼0.67 eV for pure ZnO to ∼0.41 eV for 30 at% Cu doped ZnO.

Tunable dielectric response of transition metals dichalcogenides MX2 (M=Mo, W; X=S, Se, Te): Effect of quantum confinement

Abstract: The first principle calculations have been performed to study the influence of number of layers on the dielectric properties of dichalcogenides of Mo and W for in-plan ( E ⊥ c ) as well as out-of-plan polarization ( E ∥ c ) . We have taken bulk, mono, bi, four and 6-layer setup for this study. The EELS shows significant red shift in the energies of π plasmons, while prominent red shift has been found for the energies of ( π + σ ) plasmons of all the studied materials by reducing the number of layers from bulk to monolayer limit. The ϵ s has been found to red shifted by 62.5% (66.3%), 48.5% (62.1%), 52.7% (66.2%), 61.7% (64.6%), 61.5% (66.7%) and 62.5% (70.5%) from bulk values of MoS2, MoSe2, MoTe2, WS2, WSe2, WTe2 respectively for E ⊥ c ( E ∥ c ) as one goes from bulk to monolayer of these materials. The interband transitions are found to remain independent of the number of layers, however their intensity decreases with decrease in the number of layers. The dielectric functions are highly anisotropic in low energy range and becomes isotropic in high energy range.

Infrared and structural studies of Mg1–xZnxFe2O4 ferrites

Abstract: Compositions of polycrystalline Mg–Zn mixed ferrites with the general formula Mg1−xZnxFe2O4 (0≤x≤1) were prepared by the standard double sintering ceramic method. The structural properties of these ferrites have been investigated using X-ray diffraction and infrared absorption spectroscopy. The lattice parameter, particle size, bonds length, force constants, density, porosity, shrinkage and cation distribution of these samples have been estimated and compared with those predicted theoretically. Most of these values were found to increase with increasing Zn content. The energy dispersive (EDS) analysis confirmed the proposed sample composition. The scanning electron microscope (SEM) and transmission electron microscope (TEM) micrographs showed aggregates of stacked crystallites of about 200–800 nm in diameter. Far infrared absorption spectra showed two significant absorption bands. The wave number of the first band, ν1, decreases with increasing Zn content, while the band, ν2 shifts linearly towards higher wave numbers with Zn contents, over the whole composition range. The room temperature electrical resistivity was found to decrease as Zn-content increases. Values of the vacancy model parameters showed that the packing factors Pa and Pb decrease, the fulfillment coefficient, α, remains almost constant and the vacancy parameter, β, strongly increases with increasing Zn content in the sample. The small values of Pa, Pb, α and the strong increase of the vacancy parameter, β, indicate the presence of cation or anion vacancies and the partial participation of the Zn2+ vacancies in the improvement of the electrical conductivity in the Mg–Zn ferrites.

First-principles study on the structural, electronic and optical properties of BiOX (X=Cl, Br, I) crystals

Abstract: In order to investigate systematically the structural, electronic and optical properties of bismuth oxyhalides BiOX (X=Cl, Br, I) semiconductors, the lattice constants, structural characteristics, band structures, densities of states, atomic charge populations and optical properties of BiOX crystals have been calculated using first-principles based on DFT. The calculated indirect band gaps of BiOCl, BiOBr and BiOI crystals are 2.50, 2.10 and 1.59 eV, respectively. The analysis of densities of states and atomic charge populations for BiOX crystals indicates that, (a) the valance band maximum is mainly contributed to O 2p and X np states and the Bi 6p states dominate the conduction band minimum; (b) the contribution of X ns states obviously increases with the increase of X atomic numbers, and the dispersive energy level becomes more and more significant and (c) the sequence of covalent bonding strength between atoms is Bi–O >Bi–I>Bi–Br>Bi–Cl. In addition, the calculated absorption edges of the absorption coefficients I(ω) for BiOCl, BiOBr and BiOI crystals are 355, 448 and 645 nm, respectively, which agree well with our experimental measurements of 376, 442 and 628 nm and the previous reported results of 370, 440 and 670 nm.

