Showing papers in "Physica B-condensed Matter in 2016"

Growth, spectral, thermal, laser damage threshold, microhardness, dielectric, linear and nonlinear optical properties of an organic single crystal: L-phenylalanine DL-mandelic acid

Abstract: Single crystals of L -phenylalanine dl-mandelic acid [C9H11NO2. C8H8O3], have been grown by the slow evaporation technique at room temperature using aqueous solution. The single crystal XRD study confirms monoclinic system for the grown crystal. The functional groups present in the grown crystal have been identified by FTIR and FT-Raman analyses. The optical absorption studies show that the crystal is transparent in the visible region with a lower cut-off wavelength of 257 nm and the optical band gap energy Eg is determined to be 4.62 eV. The Kurtz powder second harmonic generation was confirmed using Nd:YAG laser with fundamental wavelength of 1064 nm. Further, the thermal studies confirmed no weight loss up to 150°C for the as-grown crystal. The photoluminescence spectrum exhibited three peaks (414 nm, 519 nm, 568 nm) due to the donation of protons from carboxylic acid to amino group. Laser damage threshold value was found to be 4.98 GW/cm2. The Vickers microhardness test was carried out on the grown crystals and there by Vickers hardness number (Hv), work hardening coefficient (n), yield strength (σy), stiffness constant C11 were evaluated. The dielectric behavior of the crystal has been determined in the frequency range 50 Hz–5 MHz at various temperatures.

Effect of an encapsulate carbon nanotubes (CNTs) on structural and electrical properties of PU/PVC nanocomposites

Abstract: Nanocomposites samples of polyurethane and polyvinyl chloride (PU/PVC) loaded both multi and single walled-carbon nanotubes were synthesized by the casting technique. The X-ray analysis indicated that decrease degree of crystallinity after addition of carbon nanotubes (CNTs) due to interaction between CNTs and PU/PVC. The Transmission electron microscope (TEM) indicated encapsulation of polymer blend on CNTs surface. The highest value of AC conductivity was observed at high content of CNTs and frequency related effective conductive network formed when CNTs loaded in the blend. Whereas molecules of CNTs bridged between localized states and potential barrier. The permittivity ( e ′ ) was decreased when the frequency increased due to direction dipoles of applied electric field. At high frequencies, the decreasing trend of permittivity seems nearly stable attributed to dipoles orientation. The higher value of dielectric loss ( e ″ ) was observed at low frequency due to the mobile charges within blend backbone. An increases of loss tangent (tan δ) with increasing in CNTs content was expected because conductivity increases with increasing CNTs. The decrease of tan δ with increasing frequency is attributed to the fact that the hopping frequency of charge carriers cannot follow any changes of externally applied electric field.

Characterization of Sm-doped CeO2 nanoparticles and their magnetic properties

Abstract: Cubic phase Sm-doped CeO 2 nanoparticles (Ce 1− x Sm x O 2 , x =0, 0.05, 0.10, 0.15 and 0.20) were synthesized by the polymer pyrolysis method. X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometer (VSM) were employed to characterize the phase, morphology, valence states and magnetic properties of the samples. The samples were calcined at the low temperature of 600 °C which resulted in crystallite sizes of 10–20 nm. Raman and XPS spectra showed the presence of Ce 4+ , Ce 3+ and Sm 3+ ions and oxygen vacancies in the samples. Magnetization curves obtained from all samples exhibited ferromagnetic behavior at room temperature (RT-FM) with a maximum value of 0.012 emu/g for x =0.15. The data exhibited a good fit to bound magnetic polaron (BMP) model curves which account for the RT-FM behaviour by having sufficient concentrations of electrons bound to oxygen vacancies to facilitate a long-range exchange interaction between Ce 3+ ions. However, the relatively low values obtained for the BMP concentrations suggest that other mechanisms may also be at play.

