مجله پژوهش فیزیک ایران
سال بیستم شماره 2 (تابستان 1399)

One of the long-term objectives in the development of high gain free electron lasers (FEL) is to reduce the necessary electron beam voltage for a strong FEL interaction at a given frequency. FEL oscillators (FELOs) play the main role to this end. In this paper, the simulation of one dimensional FELO with planar wiggler is done at the applicable Tera-hertz regime. The effect of pre-bunched electron beam on the gain improvement or on the laser length is investigated. To study the evolution of system, a set of self-consistent nonlinear differential equations are solved numerically by the Runge-Kutta method and the averaging of electron beam is done by the Simpson method.

In this paper, by using the quantum input-output relations for an anisotropic slab, we investigate the quadrature squeezing and Mandel parameter of the transmitted squeezed coherent state through a metamaterial slab with negative refractive index and also a hyperbolic metamaterial slab. It will be shown that the nonclassical properties of the aforementioned state decrease sharply in passing through the negative refractive index slab. While, the hyperbolic metamaterial slab with a small absorption coefficient can somewhat retain the nonclassical properties of the incident squeezed light.

Collagen fibers can both inhibit and promote cellular migration, based on their orientation. Physical models such as Random walk can regenerate cellular migration. As such, we proposed a model based on (biased) random walk to study migration on collagen fibers. We took mean squared displacement as the determinant factor for metastatic risk and found that direction of fibers, spatial correlations and migration mode together regulate the risk and we proposed an approach to quantify an existing prognostic approach.

In this paper, we have considered the extended version of the Kitaev model in one dimension, i.e., a long-range p-wave superconducting wire. In the long-range Kitaev chain, the superconducting hopping and pairing terms in the Hamiltonian decay, independently, in a power-law fashion  , where l is the distance between the two sites and x is some positive constant. We have studied the appearance of Majorana zero-energy edge modes and also, massive Dirac edge modes by exact diagonalization, as well as analytical computations. Exact diagonalization indicates the existence of both kinds of massless and massive edge modes in the energy spectrum. Furthermore, we obtain the phase diagram and the  topological phase transitions by calculating the winding number, which is the topological invariant.

The graphene-like phase of boron atoms was recently synthesized on Al (111) substrate. This two dimensional material which is unstable without the substrate is improved to a stable structure after being combined with oxygen. In this research, mechanical and electronic transport properties of graphene-like borophene oxide (g-B2O) has been investigated within density functional theory framework and non-equilibrium Green’s function, for this purpose total and partial density of states, energy band structure, charge density, elastic constants, Young's modulus, Poisson's ratio, quantum conductance, and current-voltage characteristics have been calculated by applying small uniaxial and biaxial strains. The results show that g-B2O is a metal and has a Dirac point like graphene with a linear dispersion energy at a position above the Fermi level. In addition, the current-voltage curves display the Ohmic behavior of this material and exhibit that positive strain reduces the current density in armchair direction ( ) and increases the current density in zigzag direction of ( ) compared to without strain . The positive biaxial and uniaxial strains in armchair direction with almost similar behavior have the most variations in and . Besides, the negative strain in zigzag direction causes the most increasing. The negative uniaxial strain in armchair direction and negative biaxial strain with almost analogous behavior caused the most (the least) changes in ( ). The anisotropic current density along zigzag and armchair directions as well as the ability to control this anisotropy by positive and negative strains make this material suitable to usage in nanoelectronic devices.

In this paper, we study the geometric phase of a two-level atom near a dielectric slab. For this purpose, by applying the Von-Neumann equation, we obtain eigenvalues and eigenvectors of the reduced density operator of the atomic system. Then, we obtain the Lamb shift and transition rates of the atomic system in term of the electromagnetic Green tensor. Finally, by calculating the electromagnetic Green tensor of the system and making use of the kinematic approach, we study the geometric phase of the atomic system near the dielectric slab. We show that the geometric phase can be used as a sensitive probe to the surface-phonon polariton waves.