Electronic structural, elastic properties and thermodynamics of Mg17Al12, Mg2Si and Al2Y phases from first-principles calculations

Abstract: Electronic structures, elastic properties and thermal stabilities of Mg17Al12, Mg2Si and Al2Y have been determined from first-principle calculations. The calculated heats of formation and cohesive energies show that Al2Y has the strongest alloying ability and structural stability. The brittle behavior and structural stability mechanism is also explained through the electronic structures of these intermetallic compounds. The elastic constants are calculated, the bulk moduli, shear moduli, Young's moduli and Poisson ratio value are derived, the brittleness and plasticity of these phases are discussed. Gibbs free energy, Debye temperature and heat capacity are calculated and discussed.

Collective resonances in plasmonic crystals: Size matters

Abstract: Periodic arrays of metallic nanoparticles may sustain surface lattice resonances (SLRs), which are collective resonances associated with the diffractive coupling of localized surface plasmons resonances (LSPRs). By investigating a series of arrays with varying number of particles, we traced the evolution of SLRs to its origins. Polarization resolved extinction spectra of arrays formed by a few nanoparticles were measured, and found to be in very good agreement with calculations based on a coupled dipole model. Finite size effects on the optical properties of the arrays are observed, and our results provide insight into the characteristic length scales for collective plasmonic effects: for arrays smaller than ∼ 5 × 5 particles, the Q-factors of SLRs are lower than those of LSPRs; for arrays larger than ∼ 20 × 20 particles, the Q-factors of SLRs saturate at a much larger value than those of LSPRs; in between, the Q-factors of SLRs are an increasing function of the number of particles in the array.

Enhancement of output performance of Cu2ZnSnS4 thin film solar cells—A numerical simulation approach and comparison to experiments

Abstract: The formation of stable, low resistance and nonrectifying contacts to Cu 2 ZnSnS 4 (CZTS) thin film photovoltaic material are the major and critical challenges associated with its effect over the output performance of fabricated solar cells. The solution of continuity equation in one dimension for a soda lime glass substrates (SLG) |Mo | CZTS | CdS | ZnO:Al cell structure is considered in the simulation of its current–voltage characteristics that is governed by the back contact material, acceptor concentration as well as thickness of the CZTS layer. Our primary simulation shows a 6.44% efficiency of the CZTS solar cell which is comparable to reported experimental data if these parameters are not optimized. However, by optimizing them a simulated conversion efficiency as high as 13.41% ( V oc =1.002 V, J sc =19.31 mA/cm 2 , fill factor ( FF )=69.35%) could be achievable. The solar cell with a back contact metal work function of 5.5 eV, an absorber layer's thickness of 2.68 μm and an acceptor concentration of 5×10 16 cm −3 were optimum. The presented optimization is ideal and subject to experimental verification with a precise control of the process parameters along with reduced surface as well as bulk recombination, secondary phases and thermalization losses.

Mechanical properties of graphdiyne sheet

Abstract: The influences of strain to the energetic and electronic properties of graphdiyne are investigated based on first-principles calculations. The elastic parameters of graphdiyne are determined by total energy calculation. Compared to graphyne, graphdiyne is softer because it has less C–C bonds. Moreover, the band gap of graphdiyne is tunable under uniform strain. It monotonously increases with increasing strain value, which originates from the decreased orbital overlap between C atoms when strain increases.

The effect of Te doping on the electronic structure and thermoelectric properties of SnSe

Abstract: SnSe1−xTex (x=0, 0.0625) bulk materials were fabricated by melting Sn, Se and Te powders and then hot pressing them at various temperatures. The phase compositions of the materials were determined by X-ray diffraction (XRD) and the crystal lattice parameters were refined by the Rietveld method performed with DBWS. XRD analysis revealed that the grains in the materials preferentially grew along the (l 0 0) directions. The structural behavior of SnSe1−xTex (x=0, 0.0625) was calculated using CASTEP package provided by Materials Studio. We found that the band gap of SnSe reduced from 0.643 to 0.608 eV after Te doping. The calculated results were in good agreement with experimental results. The electrical conductivity and the Seebeck coefficient of the as-prepared materials were measured from room temperature to 673 K. The maximum power factor of SnSe is ∼0.7 μW cm−1K−2 at 673 K.