Nonlocal vibration of axially moving graphene sheet resting on orthotropic visco-Pasternak foundation under longitudinal magnetic field

Abstract: In the present research, vibration and instability of axially moving single-layered graphene sheet (SLGS) subjected to magnetic field is investigated. Orthotropic visco-Pasternak foundation is developed to consider the influences of orthotropy angle, damping coefficient, normal and shear modulus. Third order shear deformation theory (TSDT) is utilized due to its accuracy of polynomial functions than other plate theories. Motion equations are obtained by means of Hamilton’s principle and solved analytically. Influences of various parameters such as axially moving speed, magnetic field, orthotropic viscoelastic surrounding medium, thickness and aspect ratio of SLGS on the vibration characteristics of moving system are discussed in details. The results indicated that the critical speed of moving SLGS is strongly dependent on the moving speed. Therefore, the critical speed of moving SLGS can be improved by applying magnetic field. The results of this investigation can be used in design and manufacturing of marine vessels in nanoscale.

Change Spectroscopic, thermal and mechanical studies of PU/PVC blends

Abstract: Blends of polyurethane (PU) and polyvinyl chloride (PVC) with different concentrations were prepared by casting method. The effects of PU on PVC blends was examined by Fourier transform-infrared (FTIR), Ultra-violet visible studies (UV/VIS.), X-ray diffraction (XRD), Thermogravimetric (TGA), Differential scanning calorimetry (DSC), and mechanical properties (stress–strain curve). The interaction between PU and PVC was examined by FT-IR through the absorbance of the N–H groups and was correlated to mechanical/thermal properties. Ultra-violet visible said that optical energy gap decrease with increasing concentration of PU. Differential scanning calorimetry results was observed a single glass transition temperature (Tg) for blends this confirming existence miscibility within the blends. The causes for best thermal stability of some blends may be described by measurements of interactions between C=O groups of PU and the α-hydrogen of PVC or a dipole–dipole –C=O..Cl–C– interactions. Significant alterations in FTIR, X-ray and DSC examination shows an interactions between blends had good miscibility. X-ray shows some alterations in the intensity with additional PU. PU change the mechanical behavior of PVC through of the blends. When polyurethane content increase causes polyvinyl chloride tensile strength decreases and elongation at break increase.

Effect of temperature on structural, optical and photoluminescence studies on ZnO nanoparticles synthesized by the standard co-precipitation method

Abstract: This present study brings the synthesis of Zinc oxide (ZnO) nanoparticles (NPs) by the standard aqueous chemical route technique. The impact of calcination temperature on the extent of the ZnO nanoparticles is studied for its lattice constraints. X-ray diffraction (XRD) affirms the hexagonal Wurtzite structure of the synthesized ZnO nanoparticles. From the Williamson–Hall (W–H) plot, positive slope is inferred for pure and calcined ZnO NPs and confirms the presence of tensile strain. From the SEM images it is found that the crystallinity enhances with calcination temperature. From the optical studies, it is found that the band gap energy decreases with improved transmission. The Photoluminescence (PL) spectrum reveals the UV emission is strong near the band-edge. The emission peaks around 400–480 nm result in blue emission and the peaks around 540–560 nm result in green emission. Decrease in band gap energy and enhancement in PL studies reveal the red shift of the calcined ZnO exhibiting solid quantum confinements.

Effect of solvent medium on the structural, morphological and optical properties of ZnO nanoparticles synthesized by the sol–gel method

Abstract: ZnO nanoparticles were synthesized using a sol–gel method. The effect of solvent medium on the structural, morphological and optical properties of ZnO nanoparticles were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence spectroscopy (PL), UV–vis spectroscopy (UV–vis) and Energy-dispersive X-ray spectroscopy ( EDS). The XRD patterns showed single phase hexagonal structure. The crystallite size of as prepared ZnO nanoparticles was found to decrease from 28.1 nm to 10.8 nm with the increase in volume ratio of ethanol in the solvent as peak intensities and sharpness increase with corresponding increase in volume ratio of water. SEM micrographs showed that samples prepared in water medium are fairly spherical which turned to tiny rods with increasing volume ratios of ethanol. A sharp ultraviolet (UV) emission peak centred about 385 nm and a broad green–yellow emission at about 550 nm are observed with PL measurements. The band gap of ZnO decreased from 3.31 to 3.17 eV with an increase in the ethanol composition in the solvent, implying that the optical properties of these materials are clearly affected by the synthesis medium.