The aim of this study is to investigate the  properties of the  La0.6Sr0.4Fe0.8Mn0.2O3-δ (LSFM)compound as cathode of intermediate temperature solid oxide fuel cells. The LSFM compound was synthesized by the  sol-gel process. Thestructural, electrical and electrochemical properties of LSFMwere tested via X-ray diffraction (XRD), scanning electron microscopy (SEM), high temperature four-probe resistivity measurement (HTRM) and electrochemical impedance spectroscopy (EIS). The X-ray pattern showed  that sample had a Rhombohedra structure and space group symmetry of LSFM is R-3c; also, the LSFM had  good chemical compatibility with YSZ electrolyte. The conductivity of the sample was increased with raising the temperature. The maximum electrical conductivities for the LSFM compound were  equal to  3.64 S.cm-1 in air at 739 ℃. The cathode area specific resistance of LSFM was 1.69, 1.01, 0.63, 0.52, and 0.45 Ω cm2 at 600, 650, 700, 750, and 800 ℃, respectively

Electronic and optical properties of pentagonal GeS2 monolayer are investigated by first principles calculations in the framework of the density functional theory. The stability of the nanostructure is confirmed by cohesive energy calculation, as well as phonon dispersion calculation. The electronic properties simulation indicates that GeS2 monolayer is an indirect band gap semiconductor with a band gap of about 0.9 eV. Furthermore, the optical properties investigation reveals that the material exhibits a very low absorption and reflectivity in visible region of the electromagnetic spectrum. However, it has a considerable absorption and reflectivity in the ultra violet region. The results of this study, therefore, suggest that the considered structure has a good potential application in the  new generation of opto-electronic devices, especially as a UV protection layer.

Consider an open quantum, system interacting with its environment. Whether the reduced dynamics of the system   can be given by a linear map or not is an important question in the theory of open quantum systems. Dominy, Shabani and Lidar have proposed a general framework for linear Hermitian reduced dynamics. In addition, it has been shown that their framework is, in fact, the most general one: The reduced dynamics of the system is linear if and only if it can be formulated within the Dominy-Shabani-Lidar framework. This result has been given in a rather abstract way. Here, we want to give another proof for it, in a more illustrative manner.

ZnO/g-C3N4 hybrid nanofibers containing different concentrations of g-C3N4 nanosheets  were  prepared using electrospinning technique; this was  followed by annealing at 460 ˚C for one hour in a box furnace. Based on scanning electron microscopy (SEM) image analysis,  the mean diameter of the nanofibers was measured to be  ~ 55 nm. Fourier transform infrared (FTIR) spectroscopy  confirmed the presence of ZnO and g-C3N4 in the prepared nanofibers. The photocatalytic activity of the nanofibers was examined under UV photoirradiation, showing  that the ZCN0.5 nanofibers containing 0.5 wt% of g-C3N4 exhibited the highest performance, as compared to other photocatalysts. The observed improvement in photodegradation over the optimized photocatalyst could  be due to retardation in the  charge carriers’ recombination rate in the ZCN0.5 photocatalyst sample,  as compared with the pure ZnO and pure g-C3N4.

It has been known that by encoding the  Boltzmann weights of a classical Ising model in the amplitudes of the wave function of the ground state of the toric code model, the classical phase transition in Ising model is mapped to a topological phase transition in a perturbed toric code model. Since such topological phase transitions cannot be characterized by any local order parameter, it will be an important challenge to find an order parameter which describes the above topological phase transition. In this paper, using a simple technic based on mapping between classical Ising model and the ground state of the toric code model, we find a non-local order parameter which well reveals the topological nature of the above phase transition. We show that such an order parameter is, in fact, a kind of string order parameter which has been recently introduced for some topological phase transitions.

