Sharif Digital Repository

Screening length at doped nanotube junctions, which is of primary importance in device applications, has been a controversial issue. Employing a fully self-consistent tight-binding approach, we show that, for both sharp and smooth external potential drops, the screening length is of the order of a few carbon-ring separations. This rather strong screening of the Coulomb interaction is accompanied by the oscillation of transferred charge at the junction, and is observed for both semiconducting and metallic tubes despite minor differences  

First principles density functional theory (DFT)-based molecular dynamics (MD) is used to study some physical and electronic properties of amorphous silicon (a-Si) samples, as-quenched and annealed containing dangling and floating bonds (pertinent to the threefold- and fivefold-coordinated defects, respectively) as well as distorted tetrahedral bonds. Surprisingly, except for the work of Pantelides (1986) who gave a rough estimate for the effective electron correlation energy, Ueff of a floating bond on the fivefold-coordinated Si, to date, there are no theoretical studies in the literature for the calculation of Ueff pertinent to this type of defect. In this work, Ueff for each type of... 

Theoretical studies on the spin-dependent transmission and current-voltage characteristic in a mesoscopic system, which consists of two semi-infinite ferromagnetic (FM) leads (as source and drain) separated by a typical periodic quantum dot (QD) are presented. The calculations are based on the tight-binding model and transfer matrix method, and investigate the magnetoresistance (MR) and the spin polarization within the Landauer-Büttiker formalism. The spin-dependent transport behavior can be controlled via a gate voltage and an applied bias in the ballistic regime. The numerical results are shown for a periodic polymer chain with nonmagnetic (NM) and FM leads, and also, with two FM leads.... 

In this work, we study the conductance of a general periodic quantum dot (QD) attached to ideal semi-infinite uniform metallic leads (nanocrystals), fully analytically. We propose a new general formula which relates conductance to transfer matrix (TM) for an isolated cell in the periodic dot. The equation describes exactly the dependence of the transmission coefficient (TC) on Fermi energy, dot-size, dot-lead coupling, and gate voltage for an arbitrary periodic dot. Then, we derive a nonlinear equation which gives the resonance, bound, and surface state energies. Finally, the TC has been calculated for gapless, single, and double gap models exactly. Moreover, we have also calculated the... 

The localization behavior of the one-dimensional Anderson model with correlated and uncorrelated purely off-diagonal disorder is studied. Using the transfer matrix method, we derive an analytical expression for the localization length at the band center in terms of the pair correlation function. It is proved that for long-range correlated hopping disorder, a localization- delocalization transition occurs at the critical Hurst exponent Hc=1 2 when the variance of the logarithm of hopping "σln(t)" is kept fixed with system size N. Numerically, this transition can be expanded to the vicinity of the band center. Based on numerical calculations, finite-size scaling relations are postulated for... 

A generalized Hubbard model based on a molecular approach is used to calculate many electron wavefunctions of diamond vacancies. We have calculated the oscillator strength of the dipole transition rates from the ground states of the neutral and negatively charged vacancies. The ratio of the oscillator strengths is in very good quantitative agreement with the reported optical spectroscopic data. Electronic configurations in the ground and dipole allowed excited states are presented. With the proposed picture, the much larger oscillator strength of the negatively charged vacancy with respect to other experimentally investigated color centers N-V, H3, X3 and H4 is explained  

We study phonon delocalization in disordered media in the presence of nonlinearity. By considering the Fermi-Pasta-Ulam β-model, we show that regardless of whether the initial state of the linear system is localized or not, the final state will be an extended mode after turning on the nonlinear term. We report on the results of an extensive dynamical simulation of a disordered nonlinear system, which show that, independent of the initial mode frequency, in the final state the energy spectrum is excited according to the Kolmogorov spectrum E(ω)∼ω-5/3. Finally, we show that disorder will not cause delocalization of intrinsic localized modes. © 2020 American Physical Society  

Two-dimensional (2D) electron systems in the presence of disorder are of interest in connection with the observed metal-insulator transition in such systems. We use density functional theory in its local-spin density approximation (LSDA) to calculate the ground-state energy of a 2D electron system in the presence of remote charged impurities which up on averaging provides disorder. The inverse compressibility calculated from the ground-state energy exhibits a minimum at a critical density controlled by the disorder strength. Our findings are in agreement with experimental results. © World Scientific Publishing Company  

Many-electron calculations based on a generalized Hubbard Hamiltonian for electronic states of the diamond vacancies are reported. The model does not use the configuration interaction (CI) method and proper tetrahedral symmetry and spin properties of the defect are included in the Hamiltonian. Atomic orbital bases are introduced for the Hamiltonian calculation. Excited states of both neutral and negatively charged vacancies in diamond are calculated. The calculated values for the experimentally observed first dipole transition energies of the vacancies in diamond, GR1 and ND1 bands, are in good agreement with experiment. To obtain these results, we used a semi-empirical Hamiltonian... 

A new model to calculate electronic states of the diamond vacancies has been developed using many body techniques. This model is based on physical assumptions of previous molecular models but does not use configuration interaction. Present model allows an accurate and unified treatment of electronic levels and related eigen functions for diamond vacancies, in addition to transition energies of the first dipole-allowed transitions in the neutral (V0) and negatively charged (V-) vacancies, GR1 and ND1 band. For the first time, we calculated their optical transition intensities. For obtaining these results, we solved a generalized form of the Hubbard Hamiltonian, which consists of all... 