Frequency and voltage dependence dielectric properties, ac electrical conductivity and electric modulus profiles in Al/Co 3 O 4 -PVA/p-Si structures

Abstract: In this research a simple microwave-assisted method have been used for preparation of cobalt oxide nanostructures. The as-prepared sample has been investigated by UV–vis spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM). On the other hand, frequency and voltage dependence of both the real and imaginary parts of dielectric constants (e′, e″) and electric modulus (M′ and M″), loss tangent (tanδ), and ac electrical conductivity (σac) values of Al/Co3O4-PVA/p-Si structures were obtained in the wide range of frequency and voltage using capacitance (C) and conductance (G/ω) data at room temperature. The values of e′, e″ and tanδ were found to decrease with increasing frequency almost for each applied bias voltage, but the changes in these parameters become more effective in the depletion region at low frequencies due to the charges at surface states and their relaxation time and polarization effect. While the value of σ is almost constant at low frequency, increases almost as exponentially at high frequency which are corresponding to σdc and σac, respectively. The M′ and M″ have low values at low frequencies region and then an increase with frequency due to short-range mobility of charge carriers. While the value of M′ increase with increasing frequency, the value of M″ shows two peak and the peaks positions shifts to higher frequency with increasing applied voltage due to the decrease of the polarization and Nss effects with increasing frequency.

Analysis of temperature-dependant current–voltage characteristics and extraction of series resistance in Pd/ZnO Schottky barrier diodes

Abstract: We report on the analysis of current voltage ( I–V ) measurements performed on Pd/ZnO Schottky barrier diodes (SBDs) in the 80–320 K temperature range. Assuming thermionic emission (TE) theory, the forward bias I–V characteristics were analysed to extract Pd/ZnO Schottky diode parameters. Comparing Cheung’s method in the extraction of the series resistance with Ohm’s law, it was observed that at lower temperatures (T 200 K). The barrier height and the ideality factor decreased and increased, respectively, with decrease in temperature, attributed to the existence of barrier height inhomogeneity. Such inhomogeneity was explained based on TE with the assumption of Gaussian distribution of barrier heights with a mean barrier height of 0.99 eV and a standard deviation of 0.02 eV. A mean barrier height of 0.11 eV and Richardson constant value of 37 A cm − 2 K −2 were determined from the modified Richardson plot that considers the Gaussian distribution of barrier heights.

Composition-dependent magnetic properties of melt-spun La or/and Ce substituted nanocomposite NdFeB alloys

Abstract: Aiming at high-performance low-cost NdFeB magnets, the magnetic properties and microstructure for melt spun nanocrystalline (Nd 1− x M x ) 10 Fe 84 B 6 (M=La, Ce, or La 0.5 Ce 0.5 ; x =0–0.7) alloys were investigated. Relatively, LaCe-substituted alloys show high values of the remanent magnetization M r , the maximum energy product ( BH ) max and the coercivity H c , up to 114 emu/g (1.07 T), 147 kJ/m 3 and 471 kA/m, respectively, at x =0.1. The unusual increase in coercivity for the alloys with 10% La or 10% La 0.5 Ce 0.5 substitution is possibly attributed to the phase segregation in alloys with certain La or LaCe contents. The reduced Curie temperature and spin-reorientation temperature were obtained for La, Ce or LaCe substituted alloys. Transmission electron microscope analysis has revealed that a fine and uniform distributed grain structure leads to remanence enhancement for La 0.5 Ce 0.5 substituted alloys. The present results indicate that partially substituting Nd by La or/and Ce cannot only effectively reduce the cost of nanocrystalline NdFeB based magnetic powders but also can maintain a relatively good combination of magnetic properties.

