International Journal of Modern Physics B

With the plane wave expansion method, we introduced the concept and theory of two-dimensional function photonic crystal, and compared the theoretical difference of two-dimensional function photonic crystal with the conventional photonic crystal. In Numerical calculation, with COMSOL software based on finite element method, we have studied the bandgap structure, defect mode and eigenfield distribution for the two-dimensional function photonic crystal with and without point defect. For the two-dimensional function photonic crystal, the dielectric constant of dielectric column is the space coordinate function, which can be realized by the electro-optic effect and the Kerr effect. In the paper, we studied the dielectric constant function form is $𝜀$(r) = kr + b, and the parameters k and b can be adjusted by changing the electric field or light intensity applied to the dielectric column. By changing the parameters k or b of the dielectric column dielectric constant, we found the two-dimensional function photonic crystal transmission characteristics have the adjustability, i.e., the bandgap structure, the bandgap position and width, the defect mode position, width and number and the defect mode eigenfield distribution can be changed by adjusting the parameters k or b, which can be used to conveniently design the bandgap structure that we need. These conclusions will provide important theoretical basis and methods for optical devices design.

A new metamaterial absorber (MA) having distinct properties than those given in the literature is investigated. Although several designs have been studied for achieving absorption characteristics in single-band, dual-band and multiple bands within the whole spectrum of solar light, there has been limited number of researches examining the broadband MA in the visible light section of the spectrum. The designed structure is composed of the combination of three layers having different thicknesses including a metallic substrate, dielectric and a metal layer. Due to the sandwich-like structure, it can support the plasmonic resonance. The proposed structure, which provides a maximum absorption level of 99.42% at 579.26 THz, has a high absorption rate of 99% between the frequency band 545 and 628 THz. Numerical results indicate that the proposed structure has perfect absorption which is greater than 90.98% through the whole working frequency band. The dependency of the designed structure on the polarization angle is investigated for different incident angles with TE and TM polarizations as well as the TEM mode. In addition to its potential applications such as solar cells and cloaking, the designed structure can also be considered as a color sensor and an optical frequency sensor.

The Wigner–Weyl transform and phase space formulation of a density matrix approach are applied to a non-Hermitian model which is quadratic in positions and momenta. We show that in the presence of a quantum environment or reservoir, mean lifetime and decay constants of quantum systems do not necessarily take arbitrary values, but could become functions of energy eigenvalues and have a discrete spectrum. It is demonstrated also that a constraint upon mean lifetime and energy appears, which is used to derive the resonance conditions at which long-lived states occur. The latter indicate that quantum dissipative effects do not always lead to decay but, under certain conditions, can support stability of a system.

Single-phase polycrystalline Bi${}_{0.85}$Ho${}_{0.05}$Sm${}_{0.1}$FeO₃ ceramic is prepared by conventional solid state route. The co-doping of Sm and Ho (via Bi site) in BiFeO₃ controls the formation of secondary phases. The Rietveld refinement analysis shows an increasing trend in tilt angle due to the rotation of FeO₆ octohedra with respect to host BiFeO₃. The remanent polarization and the magnetization of Bi${}_{0.85}$Ho${}_{0.05}$Sm${}_{0.1}$FeO₃ ceramic are found to be significantly improved than BiFeO₃ and Bi${}_{0.9}$Sm${}_{0.1}$FeO₃ at room-temperature. Considerable variations in the remanent polarization (0.18 to 0.11 $\mu$C/cm²) on magnetic poling and a dielectric anomaly in the vicinity of the antiferromagnetic transition temperature are due to the intrinsic magnetoelectric coupling effect in Bi${}_{0.85}$Ho${}_{0.05}$Sm${}_{0.1}$FeO₃ ceramic. The dielectric permittivity increases with increase in applied magnetic field and the coupling coefficient of Bi${}_{0.85}$Ho${}_{0.05}$Sm${}_{0.1}$FeO₃ ceramic is found to be 0.91% at 4 kOe.

The effect of a combined nanochannel structure on the permeation of water molecules across a nanochannel is studied by molecular dynamics simulations. The simulation results show that a combined structure has an effect on enhancing water permeation ability. We obtain a maximal enhancement ratio which can reach a value of 4.381 without changing the entrance radius of the nanochannel. First, we find that the enhancement behavior of water molecules across a combined nanochannel is related to the radius ratio and the radius of the entrance pore. These simulation results will be beneficial for understanding the effect of a combined nanochannel structure on water transfer ability.