Linear and nonlinear optical properties in a disk-shaped quantum dot with a parabolic potential plus a hyperbolic potential in a static magnetic field

Abstract: Linear and nonlinear optical properties in a disk-shaped quantum dot (DSQD) with a parabolic potential plus a hyperbolic potential in a static magnetic field are theoretically investigated within the framework of the compact-density-matrix approach and iterative method The energy levels and the wave functions of an electron are obtained by three kinds of approximation methods It is found that optical absorption coefficients and refractive index changes are not only by the characteristic parameters of the hyperbolic potential and the confinement frequency, but also by the magnetic field

Optical properties of lithium magnesium borate glasses doped with dy3+ and sm3+ ions

Abstract: Several studies showed the interesting properties of trivalent lanthanide ions when doped in various types of glasses. Optical and physical properties of lithium magnesium borate glasses doped with Dy3+ then with Sm3+ ions were determined by measuring their absorption and luminescence spectra in the visible region. The absorption spectra of Dy3+ showed eight absorption bands with hypersensitive transition at 1265 nm (6H15/2→6F11/2-6H9/2) and three PL emission bands at 588 nm (4F9/2→6H15/2), 660 nm (4F9/2→6H13/2) and 775 nm (4F9/2→6H11/2). Regarding the Sm3+, nine absorption bands were observed with hypersensitive transition at 1237 nm (6H5/2–6F7/2); the PL spectrum showed four prominent peaks at 4G5/2→6H5/2 (yellow color), 4G5/2→6H7/2 (bright orange color), 4G5/2→6H9/2 (orange reddish color) and 4G5/2→6H11/2 (red color), respectively. Finally, a series of physical parameters such as the oscillator strengths, refractive index, ions concentration, Polaron radius and other parameters were calculated for each dopant.

First-principles study of electronic structures and optical properties of Cu, Ag, and Au-doped anatase TiO2

Abstract: We perform first-principles calculations to investigate the band structure, density of states, optical absorption, and the imaginary part of dielectric function of Cu, Ag, and Au-doped anatase TiO2 in 72 atoms systems. The electronic structure results show that the Cu incorporation can lead to the enhancement of d states near the uppermost of valence band, while the Ag and Au doping cause some new electronic states in band gap of TiO2. Meanwhile, it is found that the visible optical absorptions of Cu, Ag, and Au-doped TiO2, are observed by analyzing the results of optical properties, which locate in the region of 400–1000 nm. The absorption band edges of Cu, Ag, and Au-doped TiO2 shift to the long wavelength region compared with the pure TiO2. Furthermore, according to the calculated results, we propose the optical transition mechanisms of Cu, Ag, and Au-doped TiO2. Our results show that the visible light response of TiO2 can be modulated by substitutional doping of Cu, Ag, and Au.

Pt–Al2O3 nanocoatings for high temperature concentrated solar thermal power applications

Abstract: Nano-phased structures based on metal–dielectric composites, also called cermets (ceramic–metal), are considered among the most effective spectral selective solar absorbers. For high temperature applications (stable up to 650 °C) noble metal nanoparticles and refractory oxide host matrices are ideal as per their high temperature chemical inertness and stability: Pt/Al2O3 cermet nano-composites are a representative family. This contribution reports on the optical properties of Pt/Al2O3 cermet nano-composites deposited in a multilayered tandem structure. The radio-frequency sputtering optimized Pt/Al2O3 solar absorbers consist of stainless steel substrate/ Mo coating layer/ Pt–Al2O3/ protective Al2O3 layer and stainless steel substrate/ Mo coating layer /Pt–Al2O3 for different composition and thickness of the Pt–Al2O3 cermet coatings. The microstructure, morphology, theoretical modeling and optical properties of the coatings were analyzed by the x-ray diffraction, atomic force, microscopy, effective medium approximation and UV–vis specular and diffuse reflectance.