Effect of doping of trivalent cations Ga 3+ , Sc 3+ , Y 3+ in Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) system on Li + ion conductivity

Abstract: We report the effect of trivalent cations dopants in the Li 1.3 Al 0.3− x R x Ti 1.7 (PO 4 ) 3 (R=Ga 3+ , Sc 3+ , Y 3+ ) NASICON ceramic system in the concentration range x =0.01,0.03,0.05,0.07, on the Li + ion conducting properties using impedance spectroscopy. The samples were prepared by solid state reaction method and characterized by X-Ray Diffraction and density measurements. The electrical properties were studied using impedance spectroscopy in frequency range 10 Hz to 20 MHz and temperature range 303 K to 423 K. Although the porosity of the material decreased with doping, the overall Li + ion conductivity of the system did not improve with doping. Ionic radii of the dopant cations was found to be an important factor in formation of impurity phases and low Li + ion conductivity. Gallium doped samples exhibited a higher Li + ion conductivity compared to its scandium and yttrium doped counterparts.

Morphological, Raman, electrical and dielectric properties of rare earth doped X-type hexagonal ferrites

Abstract: The influence of rare-earth metals (La, Nd, Gd, Tb, Dy) on morphology, Raman, electrical and dielectric properties of Ba2NiCoRExFe28−xO46 ferrites were studied. The scanning electron microscopy (SEM) exhibited the platelet like structure of these hexagonal ferrites. The surface morphology indicated the formation of ferrite grains in the nano-regime scale. The bands obtained at lower wave number may be attributed to the metal-oxygen vibration at octahedral site which confirm the development of hexagonal phase of these ferrites. The resonance peaks were observed in dielectric constant, dielectric loss factor and quality factor versus frequency graphs. These dielectric parameters indicate that these ferrites nano-materials are potential candidates in the high frequency applications. The enhancement in DC electric resistivity from 2.48×108 to 1.20×109 Ω cm indicates that the prepared materials are beneficial for decreasing the eddy current losses at high frequencies and for the fabrication of multilayer chip inductor (MLCI) devices.

Maximum hardness and minimum polarizability principles through lattice energies of ionic compounds

Abstract: The maximum hardness (MHP) and minimum polarizability (MPP) principles have been analyzed using the relationship among the lattice energies of ionic compounds with their electronegativities, chemical hardnesses and electrophilicities. Lattice energy, electronegativity, chemical hardness and electrophilicity values of ionic compounds considered in the present study have been calculated using new equations derived by some of the authors in recent years. For 4 simple reactions, the changes of the hardness (Δη), polarizability (Δα) and electrophilicity index (Δω) were calculated. It is shown that the maximum hardness principle is obeyed by all chemical reactions but minimum polarizability principles and minimum electrophilicity principle are not valid for all reactions. We also proposed simple methods to compute the percentage of ionic characters and inter nuclear distances of ionic compounds. Comparative studies with experimental sets of data reveal that the proposed methods of computation of the percentage of ionic characters and inter nuclear distances of ionic compounds are valid.

Nonlinear optical parameters of nanocrystalline AZO thin film measured at different substrate temperatures

Abstract: The 2.2 wt% of aluminum (Al)-doped zinc oxide (AZO) transparent and preferential c -axis oriented thin films were prepared by using radio frequency (DC/RF) magnetron sputtering at different substrate temperature ranging from room temperature to 200 °C. For structural analysis, X-ray Diffraction (XRD) and Atomic Force Electron Microscope (AFM) was used for morphological studies. The optical parameters such as, optical energy gap, refractive index, extinction coefficient, dielectric loss, tangent loss, first and third order nonlinear optical properties of transparent films were investigated. High transmittance above 90% and highly homogeneous surface were observed in all samples. The substrate temperature plays an important role to get the best transparent conductive oxide thin films. The substrate temperature at 150 °C showed the growth of highly transparent AZO thin film. Energy gap increased with the increased in substrate temperature of Al doped thin films. Dielectric constant and loss were found to be photon energy dependent with substrate temperature. The change in substrate temperature of Al doped thin films also affect the non-liner optical properties of thin films. The value of χ (3) was found to be changed with the grain size of the thin films that directly affected by the substrate temperature of the pure and Al doped ZnO thin films.

