Sharif Digital Repository

Emergence of high pseudo-magnetic field as a result of mechanical deformations is one of the intriguing characteristics of the graphene honeycomb structure. For a time-dependent nonuniform strain and in the context of Berry curvature approach, the topological charge pumping is studied theoretically. Calculations have been performed in time–momentum parametric space. Within the Berry curvature approach, it has been revealed that time-dependent deformations result in nonzero valley-dependent charge pumping in the gapped graphene when there is a population imbalance between the valleys. This indicates that the valley polarization can be measured by the amount of topological charge pumping in... 

Set of experiments has been developed to study existing runaway electrons in "Damavand" tokamak plasma upon characteristics of hard x-ray emissions produced by collision of the runaway electrons with the plasma particles and limiters. As a first step, spatial distribution of hard x-ray emissions on the equatorial plane of the torus was considered. Obtained spectra of hard x-ray emissions for different alignments of shielded detector indicate isotropic emissivity in the equatorial plane. This is in agreement with wide angle cone of bremsstrahlung radiations, deduced from the mean value of energy of the runaway electrons. The mean energy was calculated from the slope of the energy spectrum of... 

This paper presents an efficient mixed-integer linear formulation for long term maintenance schedule of overhead lines. The proposed formulation is based on risk management approach and utilizes the model of decoupled risk factors. The proposed methodology yields a significant computational saving comparing to the previously reported dynamic programming. Risk management approach enables the asset managers to consider the actual condition of electrical equipment and expected consequence of their failures. Furthermore, the decoupled risk strategy in conjunction with the mixed-integer linear programming formulation establishes a precise description of time-dependent deterioration failure rate... 

This paper describes a numerical model developed for the computation of creep damages in a thick-walled sphere subjected to an internal pressure and a thermal gradient. The model predicts the creep damage histories during the life of the sphere, owing to variations in stresses with time and through-thickness variations. The creep damage fraction is based on the Robinson's linear life fraction damage rule, which has been incorporated in a nonlinear time-dependent stress analysis. Following the stress histories, the effective stress histories are obtained and the creep damages are calculated and summed during the life of the sphere. The material long-term creep properties up to the rupture and... 

This paper proposes a new method for solving the time-dependent neutron transport equation based on nodal discretization using the MCNPX code. Most valid nodal codes are based on the diffusion theory with differences in approximating the leakage term until now. However, the Monte Carlo (MC) method is able to estimate transport parameters without approximations usual in diffusion method. Therefore, improving the nodal approach via the MC techniques can substantially reduce the errors caused by diffusion approximations. In the proposed method, the reactor core is divided into nodes of arbitrary dimensions, and all terms of the transport equation e.g. interaction rates and leakage ratio are... 

A highly reliable tool for transient simulation is vital in the safety analysis of a nuclear reactor. Despite this fact most tools still use diffusion theory and point-kinetics that involve many approximation such as discretization in space, energy, angle and time. However, Monte Carlo method inherently overcomes these restrictions and provides an appropriate foundation to accurately calculate the parameters of a reactor. In this paper fundamental parameters like multiplication factor (K eff ) and mean generation time (t G ) are calculated using Monte Carlo method and then employed in transient analysis for computing the neutron population, proportional to K eff , during a generation time... 

Transient analysis which is vital in safety analysis requires a reliable calculation method. Most valid tools use diffusion theory with many approximations by now. However, the Monte Carlo method inherently overcomes these approximations and accurately calculates the parameters of a reactor. In this paper, a new time-dependent transport approach is described to simulate the nuclear reactor dynamic correctly using the MCNPX code. In this approach the fundamental parameters of a nuclear reactor like multiplication factor (K eff ) and mean generation time (t G ) are calculated using MCNPX code. They are then employed in the formulas to compute neutron population, proportional to K eff , during... 

The discrete-time quantum walk dynamics can be generated by a time-dependent Hamiltonian, repeatedly switching between the coin and the shift generators. We change the model and consider the case where the Hamiltonian is time-independent, including both the coin and the shift terms in all times. The eigenvalues and the related Bloch vectors for the time-independent Hamiltonian are then compared with the corresponding quantities for the effective Hamiltonian generating the quantum walk dynamics. Restricted to the non-localized initial quantum walk states, we optimize the parameters in the time-independent Hamiltonian such that it generates a dynamics similar to the Hadamard quantum walk. We... 

Symmetrical and unsymmetrical biscyclometalated platinum(II) complexes [Pt(ptpy)2], 1, and [Pt(ptpy)(bppy)], 2, in which ptpy = deprotonated 2-(p-tolyl)pyridine and bppy = deprotonated 2-(3-bromophenyl)pyridine, have been prepared from the reaction between tris(pentafluorophenyl)borane, B(C6F5)3 and 1 equiv of monocyclometalated complexes [PtMe(ptpy)(ptpyH)], B, and [PtMe(ptpy)(bppyH)], C, respectively. The solid-state structures of 1 and 2 have been determined by X-ray crystallography. The reaction of 1 with 1 equiv of TlPF6 or [Au(PPh3)]OTF resulted in the production of heteronuclear complexes [Pt(ptpy)2Tl]PF6, 3 and {Pt(ptpy)2[Au(PPh3)]}OTF, 4, respectively. X-ray diffraction data showed... 

Most real world time series have transient behaviours and are non-stationary. They exhibit different type of non-stationarities, such as trends, cycles, random-walking, and generally exhibit strong intermittency. Therefore local stochastic characteristics of time series, such as the drift and diffusion coefficients, as well as the jump rate and jump amplitude, will provide very important information for understanding and quantifying “real time” variability of time series. For diffusive processes the systems have a longer memory and a higher correlation time scale and, therefore, one expects the stochastic features of dynamics to change slowly. In contrast, a rapid change of dynamics with... 

