جستجوی مقالات مرتبط با کلیدواژه « ساختار نواری » در نشریات گروه « فیزیک »

In this paper, the NixMg1-x O (0.1 ≤ x ≤ 0.4) solid solution nano-powder was synthesized by new and soft non-alkoxide sol-gel self-combustion method. In this method, Ni(NO3)3·6H2O, Mg(CH3COO)2·4H2O and Citric acid (CA), were used as Ni2+, Mg2+ ion and gelling and combusting source, respectively. Then, by thermal gravimetric analysis (TGA) the chemical reaction and the appropriate temperature to form a stable compound were determined. The influences of molar ratio of component (x) on structural and optical properties of samples have been investigated by X-ray diffraction (XRD), field-emission scanning electron microscope (FESEM), diffuse reflectance spectroscopy (DRS), photoluminescence (PL), and Fourier transform infrared (FTIR) analysis. By increasing in x, all the samples are shown decreasing ecreasing in lattice parameter (a) and crystallite size (D), which indicates the contamination of magnesium oxide with nickel and the formation of NixMg1-xO solid solution. The Band gap was decreased by increasing in x which shows that Ni2+ ions in MgO structure causes some modifications in the energy levels and the optical absorbance characteristics associated with F centers due to oxygen defect centers.

Arsenene is one of the members of a large group of two-dimensional structures that in the present project, we have investigated the single layer structure and its nanoribbons based on density functional theory. In this study, after calculating the single layer band structure, we discussed the effect of width on the band structure of nanoribbon from the angle of quantum limiting phenomenon. The results show the different effect of quantum limiting phenomenon on different points of each band in the band structure, which is the cause of indirect-direct gap transition due to the reduction of the width of armchair nanoribbon. Also, the electrical conductance properties of arsenene single layer have been obtained by calculating the mobility of load carriers and the effect of uniaxial strain of crystalline structure on them has been investigated. In order to investigate the mobility, crystalline structure defects have been neglected and only phonon dispersion is considered based on Takagi relationship. In this process, the focus is not on the quantitative accuracy of the obtained values for the mobility of load carriers and the existing anisotropy between mobility in the two directions of armchair and zigzag has been the focus of the discussion. The results of the calculations show a significant anisotropy of mobility in the two directions of armchair and zigzag and the effect of uniaxial strain of crystal structure on it. Also, a significant difference between the mobility of electrons and holes in the direction of armchair is one of the key results.

In this paper, the electrical characteristics and resonant tunneling phenomenon in nanoscale double gate field effect Schottky transistor with InP (Indium Phosphide) as the channel material is investigated via non-equilibrium Green's function formalism. Unlike the conventional field effect transistor with doped source/drain, Schottky transistor possesses metallic source/drain regions and direct tunneling from source to channel is the main current mechanism of this device. The bandstructure of double gate device is calculated based on sp3d5s* tight binding approach employing thickness dependant two dimensional Hamiltonian. Reducing the channel thickness results in the increment of the carrier effective mass and shift of energy of subbands to higher values, in comparison with the related bulk values. In addition, by scaling down the channel thickness, gate control over the channel is enhanced that results in the improvement of the device electrical characteristics. Next, due to the increment of the effective Schottky barrier that is originated from quantum effects, a quantum well profile is created along the channel length from source to drain at low drain voltages. In this situation and for reduced values of temperature, resonant tunneling occurs in the proposed device. Different physical and structural parameters that may affect resonant tunneling are thoroughly investigated.

In this paper, a two-dimensional phononic crystal comprising of steel rod in water is investigated. Three cross- sections for this rod are considered using finite element method (EFM). We plot the equifrequency surface of the first band, because of equifrequency surface convex around the edge of the first Brillouin Zone, we guess the negative effective phononic mass and so negative refraction. Moreover, the negative refraction behaviors are demonstrated by the simulation of a phononic crystal slab. The results show the maximum intensity for the phononic crystal with the circle, square, and triangle cross- section, of 1.15 MHz, 1.11MHz and 1.27MHz, respectively. The dependence of intensity and focal point distance on the slab thickness and between the source and the slab on the negative refractive is investigated. The results are in good agreement with other works.

The GaAs nano-layer has received much attention due to its wide application. Due to the importance of the GaAs nano-layer, in this paper, using two Mn and Fe impurities, the probabily of the metallic to semiconductor phase transition and vice versa, as well as the displacements of the optical coefficients peaks of this nano-layer are investigated. For this purpose, the structural, electronic and magnetic properties of pure GaAs nano-layer and this nano-layer with Mn (GaAs+Mn) and Fe (GaAs+Fe) impurities located at the nanolayer surface are investigated using the density functional theory. The electron density of states, linear coefficients of electronic specific heat, band structures and the total and local magnetic moment at impurity atomic position of these nano-layers are calculated and compared. The real and imaginary parts of dielectric function, static dielectric functional, uniaxial anisotropy, reflectivity, absorption, electron energy loss function and optical conductivity of pure GaAs , GaAs+Mn and GaAs+Fe nano-layers for electric field parallel and perpendicular to nano-layer surface within GGA and GGA_EV approaches are investigated and compared.