0 - π Quantum transition in a carbon nanotube Josephson junction: Universal phase dependence and orbital degeneracy

Abstract: In a quantum dot hybrid superconducting junction, the behavior of the supercurrent is dominated by Coulomb blockade physics, which determines the magnetic state of the dot. In particular, in a single level quantum dot singly occupied, the sign of the supercurrent can be reversed, giving rise to a π -junction. This 0 − π transition, corresponding to a singlet-doublet transition, is then driven by the gate voltage or by the superconducting phase in the case of strong competition between the superconducting proximity effect and Kondo correlations. In a two-level quantum dot, such as a clean carbon nanotube, 0– π transitions exist as well but, because more cotunneling processes are allowed, are not necessarily associated to a magnetic state transition of the dot. In this proceeding, after a review of 0– π transitions in Josephson junctions, we present measurements of current-phase relation in a clean carbon nanotube quantum dot, in the single and two-level regimes. In the single level regime, close to orbital degeneracy and in a regime of strong competition between local electronic correlations and superconducting proximity effect, we find that the phase diagram of the phase-dependent transition is a universal characteristic of a discontinuous level-crossing quantum transition at zero temperature. In the case where the two levels are involved, the nanotube Josephson current exhibits a continuous 0 − π transition, independent of the superconducting phase, revealing a different physical mechanism of the transition.

Enhanced photocatalytic activity of ZnS nanoparticles loaded with MoS 2 nanoflakes by self-assembly approach

Abstract: A hybrid consisting of ZnS nanoparticles supported on layered MoS 2 −ZnS was synthesized by a hydrothermal method based on self-assembly technique without using a template. XRD, SEM-EDX, TEM, HR-TEM, TG-DTA, XPS, N 2 adsorption-desorption, and UV–Vis spectroscopies were used to characterize the structural features, morphology, and composition of the MoS 2 –ZnS hybrid. The results show that the MoS 2 –ZnS hybrid is mainly ZnS nanoparticles on layered MoS 2 with a thickness of ca. 5–20 nm. The combination of the MoS 2 and ZnS hybrid structure is beneficial for enhancing the photocatalytic degradation of rhodamine B (RhB) under visible light irradiation. A possible photoreaction mechanism of the MoS 2 –ZnS hybrid in the degradation is proposed. The photoexcited electrons from the ZnS could easily transfer to the conduction band of MoS 2 , thus decreasing the recombination of photoinduced carriers and enabling the degradation of RhB under visible light irradiation.

An investigation of the corrosion of polycrystalline iron by XPS, TMS and CEMS

Abstract: The room temperature studies of polycrystalline iron exposed to air at various temperatures were performed using: the transmission Mossbauer spectroscopy (TMS), the conversion electron Mossbauer spectroscopy (CEMS) and the X-ray photoelectron spectroscopy (XPS). The unique combination of these techniques allows to determine changes of chemical composition and content of iron oxides simultaneously on the surface region, the 300 nm pre-surface region and the bulk of the samples. The results show that the chemical composition of samples changes significantly and it is strongly dependent on temperature at which the iron sample is exposed to air as well as on investigated region.

Experimental and first principles investigation of the multiferroics BiFeO3 and Bi0.9Ca0.1FeO3: Structure, electronic, optical and magnetic properties

Abstract: We propose first-principles methods to study the structure, electronic, optical and magnetic properties of BiFeO3 (BFO) and Bi0.9Ca0.1FeO3 (BCFO). The morphology, optical band gap as well as magnetic hysteresis also have been investigated using experimental methods. X-ray diffraction data shows that Bi-site doping with Ca could result in a transition of crystal structure (from single phase rhombohedral (R3c) to two phase coexistence). Changing of Fermi level and decreasing of band gap indicating that the Ca-doped BFO exhibit a typical half-metallic nature. The optical absorption properties are related to the electronic structure and play the key role in determining their band gaps, also we have analyzed the inter-band contribution to the theory of optical properties such as absorption spectra, dielectric constant, energy-loss spectrum, absorption coefficient, optical reflectivity, and refractive index of BCFO. Enhancement of magnetic properties after doping is proved by both experimental and calculated result, which can be explained by size effect and structural distortion.

