International Journal of Modern Physics B

We study the unconventional transmission properties of a wave packet through a PT-symmetric potential region as describing the actual electromagnetic wave propagation along a waveguide filled with gain and loss media. The nontrivial behavior of the transmission probability manifests in the giant amplification of the incident electromagnetic signal of given wavelengths for well-defined configurations, depending on the gain/loss contrast. Maximum transmission peaks are related to spectral singularities and a strict correlation exists between the “resonant” wavelengths and the gain/loss contrast. The transit times are as well calculated, showing their surprising vanishing in the opaque barrier limit, independently of the gain/loss contrast, which is reminiscent of some sort of Hartman effect. Also, nonlocal effects manifest in the presence of negative delay times for given configurations, while a correlation is apparent between maximum delay times and transmission probability peaks, though appreciably depending on the gain/loss contrast.

The majority vote model with two states on regular and small-world networks is considered under the influence of periodic driving. Monte Carlo simulations show that the time-dependent magnetization, playing the role of the output signal, exhibits maximum periodicity at nonzero values of the internal noise parameter $q$, which is manifested as the occurrence of the maximum of the spectral power amplification; the location of the maximum depends in a nontrivial way on the amplitude and frequency of the periodic driving as well as on the network topology. This indicates the appearance of stochastic resonance in the system as a function of the intensity of the internal noise. Besides, for low frequencies and for certain narrow ranges of the amplitudes of the periodic driving double maxima of the spectral power amplification as a function of $q$ occur, i.e., stochastic multiresonance appears. The above-mentioned results quantitatively agree with those obtained from numerical simulations of the mean-field equations for the time-dependent magnetization. In contrast, analytic solutions for the spectral power amplification obtained from the latter equations using the linear response approximation deviate significanlty from the numerical results since the effect of the periodic driving on the system is not small even for vanishing amplitude.

In this paper, we study single and double ionizations of N₂O in a short elliptically polarized 800 nm laser pulse using the COLTRIMS technique. The molecular-frame photoelectron angular distribution and the ion sum-momentum distribution of single and double ionizations of N₂O molecules are reported for the single ionization dissociative channel NO${}^{+}$ + N and the double ionization dissociative channel NO${}^{+}$ + N${}^{+}$. The ionizations of multiple orbitals for the two studied dissociative channels were identified via studying the orientation dependent ionization rates for their KERs. The results show that the shape of the ionizing orbitals governs the single and double ionization processes of N₂O.

We use the first-principles calculation based on density functional theory (DFT) to investigate the hydrogen storage of Li decorated coronene graphene. Our result indicates that single Li atom can adsorb three H₂ molecules and the adsorption energy per H₂ is −0.224 eV. When four Li atoms doped, the largest hydrogen gravimetric density is 6.82 wt.% and this is higher than the 2017 target by the US department of energy (DOE). Meanwhile, the adsorption energy per H₂ is −0.220 eV, which is suitable for H₂ molecules to store. Therefore, Li decorated coronene graphene will be a candidate for hydrogen storage materials in the future.

Motivated by the chiral anomaly steering negative longitudinal magnetoresistance in GdBiPt under external magnetic field, we studied the electronic structures of GdBi with paramagnetism, antiferromagnetism and ferromagnetism by first-principles calculations with modified Becke and Johnson local density approximation plus Hubbard $U$. Our calculated results reveal that paramagnetic GdBi is semiconducting, while the antiferromagnetic GdBi is a topological nontrivial compensation metal. We also predict the presence of a pair of Weyl fermions in ferromagnetic GdBi and GdSb. The band crossing along the direction of magnetization is protected by the fourfold rotation symmetry, and the topological charge associating with each $d/p$ band crossing point is $\pm 1$.

In this work, the conducting properties of graphene lattice with a particular concentration of defect (5% and 10%) has been studied. The real space block recursion method introduced by Haydock et al. has been used in presence of the random distribution of defects in graphene. This Green function based method is found to be more powerful than the usual reciprocal based methods which need artificial periodicity. Different resonant states appear because of the presence of topological and local defects are studied within the framework of Green function.

In previous studies, it is found that the frame and pore in porous media both possess the fractal geometric character. So the permeability and porosity models of bi-fractal porous media are derived based on the assumption that a porous media consists of fractal solid clusters and capillary bundles. The expressions of presented models are constituted by the fractal parameters of solid cluster and those of capillary bundle. Good agreement between model predictions and experimental data is obtained. This verifies the validity of the permeability and porosity models for bi-fractal porous media. The sensitive parameters that influence the permeability and porosity are specified, and their effects on the relationship between permeability and porosity are discussed.