Defect mode properties in a one-dimensional photonic crystal

Abstract: Based on the transfer matrix method (TMM), the interaction of electromagnetic waves with one-dimensional (1D) defective photonic crystal in ultraviolet (UV) frequency region had been studied. With the calculated transmittance characteristics in the wavelength domain, it can be found that the defect mode can be generated within the photonic band gap (PBG) at the central wavelength. Also the effects of many parameters such as the angle of incidence, the state of polarization and the defect layer thickness have been taken in account. A significant effect in generating multiple defect peaks within the PBG has been illustrated.

Carrier transport and bandgap shift in n-type degenerate ZnO thin films: The effect of band edge nonparabolicity

Abstract: Contribution of band edge nonparabolicity to the charge carrier transport in degenerate n-type zinc oxide thin films has been investigated theoretically in order to understand the fundamental aspects of electron scattering in such thin films regardless of precise details of the preparation procedure. To conduct this, the theoretical evaluated results have been compared to the experimental values taken from literatures. The results indicate that the nonparabolicity (introducing through effective mass of charge carriers) has a strong effect on the total mobility of carriers in zinc oxide films so that a satisfactory agreement with experimental data is fulfilled. The dependence of nonparabolicity on bandgap shift is also discussed. Studying the optoelectronic properties of numerous moderately and heavily doped samples revealed that their optical bandgap has lower blueshift than the theoretical value obtained from the well-known Burstein–Moss effect. So, the observed bandgap shift was dependent on the carrier concentration and the total shift of bandgap was evaluated by combining the Burstein–Moss and bandgap narrowing effects. Two different cases were also examined; parabolic and nonparabolic (modified) Burstein–Moss effects. The results show that the modified Burstein–Moss effect leads to great agreement with experimental data.

Nonlocal vibration of coupled DLGS systems embedded on Visco-Pasternak foundation

Abstract: In this paper, vibration analysis of the coupled system of double-layered graphene sheets (CS-DLGSs) embedded in a Visco-Pasternak foundation is carried out using the nonlocal elasticity theory of orthotropic plate. The two DLGSs are coupled by an enclosing viscoelastic medium which is simulated as a Visco-Pasternak foundation. Considering the Von Karman nonlinear strain-displacement-relations, the motion equations are derived using the Hamilton's principle. Differential quadrature method (DQM) is applied to obtain the frequency ratio for various boundary conditions. The detailed parametric study is conducted, focusing on the combined effects of the nonlocal parameter, aspect ratio, graphene sheet's size, boundary conditions and the elastic and viscoelastic medium coefficients on the frequency ratio of CS-DLGSs. In this coupled system, two case of DLGSs vibration are investigated and compared with each other: (1) In-phase vibration (2) Out-of-phase vibration. The results indicate that the frequency ratio of the CS-DLGSs is more than the single-layered graphene sheet (SLGS). The results are in good agreement with the previous researches.

A study of energy band gap versus temperature for Cu2ZnSnS4 thin films

Abstract: The temperature dependent band gap energy of Cu2ZnSnS4 thin film was studied in the temperature range of 77–410 K. Various relevant parameters, which explain the temperature variation of the fundamental band gap, have been calculated using empirical and semi-empirical models. Amongst the models evaluated, the Varshni and Passler models show the best agreement with experimental data in the middle temperature range. However, the Bose–Einstein model fits reasonably well over the entire temperature range evaluated. The calculated fitting parameters are in good agreement with the estimated value of the Debye temperature calculated using the Madelung–Einstein approximation and the Hailing method.

Electrical conduction and dielectric studies of ZnO pellets

Abstract: A series of Zinc Oxide pellets sintered at different temperatures was studied by means of dielectric spectroscopy in the wide frequency range of 1–106 Hz and temperature interval from −100 °C to 30 °C. Electrical conductivity was analysed using Jonsher's universal power law, and the values of s were found to decrease with the increase in temperature, which agrees well with the correlation barrier hopping (CBH) model. As the temperature increased, energy activation Edc became less than 0.39 eV and dc conductivity (σdc) values in the range of 1.9×10−14–9.7×10−10 Ω m−1 were observed. The dielectric modulus showed ionic polarisation at the intermediate and high frequencies related to oxygen interstitial Oi, oxygen vacancy VO and Zinc interstitial Zni. At low frequency, it revealed a Maxwell–Wagner–Sillars relaxation with barrier heights of grain boundaries between 0.74 and 0.88 eV for all the studied pellets.