Using a generalized Hubbard Hamiltonian, many-electron calculations of energy levels and corresponding wave functions of negatively charged and neutral nitrogen-vacancy centers in diamond were reported. The energies, orbital, and spin symmetries of the ground and excited states are in good quantitative agreement with available optical and electron paramagnetic resonance measurements. The many-electron wave functions were employed to predict the spin density on the N and C atoms in the ground and excited states. The present model explains the recent, experimentally observed definite nonzero spin density on N atom for the 4A2 excited state of the neutral charge state of NV (NV0) based on the... 

Growth mechanism of a single wall carbon nanotube on the surface of a nickel nanoparticle in the CVD method has been investigated by classical molecular dynamics method. Using first principles methods, we have first constructed a classical potential to describe the interaction between a carbon atom and the nickel surface. The important ingredient in this potential is its coordination number dependence, which also provides the key to the growth mechanism of the nanotube. From the simulations, it is proposed that the growth of an armchair nanotube takes place via attachment of dimers to its end which is in contact with the nickel surface. The effect of nickel nanoparticle's radius on the... 

Thermoelectric properties of a nanocontact made of two capped single wall carbon nanotubes (SWCNT) are calculated within the tight-binding approximation and by using Green's function method. It is found that doped semiconducting nanotubes can have high Seebeck coefficients. This in turn leads to very high figures of merit (ZT) for p -doped tubes which turn out to have also a large electrical to thermal conductivity ratio. Transport in the nanocontact device is dominated by quantum interference effects, and thus it can be tuned by doping (charge transfer and/or impurity potential) or application of a (nano-)gate voltage, or a magnetic field. Another reason for high ZT in this device is the... 

Effect of doping of carbon nanotubes by magnetic transition metal atoms has been considered in this paper. In the case of semiconducting tubes, it was found that the system has zero magnetization, whereas in metallic tubes the valence electrons of the tube screen the magnetization of the dopants: the coupling to the tube is usually antiferromagnetic (except for Cr)  

Symmetric lattice relaxation around a vacancy in diamond and its effect on many electron states of the defect have been investigated. A molecular approach is used to evaluate accurately electron-electron (e-e) interaction via a semiempirical formalism which is based on a generalized Hubbard Hamiltonian. Coupling of the defect molecule to surrounding bulk is also considered using an improved Stillinger-Weber potential for diamond. Strong dependence of the electronic energy levels to the relaxation size of the nearest neighbor (NN) atoms indicates that in order to obtain quantitative results the effect of lattice relaxation should be considered. Except for the high spin state of the defect 5A... 

Transport properties of doped nanotube-based double junctions forming a nanotransistor and investigated within the tight binding formalism. The effects of doping gate length and gate soften have been considered. It is found that in addition to the importance of rotational symmetry in determining transport properties, large gains can be achieved for semiconducting doped tubes  

The nonlinear capacitance in doped nanotube junctions is calculated self consistently. A negative differential capacitance is observed when the applied bias becomes larger than the pseudogap of the metallic armchair nanotube. For this device, one can deduce a relaxation time of approximately 0.1 fs. Because of its negative differential resistance (NDR), a switching time of less than a femtosecond, i.e., at least three orders smaller than present-day switching times, can also be estimated. This effect is important in designing ultrafast nano-electronic components. ©2005 The Physical Society of Japan  

Al-Zn-Mg-Cu alloys (7xxx series Al alloys) are extensively used for their superior mechanical and corrosion performance. These properties are microstructure-sensitive and highly dependent on the formation, growth and coarsening of precipitates. To date, a wide variety of ageing procedures have been developed to tailor the evolved microstructures so as to yield a good combination of mechanical capacity and corrosion resistance of 7xxx series Al alloys. Among these methods, isothermal ageing, multi-stage ageing, non-isothermal ageing, retrogression and re-ageing (RRA), and stress ageing (i.e. creep ageing) are the most significant. In the present review, all of these approaches are... 

After an earthquake, the residual stiffness and strength of structural elements are typically estimated based on a qualitative visual inspection of cracks that is prone to error. In this paper a new approach is proposed to automatically estimate the updated stiffness and strength of damaged unreinforced masonry walls by characterization of crack patterns by a mathematical index. It is shown that structural and textural fractal dimensions of a crack pattern reflect the extent of cracking and the type of cracking or crushing, i.e., whether the cracks pass through joints or whether bricks have been damaged and crushed. Using results of six quasi-static cyclic tests on unreinforced brick masonry... 

In order to optimize the aging treatment of Mg-1.8Zn-0.7Si-0.4Ca alloy, different times and temperatures of solid solution and age hardening were applied to the alloy specimens. Microstructures and mechanical properties of the specimens were investigated using the optical microscopy, field emission scanning electron microscopy equipped with an energy dispersive x-ray spectrometer, x-ray diffraction, hardness, and shear punch tests. The lowest hardness and strength were achieved by solution treating of the alloy at 500 °C for 8 h, presenting the optimal condition for solution treatment of the alloy. The microstructural examinations revealed three different precipitates consisting of CaMgSi,...