Enhanced high harmonics are generated by local and global optimization approaches to achieve a supercontinuum spectrum. Based on time-dependent density functional theory calculations, the optimum convolution of a two-color chirped pulse from an N2O molecule implements a significant enhancement of cutoff frequency and high harmonic yield. The optimization is done by controlling the effective chirp parameters and the carrier-envelope phase of the designed laser field. Indeed, all of the effective parameters are adjusted simultaneously for the global optimization; whereas, just two variables are tuned to obtain the desired cutoff frequency based on the local optimization. The results show that... 

Enhanced high harmonics are generated by local and global optimization approaches to achieve a supercontinuum spectrum. Based on time-dependent density functional theory calculations, the optimum convolution of a two-color chirped pulse from an N2O molecule implements a significant enhancement of cutoff frequency and high harmonic yield. The optimization is done by controlling the effective chirp parameters and the carrier-envelope phase of the designed laser field. Indeed, all of the effective parameters are adjusted simultaneously for the global optimization; whereas, just two variables are tuned to obtain the desired cutoff frequency based on the local optimization. The results show that... 

Capillary imbibition is an important recovery mechanism in naturally fractured reservoirs when water-filled fractures surround water-wet matrix blocks. A large amount of studies of imbibition process is simply total or partial immersion of nonwetting phase saturated rock in aqueous wetting phase. However, water advance in fractures during water flooding or water encroachment from an active aquifer introduces time dependent boundary conditions where invariant exposure of rock surface to water is not representative. In this work, a laboratory simulated matrix-fracture system was used to investigate different aspects of imbibition in the presence of fracture fluid flow (namely dynamic... 

Most real world time series have transient behaviours and are non-stationary. They exhibit different type of non-stationarities, such as trends, cycles, random-walking, and generally exhibit strong intermittency. Therefore local stochastic characteristics of time series, such as the drift and diffusion coefficients, as well as the jump rate and jump amplitude, will provide very important information for understanding and quantifying “real time” variability of time series. For diffusive processes the systems have a longer memory and a higher correlation time scale and, therefore, one expects the stochastic features of dynamics to change slowly. In contrast, a rapid change of dynamics with... 

We study the quantum plasmonic features of gold and silver nanoparticles using TD-DFT+TB, a new density functional theory approach to the calculation of excited states, which combines a full DFT ground state with tight-binding approximations in the linear response calculation. In this framework, the optical properties of closed-shell Ag, Au and bimetallic Ag-Au nanoparticles with tetrahedral symmetry (with 20, 56, 120, and 165 atoms) and icosahedral structure (with 13, 55, and 147 atoms) were obtained and compared to full linear response time-dependent density functional theory (TD-DFT) as a reference and also to time-dependent density functional based tight binding (TD-DFTB) as a low-cost... 

We consider the homogenization of a Robin boundary value problem in a locally periodic perforated domain which is also time-dependent. We aim at justifying the homogenization limit, that we derive through asymptotic expansion technique. More exactly, we obtain the so-called corrector homogenization estimate that specifies the convergence rate. The major challenge is that the media is not cylindrical and changes over time. We also show the existence and uniqueness of solutions of the microscopic problem. © 2020 American Institute of Mathematical Sciences. All rights reserved  

In this paper a probabilistic methodology based on core nodalization is proposed to estimate the core power in the presence of xenon oscillation. A time-dependent Monte Carlo neutron transport code named MCSP-NOD is developed for dynamic analysis in arbitrary 3D geometries to simulate xenon oscillations as well as sub-critical condition with feedbacks. The new code is based on the approach adopted in MCNP-NOD which was previously introduced as a tool for core transient analysis using the MCNPX platform. As before, the core is divided into nodes of arbitrary dimensions, and all terms of the transport equation e.g. interaction rates, leakage ratio are estimated using the MC techniques.... 

Despite several different measures of efficiency that are applicable to the photosynthetic systems, a precise degree of efficiency of these systems is not completely determined. Introducing an efficient model for the dynamics of light-harvesting complexes in biological environments is a major purpose in investigating such systems. Here, we investigate the effect of macroscopic quantum behavior of a system of two pigments on the transport phenomena in this system model which interacts with an oscillating environment. We use the second-order perturbation theory to calculate the time-dependent population of excitonic states of a two-dimensional Hamiltonian using a non-master equation approach.... 

The nonlocal nature of unpaired Majorana bound states (MBSs) in topological superconductors can be exploited to create topologically protected qubits and perform gate operations fault-tolerantly via braidings. However, the time-dependent noises induced by coupling to an environment which is inevitable in any realistic system could spoil the topological protection. In this work, we study the effects of various dynamical noises such as Lorentzian, thermal, and quantum point contact on the MBSs in the recently proposed one-dimensional topological superconductors. We begin by investigating the Kitaev p-wave superconductors and examine the effects of long-range hopping and pairing on the... 

The safety and optimal performance of nuclear reactors require online monitoring in the core. The present paper describes a method that avoids the solution of the time-dependent neutron diffusion equation, and it uses online readings of the fixed in-core neutron detectors to reconstruct the three-dimensional (3D) neutron flux distribution. The essential idea of the nodal synthesis method is the separation of time and space-dependence of the neutron flux distribution. The time-dependent section of the flux distribution is determined by in-core neutron detector readings, and the space-dependent section is obtained from pre-computed harmonics of the neutron diffusion equation. In online...