Electrical conductivity and complex electric modulus of titanium doped nickel–zinc ferrites

Abstract: The samples Ni1+x−yZnyTix Fe2−2xO4; y=0.1, 0.0≤x≤0.5 were prepared in a single-phase spinel structure as indicated from X-ray analysis. Electrical conductivity and dielectric measurements at different temperatures from 300 K to 600 K in the frequency range from 42 Hz to 5 MHz have been analyzed. The relation of conductivity with temperature revealed a semiconductor to semimetallic behavior as Ti4+ concentration increases. The conduction mechanism depends mainly on the valence exchange between the different metal ions in the same site or in different sites. The dielectric constant as a function of temperature and frequency showed that there is a strong dependence on the compositional parameter x. The electrical modulus has been employed to study the relaxation dynamics of charge carriers. The result indicates the presence of correlation between motions of mobile ion charges. The activation energies extracted from M′(ω) and M″(ω) peaks are found to follow the Arrhenius law. The electrical conductance of the samples found to be dependent on the temperature and frequency.

Substitutional effect of Cr3+ ions on the properties of Mg–Zn ferrite nanoparticles

Abstract: The effect of Cr 3+ substitution in Mg–Zn ferrite, with a chemical formula Mg 0.5 Zn 0.5 Cr x Fe 2− x O 4 ( x =0.0–1.0), synthesized by a sol–gel auto-combustion reaction is presented in this paper. The resultant powders were investigated by various techniques, including X-ray diffractometry (XRD), transmission electron microscopy (TEM), infrared spectroscopy (IR), vibrating sample magnetometry (VSM), and DC resistivity. The XRD pattern revealed that the cubic spinel structure is maintained for the all the compositions. The particle sizes measured from XRD and TEM are in good agreement with each other. The cation distribution suggests that Mg 2+ , Cr 3+ and Fe 3+ have strong preference towards octahedral B-site. The theoretical lattice constant and experimental lattice constant match each other very well. The IR analysis supports the presently accepted cation distribution. The saturation magnetization decreases linearly with increasing Cr 3+ content. Curie temperatures are obtained by the Laoria and AC susceptibility techniques. The dc resistivity has been investigated as a function of temperature and composition.

Buckling analysis and smart control of SLGS using elastically coupled PVDF nanoplate based on the nonlocal Mindlin plate theory

Abstract: This study presents an analytical approach for buckling analysis and smart control of a single layer graphene sheet (SLGS) using a coupled polyvinylidene fluoride (PVDF) nanoplate. The SLGS and PVDF nanoplate are considered to be coupled by an enclosing elastic medium which is simulated by the Pasternak foundation. The PVDF nanoplate is subjected to an applied voltage in the thickness direction which operates in control of critical load of the SLGS. In order to satisfy the Maxwell equation, electric potential distribution is assumed as a combination of a half-cosine and linear variation. The exact analysis is performed for the case when all four ends are simply supported and free electrical boundary condition. Adopting the nonlocal Mindlin plate theory, the governing equations are derived based on the energy method and Hamilton's principle. A detailed parametric study is conducted to elucidate the influences of the small scale coefficient, stiffness of the internal elastic medium, graphene length, mode number and external electric voltage on the buckling smart control of the SLGS. The results depict that the imposed external voltage is an effective controlling parameter for buckling of the SLGS. This study might be useful for the design and smart control of nano-devices.

Linear and nonlinear intersubband optical absorption in a disk-shaped quantum dot with a parabolic potential plus an inverse squared potential in a static magnetic field

Abstract: The linear and nonlinear optical absorption in a disk-shaped quantum dot (DSQD) with parabolic potential plus an inverse squared potential in the presence of a static magnetic field are theoretically investigated within the framework of the compact-density-matrix approach and iterative method. The energy levels and the wave functions of an electron in the DSQD are obtained by using the effective mass approximation. Numerical calculations are presented for typical GaAs/AlAs DSQD. It is found that the optical absorption coefficients are strongly affected not only by a static magnetic field, but also by the strength of external field, the confinement frequency and the incident optical intensity.