In this paper, asymptotically AdS black hole solutions of massive gravity in the presence of nonlinear electromagnetic field arisen from the power theory of Maxwell invariant are investigated and the associated Euclidean on-shell action is presented. Using the Euclidean on-shell action, the gravitational partition function in the canonical ensemble is computed in arbitrary dimensions and then thermodynamic quantities of topological black holes are obtained. By extending the thermodynamic phase space, i.e., treating negative cosmological constant as thermodynamic pressure, the first law of thermodynamics as well as associated Smarr formula are examined. Next, the equation of state of topological black holes is obtained and it is proven that the critical point equation of these solutions can exhibit black hole phase transitions similar to those of van der Waals, van der Waals like and solid/liquid/gas (related to triple point) phase transitions in usual thermodynamic systems. Especially, the van der Waals phase transition is observed in 4 and higher dimensions, van der Waals type phase transition can be seen in 6 and higher dimensions, and phase transitions associated with triple point, i.e., small/intermediate/large black hole phase transition may happen in 6 and higher dimensions.

In this study, we investigate the density-wave instability and impurity screening in graphene-based double-layer structures. We assume that at least one of two layers is graphene, while the second layer could be made of graphene, conventional two-dimensional electron gas, or any other two-dimensional structure. We calculate the static dielectric function of this graphene-based double-layer structure whitin the random phase approximation, and look for the possibility of density-wave instability, changing different system parameters like the electronic density and layer spacing between two layers. In this investigation we have not seen any instability in double-layer structures consisting of coupled layers of graphene, the 2D electron gas and the gapped graphene. Finally, using static dielectric function and considering a charged impurity placed in the first layer, we show that how this impurity is screened in the first and second layers.

This study investigates  the nuclear surface tension coefficient, γ, of the proximity formalism by using the microscopic double-folding (DF) model with the realistic density-dependent (DD) nucleon-nucleon interaction of the effective M3Y forces type (including DDM3Y1, CDM3Y4 and BDM3Y1) for the ground-state to ground-state α transition of 230 parent nuclei with Z = 61-99. In fact, the present work can be considered as an expansion of the previous study which has been performed by Gharaei and Mohammadi using CDM3Y6 version in 2019. Within the propsed approach, we have tried to present a new approach for  the calculation of the surface energy coefficient, γ, in alpha-decay by integrating the proximity and DF potential models. In addition, we present a new dependency of the surface energy coefficient, γ, on the asymmetry parameter, , of the considered α-nuclei systems by fitting all of the calculated values. The obtained results suggest a new formalism for the coefficient γ that is dependent directly on the selection of the interaction type. We also test the validity of the suggested formula. To this aim,  by using the the obtained formula of the coefficient γ in the original version of the proximity potentials, we calculate the theoretical values of the alpha-decay half-lives for different nuclei in the framework of the WKB approximation. The calculated results are compared with the corresponding experimental data and those obtained from the original proximity potential 1977. It is shown that the modified forms of the proximity potential model, labeled as Prox. New (DDM3Y1), Prox. New (CDM3Y4) and Prox. New (BDM3Y1), provide better descriptions of the experimental α-decay half-lives than the proximity potential 1977 (Prox. 77). Further, the best results are obtained using the Prox. New (CDM3Y4) potential model for our selected mass range. Using the modified forms of the proximity potential, we examined the closed-shell effects in nuclei and the validity of the Geiger-Nuttall law. Additionally, the results of the Prox. New (CDM3Y4) potential model are compared with the various empirical formulas for alpha decay half-lives. Ultimately, the prediction of alpha decay half lives is made for superheavy nuclei with Z=117-120.

In this work, the stability, structure and electronic properties of the nanoclusters of germanium (Gen) and europium atom doped germanium clusters (EuGen-1) with n=2 to 12, 15 and 20 were investigated. First, the stability of nanoclusters such as Gen and EuGen-1 was addressed using FHI-aims as a software package based on the density functional theory. Then the lowest-energy structures were selected for calculating  the first vertical ionization with the symmetry adapted cluster-configuration interaction General-R (SAC-CI-General-R) method. The results of this research show that there is a good agreement between calculation and experiment ionization potential for Gen nanoclusters. Generally, the analyses of binding energies show that increasing the size of nanoclusters leads to more stability for nanoclusters. The most stable nanoclusters for EuGen can be created with exchanging the Eu atom in the most stable Gen+1 nanoclusters, but there is an exception for n=11 case. Here,  the second difference in energy (∆2E) and gap energy are computed for the stable nanoclusters. The results of ionization energy and second difference in energy confirm that Ge7 and Ge10 also EuGe8 and EuGe10 have the most stability.