Monte Carlo study of internal energy and specific heat of a nano-graphene bilayer in a longitudinal magnetic field

Abstract: The thermodynamic properties of a nano-graphene bilayer, consisting of the upper layer A of spin-3/2 with antiferromagnetic intralayer exchange coupling and the bottom layer B of spin-5/2 with ferromagnetic intralayer exchange coupling, have been studied by the use of Monte Carlo simulation. We find a number of characteristic phenomena. The effects of the exchange coupling, the single-ion anisotropy and the longitudinal magnetic field on the internal energy, the specific heat and the blocking temperature of the mixed-spin bilayer system have been investigated in detail. The internal energy and the specific heat profiles are clarified. In particular, we have found that the specific heat curve may show two peaks phenomenon for appropriate values of the system parameters.

Structural and optical study of CaF 2 nanoparticles produced by a microwave-assisted hydrothermal method

Abstract: CaF2 nanoparticles were synthesized by the microwave-assisted hydrothermal method. With the addition of the ethylenediamine (EDA) as chelating agent, the size of the particles was reduced. The CaF2 exhibit single phase identified for X-ray diffraction (XRD) and confirmed by Rietveld refinement. Scanning electron microscopy (SEM) images showed nanoparticles with non-uniform morphology and statistical analysis of collections of particles reviewed that the EDA decreases both the average particle size and average aspect ratio of the particles. The chemical composition of the surface of the particles was investigated by X-ray Photoelectron Spectroscopy (XPS) and the results indicated the presence of reasonable amounts of hydroxyl groups and oxygen ions in the samples produced with EDA. Radioluminescence (RL) measurements showed that both types of nanoparticles presented intrinsic scintillation emission formed by two main bands and that the CaF2 samples produced without EDA presented higher emission intensity. The broad RL band centered at approximately 293 nm is related to self-trapped exciton (STE) emission of calcium fluoride, while the band at 428 nm can be due to the presence of F centers. The STE excitation and optical band gap were measured through photoluminescent excitation spectra in the VUV range.

Bilinear forms and soliton solutions for a fourth-order variable-coefficient nonlinear Schrödinger equation in an inhomogeneous Heisenberg ferromagnetic spin chain or an alpha helical protein

Abstract: In this paper, a fourth-order variable-coefficient nonlinear Schrodinger equation is studied, which might describe a one-dimensional continuum anisotropic Heisenberg ferromagnetic spin chain with the octuple–dipole interaction or an alpha helical protein with higher-order excitations and interactions under continuum approximation. With the aid of auxiliary function, we derive the bilinear forms and corresponding constraints on the variable coefficients. Via the symbolic computation, we obtain the Lax pair, infinitely many conservation laws, one-, two- and three-soliton solutions. We discuss the influence of the variable coefficients on the solitons. With different choices of the variable coefficients, we obtain the parabolic, cubic, and periodic solitons, respectively. We analyse the head-on and overtaking interactions between/among the two and three solitons. Interactions between a bound state and a single soliton are displayed with different choices of variable coefficients. We also derive the quasi-periodic formulae for the three cases of the bound states.

A moving approach for the Vector Hysteron Model

Abstract: A moving approach for the VHM (Vector Hysteron Model) is here described, to reconstruct both scalar and rotational magnetization of electrical steels with weak anisotropy, such as the non oriented grain Silicon steel. The hysterons distribution is postulated to be function of the magnetization state of the material, in order to overcome the practical limitation of the congruency property of the standard VHM approach. By using this formulation and a suitable accommodation procedure, the results obtained indicate that the model is accurate, in particular in reproducing the experimental behavior approaching to the saturation region, allowing a real improvement respect to the previous approach.