Oxygen-vacancy-related high-temperature dielectric relaxation and electrical conduction in 0.95K0.5Na0.5NbO3–0.05BaZrO3 ceramic

Abstract: The crystal structure and dielectric properties of 0.95K 0.5 Na 0.5 NbO 3 –0.05BaZrO 3 (KNN–BZ) ceramic have been investigated by X-ray diffraction and dielectric measurement. A rhombohedral distortion was caused and the dielectric permittivity near Curie temperature was significantly enhanced by introducing BZ into KNN. The dielectric and conductivity properties of the sample were studied by using AC impedance spectroscopy and universal dielectric relaxation law in detail. The typical high-temperature dielectric relaxation process was confirmed to be related to the oxygen vacancies inside the ceramic. The effect of lattice distortion on the activation energy for oxygen vacancy migration in KNN–BZ was discussed by comparing with KNN and KNN–BaTiO 3 .

Sol–gel auto-combustion synthesis, structural and enhanced magnetic properties of Ni2+ substituted nanocrystalline Mg–Zn spinel ferrite

Abstract: Nanocrystalline arrays of Ni2+ substituted Mg–Zn spinel ferrite having a generic formula Mg0.7−xNixZn0.3Fe2O4 (x=0.0, 0.2, 0.4 and 0.6) were successfully synthesized by sol–gel auto-combustion technique. The fuel used in the synthesis process was citric acid and the metal nitrate-to-citric acid ratio was taken as 1:3. The phase, crystal structure and morphology of Mg–Ni–Zn ferrites were investigated by X-ray diffraction, scanning electron microscopy, and Fourier transformer infrared spectroscopy techniques. The lattice constant, crystallite size, porosity and cation distribution were determined from the X-ray diffraction data method. The FTIR spectroscopy is used to deduce the structural investigation and redistribution of cations between octahedral and tetrahedral sites of Mg–Ni–Zn spinel structured material. Morphological investigation suggests the formation of grain growth as the Ni2+ content x increases. The saturation magnetization and magneton number were determined from hysteresis loop technique. The saturation magnetization increases with increasing Ni2+ concentration ‘x’ in Mg–Zn ferrite.

A model of phase transitions in double-well Morse potential: Application to hydrogen bond

Abstract: A model of phase transitions in double-well Morse potential is developed. Application of this model to the hydrogen bond is based on ab initio electron density calculations, which proved that the predominant contribution to the hydrogen bond energy originates from the interaction of proton with the electron shells of hydrogen-bonded atoms. This model uses a double-well Morse potential for proton. Analytical expressions for the hydrogen bond energy and the frequency of O–H stretching vibrations were obtained. Experimental data on the dependence of O–H vibration frequency on the bond length were successfully fitted with model-predicted dependences in classical and quantum mechanics approaches. Unlike empirical exponential function often used previously for dependence of O–H vibration frequency on the hydrogen bond length (Libowitzky, Mon. Chem., 1999, vol.130, 1047), the dependence reported here is theoretically substantiated.

FTIR structural investigation of gamma irradiated BaO–Na2O–B2O3–SiO2 glasses

Abstract: Fourier transform infrared (FT-IR) spectra of xBaO–15Na2O–(70−x)B2O3–15SiO2 glass system with x=0, 5, 10, 15 and 20 (mol%) has been measured in the spectral range 400–4000 cm−1 at room temperature in order to understand the characteristic frequencies of the chemical bonds and bonding mechanisms, which are susceptible to the structural and spectral changes. The effect of gamma irradiation in the dose range 0.1 kGy–60 kGy on the infrared absorption spectra of these glasses is also reported. The change in the glass structure due to the effect of composition is also discussed. It has been observed that irradiation of the glasses with the gamma rays increases the BO3 groups and the non bridging oxygens which make the network loose.