In this paper, we have investigated the specific heat of the monolayer graphene under the polaron effect. For this purpose, we have first considered an electron coupled to the longitudinal acoustic (LA) phonon on the surface of the graphene with Coulomb impurity. Then, we have obtained the ground state energy of the polaron by employing the variational method and unitary transformation. We have used non-extensive thermodynamics to calculate specific heat different substrates like SiC, HfO2, h-BN, and SiO2. The specific heat variation with Coulomb bound parameter, magnetic field, temperature, and charge is then studied for these different substances.

In this paper, we present a new method for measuring the spatial coherence of optical beams by utilizing the Fresnel diffraction patterns formed by a 1D phase step in reflection. The spatial coherence function is obtained from the ratio of the amplitude of Fourier transform of the intensity distribution of diffraction of the light beam of an arbitrary source from the step to the amplitude of Fourier transform of intensity distribution of the diffraction of a coherent light from the step. The advantages of this method are possibility of simultaneous study of correlation between all pairs points along a line, simple and inexpensive setup, and recording no more than two diffraction patterns. Using the introduced method, the spatial coherence of a Schell-model beam is investigated theoretically and experimentally.

In the present study, we introduce a concept to generate the spin-polarized current in armchair transition metal dichalcogenide nanoribbons (TMDNs) by  using light irradiation. For simulating the opto-spin current, we use electron-photon interaction in non-equilibrium Green's function methods. Because of the intrinsic spin-orbit coupling, light irradiation produces spin-photocurrent in TMDNs without applying any external magnetic element. Moreover, transverse electric field modifies the magnitude and position of optical absorption peaks, as well as the magnitude of the spin-photocurrent. Finally, the fully spin-polarized photocurrent, the high quantum efficiency with a maximum of approximately 50%, the wide-wavelength-range operation from ultraviolet to infrared and optical spin-filtering effects are tunable with transverse electric field indicate the high performance of this spin-photodetector based on armchair TMDNs, thereby paving the way  toward the improved design and performance of this photodetector in spin-optoelectronics.

In present work, zinc oxide (ZnO) nanorods were synthesized by a simple hydrothermal method and its methane gas sensing features was studied under different gas concentrations and various relative humidity at room temperature. ZnO nanorods characterization investigated by X-ray diffraction (XRD) and Field effect scanning electron microscopy (FE-SEM). The results were showed the wurtzite phase of the crystallized hexagonal structure with porous architecture. A high response of 77.1% was obtained under 0.15 vol% methane gas concentration at 30% relative humidity, while a low response of 32.2% was achieved toward 0.037 vol% methane gas level at 90% relative humidity. Moreover, the low response/recovery time of 95.4/45.9 s was obtained under 0.15 vol% gas concentration. The solid state sensor of the ZnO nanorods displayed high response and good selectivity to methane gas than that other air components at environmental conditions. Finally, the methane gas sensing mechanism of the ZnO nanorods sensor was discussed as well.

Detection of weak signals is one of the important issues in spectroscopy. The most important parameters of this field is the signal to noise ratio, which has a significant effect on the accuracy of the final data. In this regard, one of the most successful methods to increase the signal-to-noise ratio is to apply the intensity modulation method. In this method, first, the intensity of the initial signal is modulated, and after entering in the spectrometer, it is demodulated and amplified by a lock-in amplifier and eventually, the desired signal is recovered from various noises. The simulation allows the user to examine and optimize the related parameters for different initial conditions. The present article is dedicated to designing a lock-in amplifier by applying a LabVIEW graphical programming language.