PHỔ PLASMON TRONG HỆ 3 LỚP GRAPHENE LỚP KÉP

Recent research demonstrates that graphene has unique properties and applications in many technological fields. This paper presents results calculated within random phase approximation at zero temperature for collective excitations, an important characteristic of materials, in a three-layer structure consisting of three bilayer graphene sheets in an inhomogeneous background dielectric. Numerical calculations show that one optical and two acoustic branches exist in the system. The optical branch becomes overdamped quickly while the two acoustic branches continue and disappear at single-particle excitation boundaries. The increase in carrier density in the layers significantly decreases the frequencies of plasmon modes. The inhomogeneity of the background dielectric decreases the frequency of the higher branches but increases that of the lower branch. The effects of interlayer separation on plasmon modes are similar to those in homogeneous systems. Our results may provide more information and contribute to improving the theory of graphene.

Structural Optimization and the Study of the Electronic, Mechanical, Thermodynamic and Phonon Properties of Mg2sn from First Principle

First principles pseudopotential method based on density functional theory is used to investigate the Structural, Mechanical, Phonon, Thermodynamic and Electronic properties of Mg2Sn. The equilibrium properties including lattice constant, bulk modulus, pressure derivative cohesive energy, young modulus, shear modulus were determined. The results obtained were compared with available experimental and other available results. Mg2Sn was found to be brittle in nature with a non-metallic properties as shown by the value of the Cauchy pressure of -4.03. The Phonon dispersion curve of Mg2Sn was obtained utilizing the PBE-GGA exchange-correlation potential as employed in the Vienna Ab-Initio Simulation Package (VASP) computer code. The gap separating the acoustic and the optical branch of the curve was found to be about 50cm-1 at X-point. The thermodynamic properties of the material was investigated in the temperature of 0-800K. At room temperature, the calculated value of the specific heat capacity ( ) is 71.28J/mol which is in good agreement with experimental and other results. Mg2Sn was found to a narrow gap semiconductor with an indirect bandgap of magnitude of 0.175eV.

Phase Imbalance Optimization in Interference Linear Displacement Sensor with Surface Gratings

In interferential linear displacement sensors, accurate information about the position of the reading head is calculated out of a pair of quadrature (sine and cosine) signals. In double grating interference schemes, diffraction gratings combine the function of beam splitters and phase retardation devices. Specifically, the reference diffraction grating is located in the reading head and regulates the phase shifts in diffraction orders. Measurement diffraction grating moves along with the object and provides correspondence to the displacement coordinate. To stabilize the phase imbalance in the output quadrature signals of the sensor, we propose to calculate and optimize the parameters of these gratings, based not only on the energetic analysis, but along with phase relationships in diffraction orders. The optimization method is based on rigorous coupled-wave analysis simulation of the phase shifts of light in diffraction orders in the optical system. The phase properties of the reference diffraction grating in the interferential sensor are studied. It is confirmed that the possibility of quadrature modulation depends on parameters of static reference scale. The implemented optimization criteria are formulated in accordance with the signal generation process in the optical branch. Phase imbalance and amplification coefficients are derived from Heydemann elliptic correction and expressed through the diffraction efficiencies and phase retardations of the reference scale. The phase imbalance of the obtained quadrature signals is estimated in ellipticity correction terms depending on the uncertainties of influencing parameters.

Study of Abnormal Group Velocities in Flexural Metamaterials

Generally, it has been known that the optical branch of a simple one-dimensional periodic structure has a negative group velocity at the first Brillouin zone due to the band-folding effect. However, the optical branch of the flexural wave in one-dimensional periodic structure doesn’t always have negative group velocity. The problem is that the condition whether the group velocity of the flexural optical branch is negative, positive or positive-negative has not been studied yet. In consequence, who try to achieve negative group velocity has suffered from trial-error process without an analytic guideline. In this paper, the analytic investigation for this abnormal behavior is carried out. In particular, we discovered that the group velocity of the optical branch in flexural metamaterials is determined by a simple condition expressed in terms of a stiffness ratio and inertia ratio of the metamaterial. To derive the analytic condition, an extended mass-spring system is used to calculate the wave dispersion relationship in flexural metamaterials. For the validation, various numerical simulations are carried out, including a dispersion curve calculation and three-dimensional wave simulation. The results studied in this paper are expected to provide new guidelines in designing flexural metamaterials to have desired wave dispersion curves.

Polaron in Electric Field and Vibrational Spectrum of Polyacetylene

In the present paper, the evolution of a polaron in polyacetylene in electric field is studied in detail. It is shown that the dependence of the polaron velocity on the applied field is determined by the peculiarities of the vibrational spectrum of polyacetylene. The spectrum is calculated numerically using the Hessian, the matrix of the second derivatives of the potential energy of the system. A supersonic polaron moving in an electric field generates coherent vibrations of the optical branch of the spectrum of the polyacetylene. The phase velocity of these oscillations is equal to the polaron velocity. An additional condition for the excitation of a mode with a certain phase velocity is its correspondence to the Fourier spectrum of an impulse with polaron shape.

Plasmon modes in Dirac–Schrödinger hybrid electron systems including layer-thickness and exchange-correlation effects

We calculate the plasmon dispersion relation and damping rate of collective excitations in a double-layer system consisting of monolayer graphene and GaAs quantum well at zero temperature including layer-thickness and exchange-correlation effects. We use the generalized random-phase-approximation dielectric function and take into account the nonhomogeneity of the dielectric background of the system. We show that the effects of layer thickness, electron densities, and exchange-correlations are more pronounced for acoustic modes, while the optical branch depends remarkably on dielectric constants of the contacting media.

A Diatomic Chain with A Mass Impurity

A classic diatomic chain with one mass impurity is studied using the recurrence relations method. The momentum autocorrelation function of the impurity results from contributions of two pairs of resonant poles and three branch cuts. The pole contribution is given by cosine function(s) and the cut contribution is the acoustic and optical branches. The acoustic and optical branches are given by expansions of even-order Bessel function. The expansion coefficients are integrals of elliptic functions in the real axis in a complex plane for the acoustic branch and along a contour parallel to the imaginary axis for the optical branch, respectively. An integral 22 22 1 2 0 d / ( r sin )( r sin ) 1 1 ϕ θ − θ− θ ∫ (r2 2 >r1 2 >1) is evaluated.

Interactions of anions and cations in carbon nanotubes

We consider the insertion of alkali-halide ion pairs into a narrow (5,5) carbon nanotube. In all cases considered, the insertion of a dimer is only slightly exothermic. While the image charge induced on the surface of the tube favors insertion, it simultaneously weakens the Coulomb attraction between the two ions. In addition, the anion experiences a sizable Pauli repulsion. For a one dimensional chain of NaCl embedded in the tube the most favorable position for the anion is at the center, and for the cation near the wall. The phonon spectrum of such chains shows both an acoustic and an optical branch.