Structural, magnetic and magnetotransport behavior of La0.7SrxCa0.3−xMnO3 compounds

Abstract: A systematic investigation of the structural, magnetic and electrical properties of a series of nanocrystalline La 0.7 Sr x Ca 0.3− x MnO 3 materials, prepared by high energy ball milling method and then annealed at 900 °C has been undertaken. The analysis of the XRD data using the Win-metric software shows an increase in the unit cell volume with increasing Sr ion concentration. The La 0.7 Sr x Ca 0.3− x MnO 3 compounds undergo a structural orthorhombic-to-monoclinic transition at x =0.15. Electric and magnetic measurements show that both the Curie temperature and the insulator-to-metal transition temperature increase from 259 K and 253 K correspondingly for La 0.7 Ca 0.3 MnO 3 ( x =0) to 353 K and 282 K, respectively, for La 0.7 Sr 0.3 MnO 3 ( x =0.3). It is argued that the larger radius of Sr 2+ ion than that of Ca 2+ is the reason to strengthen the double-exchange interaction and to give rise to the observed increase of transition temperatures. Using the phenomenological equation for conductivity under a percolation approach, which depends on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the resistivity versus temperature data measured in the range of 50–320 K and found that the activation barrier decreased with the raising Sr 2+ ion concentration.

Nonlinear vibration of embedded SWBNNTs based on nonlocal Timoshenko beam theory using DQ method

Abstract: In the present work, effect of von Karman geometric nonlinearity on the vibration behavior of a single-walled boron nitride nanotube (SWBNNT) is investigated based on nonlocal piezoelasticity theory. The SWBNNT is considered as a nanobeam within the framework of Timoshenko beam (TB). Loading is composed of a temperature change and an imposed axially electric potential throughout the SWBNNT. The interactions between the SWBNNT and its surrounding elastic medium are simulated by Winkler and Pasternak foundation models. The higher order governing equations of motion are derived using Hamilton's principle and the numerical solution of equations is obtained using Differential Quadrature (DQ) method. The effects of geometric nonlinearity, elastic foundation modulus, electric potential field, temperature change and nonlocal parameter on the frequency of the SWBNNT are studied in detail.

Effective material parameter retrieval for thin sheets: Theory and application to graphene, thin silver films, and single-layer metamaterials

Abstract: An important tool in the field of metamaterials is the extraction of effective material parameters from simulated or measured scattering parameters of a sample. Here we discuss a retrieval method for thin-film structures that can be approximated by a two-dimensional scattering sheet. We determine the effective sheet conductivity from the scattering parameters and we point out the importance of the magnetic sheet current to avoid an overdetermined inversion problem. Subsequently, we present two applications of the sheet retrieval method. First, we determine the effective sheet conductivity of thin silver films and we compare the resulting conductivities with the sheet conductivity of graphene. Second, we apply the method to a cut-wire metamaterial with an electric dipole resonance. The method is valid for thin-film structures such as two-dimensional metamaterials and frequency-selective surfaces and can be easily generalized for anisotropic or chiral media.

Nonlocal vibration of SWBNNT embedded in bundle of CNTs under a moving nanoparticle

Abstract: In this study, an analytical method of the small scale parameter on the vibration of single-walled Boron Nitride nanotube (SWBNNT) under a moving nanoparticle is presented. SWBNNT is embedded in bundle of carbon nanotubes (CNTs) which is simulated as Pasternak foundation. Using Euler–Bernoulli beam (EBB) model, Hamilton's principle and nonlocal piezoelasticity theory, the higher order governing equation is derived. The effects of electric field, elastic medium, slenderness ratio and small scale parameter are investigated on the vibration behavior of SWBNNT under a moving nanoparticle. Results indicate the importance of using surrounding elastic medium in decrease of normalized dynamic deflection. Indeed, the normalized dynamic deflection decreases with the increase of the elastic medium stiffness values. The electric field has significant role on the nondimensional fundamental frequencies, as a smart controller. The results of this work is hoped to be of use in design and manufacturing of smart nano-electro-mechanical devices in advanced medical applications such as drug delivery systems with great applications in biomechanics.