A theoretical study of isolated and doubly-clustered impurities is presented for the electronic excitations in a carbon nanotube lattice. Using a matrix operator formalism and a tight-binding model where the interactions between atoms take place via nearest-neighbor hopping, the properties of the excitations are deduced. A geometry consisting of long, single-walled carbon nanotubes is assumed with the defects introduced in the form of substitutional impurity atoms, giving rise to the localized electronic modes of the nanotube as well as the propagating modes of the pure (host) material. The impurities are assumed to be in a low concentration, having the form of either a single, isolated defect or a small cluster of two defects close together. A tridiagonal matrix technique is employed within a Green’s function formalism to obtain the properties of the discrete modes of the system, including their frequencies and localization. The numerical examples show a dependence on the nanotube diameters and on the relative spatial configurations of the impurities. The results contrast with the previous studies of line impurities since there is no translational symmetry along the longitudinal axis of the nanotubes in the present case.

Generalized Dirac monopoles in momentum space are constructed in even $d+1$ dimensions from the Weyl Hamiltonian in terms of Green’s functions. In $3+1$ dimensions, the (unit) charge of the monopole is equal to both the winding number and the Chern number, expressed as the integral of the Berry curvature. Based on the equivalence of the Chern and winding numbers, a chirally coupled and Lorentz invariant field theory action is studied for the Weyl semimetal phase. At the one loop order, the effective action yields both the chiral magnetic effect and the anomalous Hall effect. The Chern number appears as a coefficient in the conductivity, thus emphasizes the role of topology. The anomalous contribution of chiral fermions to transport phenomena is reflected as the gauge anomaly with the Pfaffian invariant $(E\cdot B)$. Relevance of monopoles and Chern numbers for the semiclassical chiral kinetic theory is also discussed.

The structural and electronic transport properties of Ga_xN_y ($x+y=6$) clusters are investigated in the framework of density functional theory (DFT). To get their most stable structures, a strategy of particle swarm optimization (PSO) algorithm is adopted. It is found that the most stable cluster’s binding energy and HOMO–LUMO gap energy decrease with Ga atom’s number in cluster increasing. The electronic transport properties of the clusters connected with two Al(100) electrodes are obtained by a method of combining nonequilibrium Green’s function (NEGF) with DFT. Equilibrium conductance of all six-atom GaN cluster is low (less than 0.65 G₀), and Ga₂N₄ has the highest one (0.635 G₀). Significant negative differential resistance (NDR) phenomenon is observed in configurations with cluster Ga₂N₄, Ga₃N₃ and Ga₅N₁, and these three clusters have almost the same current value in voltage region from 0.8 V to 1.3 V.

A new type of coupled balanced-mass double-ended tuning fork resonator (CBDETF) pressure sensor is fabricated and tested. However, the low accuracy of the CBDETF pressure sensor is not satisfied to us. Based on systematic analysis and tests, the coupling effect between the operational mode and interference mode is considered to be the main cause for the sensor in accuracy. To solve this problem, the stiffness of the serpentine beams is increased to pull up the resonant frequency of the interfering mode and make it separate far from the operational mode. Finally, the accuracy of the CBDETF pressure sensor is improved from $+/-$0.5% to less than $+/-$0.03% of the Full Scale (F.S.).

In this study, the thermal and optical properties of quartet mixtures formed at different weight ratios (1:1:1:1 and 1.5:1:1:1) from liquid crystals 4-octyloxy-4${}^{\prime }$-cyanobiphenyl (8OCB), 4-hexylbenzoic acid, 4-(octyloxy)benzoic acid and 4-(decyloxy)benzoic acid were investigated by differential scanning calorimeter (DSC) and polarized optic microscopy (POM). The phase transition temperatures of the novel quartet mixtures measured in the DSC experiments are in line with the POM experiments. The experimental results clearly show that the novel liquid crystal mixtures have displayed pure liquid crystalline properties. According to the phase diagram drawn from DSC results, the nematic range of the novel mixture at the eutectic point is larger than the nematic ranges of the components. The mesomorphic structures of produced homolog complex mixtures are found to be smectic and nematic phases. But the smectic phase cannot be observed in the novel complex 1.5:1:1:1 mixture during continuous cooling. The nematic range of the novel complex 1.5:1:1:1 mixture is bigger than the nematic range of the novel complex 1:1:1:1 mixture with increasing 8OCB. Also, the nematic-to-isotropic phase transition temperature decreases with increasing the weight ratio of 8OCB in the complex quartet mixture. Another interesting result is that the produced mixtures are to be like a medical cream at room temperatures. Furthermore, order parameter and thermal stability factor of the transitions are also calculated.