Community detection in complex network analysis is a quite challenging problem spanning many applications in various disciplines such as biology, physics and social network. A large number of methods have been developed for this problem, among which the label propagation algorithm (LPA) has attracted much attention because of its advantages of nearly-linear running time and easy implementation. Nevertheless, the random updating order and tie-breaking strategy in LPA make the algorithm unstable and may even lead to the formation of a monster community. In this paper, an improved LPA called LPA-INTIM is proposed for solving the community detection problem. Firstly, an intimacy matrix is constructed using local topology information for measuring the intimacy between nodes. And then, the node importance is calculated to ensure that nodes are updated in a specific order. Finally, the label influence is evaluated for updating node label during the label propagation process. In addition, we introduce a novel tightness function to improve the stability of the proposed algorithm. By the comparison with the methods presented in the literatures, experimental results on real-world and synthetic networks show the efficiency and effectiveness of our proposed algorithm.

Present study is a computational approach focused on optical properties of Ni-doped rocksalt CdS system under PBE-GGA and GGA + U approximations. Investigation of optical properties of CdS:Ni system by fixing supercell size and increasing dopant concentration is a focus of study. Cd atoms are substituted by Ni atoms and optical properties show significant change for higher Ni concentration. Redshift in optical absorption is observed as dopant concentration is increased. Interaction of Ni and S atoms and localization of d-states are inspected near the conduction band minima or Fermi surface. GGA + U absorption plots show slight variation with increase in energy values. Ni-doping in CdS host lattice enhances its opto-electronic properties which extend its applications in optical sensors, optical and similar devices.

Nonequilibrium phase transitions of vortex matter with a strong random pinning potential in layered superconductors are investigated by the three-dimensional frustrated anisotropic XY model and resistively-shunted junction dynamics at low, middle and high-temperatures, respectively. It is found that a disorder to order phase transition driven by an external current can be obtained at a low-temperature, however, a reordering configuration does not occur at a high-temperature. With the competition between thermal noise, disorder pins and current, the vortex matter can even show the reordering process twice at an intermediate temperature, giving a clear evidence of dc driven vortex lattice reorganization.

The structural, elastic, electronic and optical properties of cubic NaNbO₃ (c-NNO) crystals in the pressure range 0–20 GPa are studied using first-principles VASP code. The influences of pressure on lattice constants, unit cell volume, elastic constants, elastic modulus, elastic anisotropy, sound velocities, elastic wave velocities, Debye temperature and Grüneisen parameters are discussed. Electronic structure calculations show that c-NNO is a wide indirect bandgap semiconductor. The band structures are similar for different pressures, except that conduction band shifts toward higher energy with increasing pressure. The distributions of density of states reveal typically covalent Nb–O bonding with strong hybridizations and significantly ionic Na–O ones. The dielectric function and linear optical properties are calculated with HSE06 scheme, and the pressure effects of the optical properties are also investigated. The optical spectra of c-NNO exhibit similar shapes for distinct pressures and blueshifts with the increase of pressure. The phonon dynamical properties are also investigated from the density functional perturbation theory (DFPT), and the calculated phonon spectra exhibit some imaginary frequencies, which reflect the negative phonon modes and hence the dynamical instability of the system.

Identifying the degree distribution of land cover networks is helpful to find analytical methods for characterizing complex land cover, including segmentation techniques of remote sensing images of land cover. After segmentation, we can obtain the geographical objects and corresponding relationships. In order to evaluate the segmentation results, we introduce the concept of land cover network and present an analysis method based on statistics of its degree distribution. Considering the object-oriented segmentation and objects merge-based spectral difference segmentation, we construct the land cover networks for different segmentation scales and spatial resolutions under these two segmentation strategies, and study the degree distribution of each land cover network. Experimental results indicate that, for the object-oriented segmentation, the degree distributions of land cover networks follow approximately a Poisson distribution, regardless of the segmentation scales and spatial resolutions. For the objects-merge method based on spectral difference segmentation, degree distributions exhibit heavy tails. Compared with all the segmentation results, the pattern spots after objects-merge better retain the integrity of geographical features and the land cover network can reflect more accurately the topological properties of real land cover when the threshold of objects merge is suitable. This study shows that we can evaluate the reliability of segmentation results objectively by analyzing the degree distribution pattern of land cover networks.