In this paper, we study the structural and electronic properties of the cubic PbTiO3 compound by using the density functional theory. For the calculation of band structure and density of states, we use the modified Becke–Johnson exchange potential proposed by Tran and Blaha (TB-mBJ), including the relativistic spin-orbit coupling (SOC). The results obtained from TB-mBJ functional and SOC calculations show that the calculated band gap is 2.18 eV. IRelast computational package is very recently implemented into the WIEN2k code and can be used to calculate the elastic constants of the crystal structures, where IR stands for Iran. We calculate the elastic constants of this compound by the IRelast code using the PBE-GGA, PBEsol-GGA, LDA, BPW91 and Engel-Vosko functionals. Then, by these elastic constants, we obtain some other related physical quantities such as shear constant, bulk modulus, Young’s modulus and Poisson’s ratio. Furthermore, we calculate the ductility of the compound under question. The calculated ductility shows that our compound is formable and not fragile. The effect of pressure on the elastic constants shows that C11, C12 and C44 are increased as the pressure is raised inside the considered pressure interval. Furthermore, the longitudinal and transverse sound velocities are derived for the compound using its calculated elastic constants. The results, therefore, show that the sound velocities, like elastic constants, are increased as pressure is raised .

In this work, the stability and electronic structure of zigzag double-walled silicon carbide nanotubes (DWSiCNTs) (6,0)@(n,0) (with n=11-17) were investigated by using ab initio Van der Waals density functional. By calculating the formation energy and the binding energy of each double walled nanotube, the best interwall distance for the outer nanotube was indicated. The results revealed that (13,0) nanotube could be the best external nanotube for the (6,0) internal nanotube with 3.53 Åinterwall distance to make (6,0)@(13,0) DWSiCNT. The structural calculations also revealed that all studied silicon carbide nanotubes were semiconductors and their energy gap decreased from the single one to the double-walled one. Moreover, with raising the nanotube diameter, the energy gap increased, such that at the most stable double-walled nanotube, its value was about 0.216 eV.

Density functional theory (DFT) and many body perturbation theory at the G0W0 level were used to investigate the change of the electron properties of Polythiophene (PT) polymer in the vicinity of graphene. The result of the analysis of the change in the density of the load compared to the pre-mutual interaction indicates a strong electric bipolarity and an absorption of the physical type at the surface. The change in the calculated electrical potential indicates the change in the work function to the value of its initial value . The results from the DFT do not show a change in the polymeric energy gap, while graphene energy gap changes from the isolated chain obtained from the results of the many body perturbation theory at the G0W0 level.

In this article, the electronic and optical properties of graphene with BC3 substrate are investigated. The calculations are performed on the basis of full potential linearized augmented plane waves in the framework of density functional theory. The total energy calculations of AA and AB configurations show that the AB stacking configuration is more stable than AA stacking configuration. The monolayer graphene has zero band gap while graphene with BC3 substrate has a small band gap of 0.15 eV at K point. The intrinsic properties of graphene such as linear dispersion of electronic bands near the K point and high carrier mobility are retained in the graphene with BC3 substrate. The dielectric function of graphene with BC3 substrate can be considered as the superposition of dielectric function of graphene and BC3 monolayer where the interaction between graphene and monolayer BC3 leads to the shift of peak positions. These results can be used in designing new opto-electronic devices such as field effect transistors.

In this research, we have studied the photonic band structure, optical properties and thermal emission spectrum of 2D Silicon photonic crystal with hexagonal structure. The band structure, band gap map and the gap size versus radius have been calculated by plane wave expansion method. The maximum band gap size of TE (TM) polarization and the complete gap size are 51% (20%) and 17% at air hole radius r=0.43a (0.50a) and r=0.48a, respectively. The optical properies have been calculated by FDTD methd in the range of 1 to 10 . The thermal emission spectrum has been obtained from absorption by Kirchhoff’s law. The obtaine results show that by engineering the band structure, the thermal emission spectrum of 2D Silicon photonic crystal can be controlled in a manner that can be used in thermophotovoltaic systems.

