مجله پژوهش فیزیک ایران
سال بیستم شماره 1 (بهار 1399)

By using concepts of topology in mathematics, quantum mechanics, and their synergetic development during the past few decades, condensed matter physicists have discovered new phases of matter and introduced general frameworks to classify them. The research includes a vast gamut from chemistry of atomic orbitals to material science, promising new applications in the growing technologies. This review article aims to provide a better understanding of these unprecedented electron systems and their underlying topological principles. The article consist of two parts. First, there is a historical review of using topological concepts in condensed matter systems. Then, in the second part, we elaborate on some basics of topology in quantum mechanics and the concept of topological invariants.

A new method is developed to calculate the mean free path of electrons in spherical nano particles and related Г(R) in the Drude model; accordingly, we have corrected ϵ(R,ω). The new dielectric function is inserted in the series expansion of extinction and absorption cross sections in the Mie Theory. After plotting the real and imaginary part of  ϵ(R,ω) in 3D graphs, and Cext and Cabs in other 3D graphs, we show that the SPR’s positions are relatively constant for two samples; meanwhile, it is displayed that the absorbance for these two samples have visible changes. At last, in two separate 3D graphs, we have plotted the variation of wave length and absorbance against radius and standard deviation to estimate the radius range for experimentally produced gold nanoparticles. We have estimated the radius to be 17 ~ 20 nm for the immediately prepared sample and 12~14 nm for the same sample illuminated with pulsed laser. These results are consistent with the experimental data of TEM images.

In this paper, the model of the new agegraphic is considered as an alternative to the teleparallel modified gravity model. First, we obtain the Friedman equations by taking dark matter and dark energy based on the existence of the bulk viscosity in the flat Friedmann–Robertson–Walker metric. So, we obtain the cosmological parameters and the function f(T) by using the power law of the scale factor and the correspondence between the agegraphic model and teleparallel gravity. By plotting the variety of the dark energy equation of state versus the redshift parameter, we describe the accelerated expansion of the universe. Finally, we investigate the stability condition by using the function of sound speed, finding the energy-weak constraints for free parameters.

In this study, the structural and electronic properties of III-V semiconductor compounds are studied using Density Functional Theory computations within the Full Potential Linearized Augmented Plane Wave (FP-LAPW) method. After considering several exchange-correlation functionals, it is determined that the SOGGA and GGA-WC functionals are suitable alternatives for calculating the structural properties of the desired compounds. For the calculation of electronic properties, particularly the energy band gap, the GGA-EV functional and the TB-mBJ exchange potential with spin-orbit correction are approved. The results show that the exchange potential TB-mBJ + SOC accurately calculates the band gap of these compounds. In the case of materials such as TlAs, which have negative band gaps, it is found that the exchange potential TB-mBJ is not able to predict this gap; in fact, the gap is set to zero. For the calculation of the effective mass, several methods are used; after comparing with experimental data, it is found that the GGA-PBE and GGA-EV functionals calculate this quantity for small band gap and large band gap materials, respectively; this is done with proper accuracy and of course,   the best effective mass results are obtained with the method of hybrid functional HSEbgfit. It is also found that the spin-orbit correction makes the calculated effective mass results closer to the experimental values.

In this paper, a two-dimensional structure of thin-layer silicon solar cells with a combination of silver nanoparticle arrays and a blazed grating is introduced. Applying Ag nanoparticles in the top surface of thin-layer solar cells imrpoves the coefficient of light transmission into the active layer and photon absorption because of the resonance surface plasmon effect. By using the FDTD method, the transmittance and absorption of light at both surfaces is investigated. The effect of such structural parameters as radius, distance of nanoparticles, angle of blazed grating and the grating constant has been reported. Finally, both surfaces are combined and the weighted mean values of the light absorbed by active layer are calculated. The results show that the light trapping efficiency can be improved under specified combinations of the structural parameters.

Crystal defects play an important role in the material strength and its mechanical properties. The Zr-1%Nb alloy, because of its low cross-section for thermal-neutron capture, corrosion resistance in water and suitable mechanical properties, is widely used in nuclear reactors. This alloy has an HCP structure at low temperatures and for low concentrations of Nb impurity. In this work, using the first-principles density-functional theory calculations, as well as molecular dynamics calculations with interatomic potentials, we have investigated the properties of vacancy and self-interstitial point defects in pure zirconium. The formation energy and formation volume are calculated; the results show a good agreement with the  experimental values. These quantities are calculated for the Zr-1%Nb alloy as well; the  results do not show any significant differences with those of the pure Zr. In addition, the interaction between two vacancies is investigated; by the calculation of the binding energies for di-vacancy clusters in different configurations, it is shown that only those clusters are stable for which the vacancies are in the first neighbor positions. Finally, the displacement energy of a vacancy in the basal plane is calculated, showing a  good agreement with experiment.