Hydrostatic-pressure effects on the pseudogap in slightly doped YBa2Cu3O7−δ single crystals

Abstract: The influence of hydrostatic pressure P up to 1.05 GPa on resistivity ρ, excess conductivity σ ′ ( T ) and pseudogap Δ ⁎ ( T ) is investigated in slightly doped single crystals of YBa2Cu3O7−δ ( T c ( P = 0 ) ≈ 49.2 K and δ ≈ 0.5 ). The critical temperature Tc is found to increase with increasing pressure at a rate dT c / dP =+ 5.1 KGPa − 1 , while ρ ( 300 K ) decreases at a rate d ln ρ / dP = ( − 19 ± 0.2 ) % GPa − 1 . Near Tc, independently on pressure, σ ′ ( T ) is well described by the Aslamasov–Larkin and Hikami–Larkin fluctuation theories, demonstrating a 3D–2D crossover with increase of temperature. The crossover temperature T0 determines the coherence length along the c-axis ξ c ( 0 ) ≃ ( 3.43 ± 0.01 ) A ˚ at P=0, which is found to decrease with increasing P. At the same time, Δ ⁎ and the BCS ratio 2 Δ ⁎ / k B T c both increase with increasing hydrostatic pressure at a rate d ln Δ ⁎ / dP ≈ 0.36 GPa − 1 , implying an increase of the coupling strength with increasing P. At low temperatures below Tpair, the shape of the Δ ⁎ ( T ) curve is found to be almost independent of pressure. At high temperatures, the shape of the Δ ⁎ ( T ) curve changes noticeably with increasing P, suggesting a strong influence of pressure on the lattice dynamics. This unusual behavior is observed for the first time.

Free vibrations analysis of carbon nanotubes resting on Winkler foundations based on nonlocal models

Abstract: In the present study, free vibrations of single walled carbon nanotubes (SWCNT) on an elastic foundation is investigated by nonlocal theory of elasticity with both beam and shell models. The nonlocal boundary conditions are derived explicitly and effectiveness of nonlocal parameter appearing in nonlocal boundary conditions is studied. Also it is demonstrated that the beam model is comparatively incapable of capturing size effects while shell model captures size effects more precisely. Moreover, the effects of some parameters such as mechanical properties, foundation stiffness, length and radius ratios on the natural frequencies are studied and some conclusions are drawn.

Dielectric relaxation analysis and Ac conductivity of polyvinyl alcohol/polyacrylonitrile film

Abstract: A film of 0.98 polyvinyl alcohol (PVA)/0.02 Polyacrylonitrile (PAN) has been prepared using casting method. The dielectric properties were measured as function of temperature and frequency. The dielectric permittivity of PVA is considerably enhanced by doping with PAN. Different relaxation processes have been recognized within the studied ranges of temperature and frequency. The frequency temperature superposition (FTS) is well verified. Frequency and temperature dependence of Ac conductivity, σac, were studied. The conduction mechanism of pure PVA and PVA doped with PAN are discussed. The activation energy either for relaxation or conduction was calculated. Comparison with similar polymeric materials is discussed.

The Schottky-type specific heat as an indicator of relative degeneracy between ground and first-excited states: The case study of regular Ising polyhedra

Abstract: The specific heat of regular Ising polyhedra is investigated in detail as a function of temperature and magnetic field. It is shown that the regular Ising polyhedra display diverse double-peak temperature dependences of the specific heat whenever the magnetic field approaches a level-crossing field. The Schottky theory of a two-level system often provides a plausible explanation of a height and position of low-temperature peak, which emerges in the specific heat of a regular Ising polyhedron due to low-lying excitations from a ground state to a first-excited state. The height and position of Schottky-type maximum depends essentially on a relative degeneracy of the ground state and first-excited state, which are in general quite distinct in geometrically frustrated Ising spin clusters. Low-temperature variations of the specific heat with the magnetic field exhibit multipeak structure with two peaks (of generally different height) symmetrically placed around each level-crossing field.

Spectroscopic properties of Pr3+ ions embedded in lithium borate glasses

Abstract: A series of lithium borate glasses with different Pr3+ contents were prepared by the melt quench technique to explore the new material for solid state light applications. We found that the addition of Pr3+ ions in the glass matrix has a profound effect on the properties of the glasses. The presence of Pr3+ ions in the glass matrix created various absorption bands compared to the base glass. These bands were due to the ground state (3H4) of the Pr3+ to the various excited states. Optical energy band gap was calculated by Tauc's method which showed a decreasing trend with an increase in the Pr3+ content. This might be due to structural changes when the glass structure became rigid due to the Pr3+ ions and this was confirmed by the density results. Rigidity of the glass structure was further confirmed by the Fourier transformed infrared results. The excitation spectra showed bands at 3H4→3P2, 3P1 and 3P0 nm. The 3H4→3P2 band was used to study the unresolved 1D2→3H4 and 3P0→3H6 transitions of the Pr3+ ions.