In this paper we studied the CdBr2 structure properties, for example lattice constant, bulk module, energy band structure, volume optimization and electronic charge. The calculation have been performed using PWscf method in the framework of density functional theory by Espresso package. The obtained results showed the existence of energy gap 2.8 eV in Γ point ,and also an ionic feature for this compound . The calculation is good agreement with other results.
In this paper we studied the CdBr2 structure properties, for example lattice constant, bulk module, energy band structure, volume optimization and electronic charge. The calculation have been performed using PWscf method in the framework of density functional theory by Espresso package. The obtained results showed the existence of energy gap 2.8 eV in Γ point ,and also an ionic feature for this compound . The calculation is good agreement with other results.In this paper we studied the CdBr2 structure properties, for example lattice constant, bulk module, energy band structure, volume optimization and electronic charge. The calculation have been performed using PWscf method in the framework of density functional theory by Espresso package. The obtained results showed the existence of energy gap 2.8 eV in Γ point ,and alsagreement with other results.

In the absence of lattice defects, pristine graphene and hydrogen-terminated armchair graphene nanoribbons are non-magnetic materials. The presence of vacancies extremely effect on the magnetic properties and they can produce remarkable spin polarization in these materials. In this study, using first principles calculations based on Kohn-Sham density functional theory (KS-DFT), the effect of the presence of vacancies on magnetic properties and spin polarization of monolayer and bilayer armchair graphene nanoribbons are investigated. The results show that the magnitude of magnetic moments depends on the number and configuration of vacancies, their distance from each other as well as from nanoribbon's edges.

In this paper, the electronic properties and topological phase of Li2AgSn nanolayer are investigated. The calculation is done based on the density functional theory within generalized gradient approximation (GGA) using full potential Linearized Augmented Plane Waves plus local orbitals . The electron density of states of this nanolayer is calculated using GGA approximation. The linear electron specific heat coefficient of Li2AgSn nanolayer is calculated. The xx and zz compounds of dielectric constant, reflection, absorption, refraction, extinction coefficients and energy loss function of this nanolayer are investigated. The topological phase and band order of this nanolayer are investigated by using the band structure.

In this paper، using plane ware expansion method the polarization-dependent band structures are investigated in two dimensional photonic crystal waveguide with a square lattice composed of GaAs elliptic elements in air background. Then، the changes of the band structure with changing ellipticity of the elliptic elements are discussed. It is observed that by increasing the ellipticity of elements the size of the photonic band gap and the guiding eigen frequency will increase

In this paper, some electronics properties of new superconductor Sr2VO3FeAs, such as density of states, band structure, density of electron cloud and bound lengths in the ground state have been calculated. According to N(Ef) in ground state CV/T value has been estimated. The calculations were performed in the framework of density functional theory (DFT), using the full potential linearized augmented plane wave (FP-LAPW) method with the general gradient approximation (GGA).

We studied the architecture of the C60 cluster to drive its atomic positions which can be seen at room temperature. We then used the obtained carbon positions as a basis set for the fcc structure to construct the fcc-C60 compound. Self consistent calculations were performed based on the density functional theory (DFT) utilizing the accurate WIEN2K code to solve the single-particle Kohen-Sham equation within the augmented plane waves plus local orbital (APW+lo) method. The cohesive energy has been found to be 1.537 eV for the fcc-C60. The calculated small cohesive energy that results from the weak Van der Waals-London interactions among a C60 cluster with its nearest neighbors is in good agreement with experiment. The electron densities of states (DOSs) were calculated for a C60 macromolecule as well as the fcc-C60 compound and the results were compared with each other. The band gap from DOS calculations has been found to be 0.7 eV. Band structures were also calculated within the generalized gradient approximation (GGA). The band structure calculation results in 1.04 eV for the direct band gap. Two kinds of σ and π bonds were determined in the band structure. Our results are in good agreement with experiment and pseudopotential calculations.

Obtaining the superconductor samples or mainly, structural phase controlling in 123 systems is a matter of special importance. As decreasing of oxygen in this structure has special effects, and mainly causing structural phase transition, by investigating the structure and thermal analysis of the system, tetragonal-orthorhombic structural phase transition was observed and optimum contents of structural phase transition temperature and oxygen were gained (ζc=630±5°c) and (7-δcΞ6.6), respectively, that are in good agreement with others’ work. In addition by band structure and density of states calculations of we gained the equilibrium position, the role and contribution of ions, especially oxygen, in total system’s density of state. It shows that the conductivity of orthorhombic phase is more in compare with tetragonal phase in normal states of 123 systems.

We have studied the effect of precursor powder size on the microstructure and intergranular behavior of polycrystalline Bi2223 superconductors using the XRD, SEM, electrical resistivity and AC susceptibility techniques. Polycrystalline Bi2223 superconductors were prepared from the powders with different milling times. The XRD results show that by decreasing the precursor powder size the Bi2223 phase fraction increases. It was found that the grain size and grain connectivity improved by decreasing the precursor powder size. Analysis of the temperature dependence of the AC susceptibility near the transition temperature (Tc) has been done employing Beans critical state model. The observed variation of intergranular critical current densities (Jc) with temperature indicates that the decreasing of precursor powder size in the Bi2223 system cases an increase in the intergranular critical current density.