Understanding thermal behavior and processes underlying the heat transport of UO2 nuclear fuel in nuclear reactor plays a key role in predicting the efficiency of the fuel. If the heat transport, which is an important parameter in temperature distribution of the fuel, does not occur properly, the continuous increase of temperature would lead to the melting of the fuel and therefore, environmental hazards. In this work, by using a non-spin-polarized calculation for the simple description of the paramagnetic state and ignoring the Hubbard correction, the thermal properties and phonon properties of bulk UO2 are calculated. These calculations are based on the density-functional theory (DFT) and density-functional perturbation theory (DFPT). To determine the lattice-vibration properties by the finite-displacement method, we have calculated the second-order and third-order force constants based on which such quantities as  constant-volume specific heat, Gruneisen parameter, three-phonon scattering rate, scattering rate due to different levels of isotopic enrichment, and cumulative thermal conductivity are calculated. The results of the calculated specific heat based on the harmonic approximation show a good agreement with the experimental values, specifically for temperatures lower than 400 Kelvin. The results obtained for three-phonon scattering rate reveal that the scattering rate increases with temperature, thereby leading to the decrease of thermal conductivity. The results related to different levels of isotopic enrichments do not show any sensible changes in the scattering rates.

In an effort to describe quantum and classical mechanics with the same language, a wave equation for a continuous transition from quantum to classical mechanics has been proposed. Furthermore, the equivalence of this nonlinear equation with a linear one, known as the scaled equation, which is just the Schrodinger equation with the scaled Planck constant instead of the usual one, has been proved. Using this equation, we'll study three interesting phenomena; these include tunneling through a rectangular barrier, total reflection from a hard wall, and the gravitational weak equivalence principle in quantum, transition and classical regions. Time-independent scaled equation for the stationary states is derived and solved for a flux of particles incident on the barrier. The relations show that tunneling probability is exactly zero in the classical regime. For the other problems, we use a Gaussian wavepacket to calculate  the expectation value of the position operator in reflection from the hard wall and to estimate the detection probability and arrival time in the problem of the gravitational weak equivalence.

The Jump-Diffusion equation is a generalization of the Langevin equation; it has been usually applied to reconstruct discontinuous stochastic processes. In this article, by using this equation, we investigate the electrocardiogram of the electric activity of the heart beat, for three groups of subjects with normal, atrial fibrillation and ventricular arrhythmia. At first, we demonstrate that the time series of electrocardiogram is a discontinuous process that can be modeled by the jump-diffusion equation. Then, by calculating the Kramers-Moyal coefficients related to this equation, we show that there is  a significant difference between the heart dynamics of the  normal subjects and the ones with heart failure exists. Finally, we introduce a measure that may be used for the diagnosis of heart failures.

Since top quark has large Yukawa coupling, investigation the Higgs-top sector is highly interesting as it looks for any deviations from the standard model predictions. In this paper, we study the Higgs boson production in the gluon-gluon fusion channel and in the presence of top quark Flavor Changing Neutral Current (FCNC) interactions at the LHC.  We utilize the standard model effective field theory framework to probe the new physics effects. The amplitude for Higgs boson production via FCNC interactions and the theoretical expression for its signal strength are calculated. Then, by comparing this theoretical expression with the experimental value reported by LHC collaborations, we find the allowed region for those FCNC couplings that play a role in Higgs boson production

Today, many different kinds of ion sources have been developed. One of the most important ones is the penning or PIG ion source. Due to its simplicity of structure and longer lifetime, these ion sources are more widely considered than other ones. In this paper, design and simulation of the parameters affecting the ion beam characteristics from a penning ion source are performed using the CST software. Among the various structures of the permanent magnet around the source, the circular arrangement is found to be better for the confinement of particles inside the source. The electric potential of the anode, the cathode and the extraction electrode are found to be 15+, -500 and -3000 Volt, respectively. The decelerator and accelerator electrodes are used as ion source extraction systems. In addition, in the beam transport systems, high efficiency focusing in Einzel lens is more than that of immersion lens.