Investigating the effect of multiple grain–grain interfaces on electric and magnetic properties of [50 wt% BaFe12O19–50 wt% Na0.5Bi0.5TiO3] composite system

Abstract: This report presents the fabrication, electrical properties along with the magnetic parameters of a composite system considering a strong ferrimagnetic (BaFe 12 O 19 ) and a ferroelectric (Na 0.5 Bi 0.5 TiO 3 ) material. Polycrystalline 50 wt% BaFe 12 O 19 (BaM)–50 wt% Na 0.5 Bi 0.5 TiO 3 (NBT) composite system was prepared by the solid state reaction method. Rietveld refinement of XRD pattern confirms the presence of BaM and NBT phases without any impurity phase. From scanning electron micrograph both the phases are also clearly identified. In this report, the electric relaxation and conductivity properties were systematically investigated and analyzed in the frequency range of 100 Hz to 1 MHz and temperature range of 30–200 °C. The presence of different type of grains and significant reduction in the resistance of the composite system were found to be responsible for the nature of electric relaxation behavior. A peculiar and interesting evolution of grain boundary conduction was detected which was argued due to the existence of three possible grain boundaries such as: (i) BaM–BaM interface, (ii) NBT–NBT interface and (iii) BaM–NBT interface. The magnetization study (M–H loop) paves that, the saturation magnetization and coercive field reduces for composite system.

Structural Rietveld refinement and vibrational study of MgCr x Fe 2-x O 4 spinel ferrites

Abstract: Spinel ferrites with the general formula MgCrxFe2−xO4 (0≤x≤1) were synthesized by the standard ceramic technique and characterized by X-ray diffraction. The XRD patterns confirmed that the mixed ferrite samples are in the cubic spinel structure which is further validated by Rietveld refinement in the space group Fd3m. The crystal structure and cell parameters were refined by Rietveld analysis. The vibrational study was achieved using Fourier Transform-InfraRed (FT-IR) and Raman spectroscopy. From FT-IR band frequencies, the force constants Kt and Ko , for tetrahedral (A) and octahedral (B) sites respectively, have been calculated and discussed with the trend of bond lengths obtained from Rietveld refinement. For all compositions, Raman spectra revealed the five active modes showing the vibration of O2− ions at both the A-site and B-site ions. The frequencies trend with chromium content of both FT-IR and Raman spectra showed a shift toward higher values for all modes.

Structural and room temperature ferromagnetic properties of Ni doped ZnO nanoparticles via low-temperature hydrothermal method

Abstract: A series of Zn 1− x Ni x O ( x =0, 1%, 3%, 5%) nanoparticles have been synthesized via a low-temperature hydrothermal method. Influence of Ni doping concentration on the structure, morphology, optical properties and magnetism of the samples was investigated by means of X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, UV–vis spectrophotometer and vibrating sample magnetometer instruments. The results show that the undoped and doped ZnO nanoparticles are both hexagonal wurtzite structures. The surface analysis was performed using X-ray photoelectron spectroscopic studies. The images of SEM reveal that the structure of pure ZnO and Ni doped samples are nanoparticles which intended to form flakes with thickness of few nanometers, being overlain with each one to develop the network with some pores and voids. Based on the ultraviolet–visible (UV–vis) spectroscopy analysis, it indicates that the band gap energy decreases with the increasing concentration of Ni. Furthermore, The Ni doped ZnO samples didn't exhibit higher ultraviolet-light-driven photocatalytic activity compared to the undoped ZnO sample. Vibrating sample magnetometer was used for the magnetic property investigations, and the result indicates that room temperature ferromagnetism property of 3% Ni doped sample is attributed to oxygen vacancy and interaction between doped ions.