In this work, we investigate the interacting electrons on a square lattice in the presence of Rashba spin-orbit coupling. We first obtain the effective spin model from the Rashba-Hubbard model in the strongly correlated limit using the second perturbation theory. The effective spin model includes isotropic Heisenberg terms, nearest- and next-nearest-neighbor interactions, as well as the anisotropic ones as Kane-Mele and Dzyaloshinski-Moriya interactions. We proceed to study the influence of Rashba spin-orbit coupling on the stability of the magnetic phases of isotropic Heisenberg using Luttinger-Tisza and variational minimization classical methods. Our classical calculations show that the anisotropic terms leads to the instability of the Neel, classical degenerate and collinear phases of the isotropic Heisenberg model on the square lattice into an incommensurate planar phase. The spiral magnetic order in the two-dimensional frustrated magnets can be disordered by considering the quantum fluctuations. In a heterostructure including a noncollinear magnet and a singlet superconductor, singlet Cooper pairs can be converted to triplet pairings due to the broken spin rotational symmetry. Therefore, we can engineer a topological superconductor using noncollinear magnet in a heterostructure system.

The presence of sufficiently light particles in the fundamental Lagrangian could trigger instability in rotating black holes, the so-called superradiance instability. In particular, axion and axion-like-particles (ALPs) are good candidates to prompt such an instability. As a result, a high-density axion cloud forms around the black hole. The system of black holes and the axion cloud surrounding it is called a gravitational atom. Examining the evolution of this gravitational atom could lead to the discovery of an axion or introduce new constraints on their parametric space. The axion cloud becomes unstable under certain conditions when axion-photon interactions and axion self-interactions are considered. The nature of these instabilities is the parametric resonance. In this paper, we obtain an upper bound for the rate of this instability. The results show that for the simplest axion models, this instability occurs at a very low rate because, before the resonance becomes effective, self-interactions cause the axion cloud to collapse. But for some exotic models, the resonance rate could be large enough to introduce observable effects. In addition, we will show that the parametric resonance caused by self-interactions never happens at a significant level.

Considering the SU(2) Yang-Mills theory in a 4-dimensional Einstein Gravity, we find a black brane solution in the AdS space. For this setup, by using the AdS/CFT holography, we find non-Abelian color conductivity of the gauge theory on the  boundary of the AdS space. Color conductivity is defined by the generalized Ohm’s law and computed by applying holography to the linear response theory.

Recently, the extended Kitaev-Heisenberg model has been proposed to describe spin-orbital Mott insulators, such as iridate oxides and ruthenium chloride with honeycomb lattice. Using mean-field theory, we obtain the linear gap equations to find all possible superconducting phases in terms of different exchanges and doping levels. Our calculation based  on the hole-doped model, in the presence of the off-diagonal exchange  , shows the spin-triplet states can be stable in a larger area related to the doped Kitaev-Heisenberg model with K<0 and JH>0. However, the finite ferromagnetic off-diagonal exchange solely cannot generate the triplet pairing instabilities in competition with the antiferromagnetic-Heisenberg and -Kitaev exchanges.

The absorption of the focused light inside aqueous electrolyte locally heats it; thus, it creates a temperature field and temperature gradient around the light-absorbing region. Due to a phenomenon known as Soret effect, positive and negative ions move in the presence of the temperature field toward the warmer or cooler region. However, this tendency and its corresponding motion are not the same for two types of ions; therefore, it ends up with a locally charged region. This means creating a pure electric charge suspended in the light absorption area. Applying an external electric field to the fluid then exerts a force to the net charge and its surrounding fluid, resulting in the fluid’s motion. We investigate this problem for an electrolyte fluid enclosed between two parallel transparent dielectric blades closely located to each other. Based on analytic and finite element methods, we calculate the temperature field created by the Gaussian beam inside and outside the electrolyte. We then obtain its induced electric potential and charge density. Finally, we calculate the fluid velocity field and the total induced current. The analytical and numerical results well verify each other.

This paper has investigated the effect of operating parameters such as ambient temperature and applied voltage on the dead time of a thin-walled Geiger-Muller (GM) counter using non-paralyzing model and two-source method. Experimental studies have been conducted using 137Cs and 90Sr sources at voltages ranging from 600 to 800 V and in the temperature range of -27-70 0C. The results of the investigations for applied voltage indicate that the dead time behavior in terms of voltage can be classified into three distinct regions. In region I (low voltages), the dead time decreases with increasing voltage, in region II (voltage close to the operating voltage), the dead time is almost constant. The dead time in region III (voltages above 740V) increases slowly and linearly with increasing voltage. The variation in the dead time in the region I is greater than region III. Region II with the minimum dead time and minimum variation of the operating voltage is the best operating region. Studies show that the variation of dead time and the range of the dead time plateau (District II) for 137Cs and 90Sr sources is different. The dead time was using 137Cs source was obtained between 58 and 78 ms and using 90Sr source between 79 and 130 ms. In general, the variations of dead time versus voltage for each of Regions I, II and III for 90Sr source are lower than 137Cs source. Experimental results also show that the dead time increases with increasing temperature.