International Journal of Modern Physics B

Magnesium (Mg) crystal structures are extensively explored using an evolutionary algorithm implemented in the USPEX code. Two structures with simple trigonal and tetragonal symmetries are discovered to possibly exist under high pressure. The stability of these symmetries is determined by elastic constants and phonon spectrum calculations. First-principle calculations are performed to investigate the structural, mechanical and electronic properties of different Mg structures under high pressure (up to 300 GPa). Above 190 GPa, the trigonal structure is more stable than the hexagonal close-packed (HCP) structure. Particularly, the trigonal structure can be considered a compromise between face-centered cubic (FCC) and HCP blocks. Interestingly, the tetragonal structure density is only 95% HCP structure. In addition, the tetragonal structure has strong directional bonding but is less stable than the HCP structure (up to 600 GPa). Pressure significantly changes the electronic properties of both structures although they remain metallic up to 300 GPa.

The investigation of random walks is central to a variety of stochastic processes in physics, chemistry and biology. To describe a transport phenomenon, we study a variant of the one-dimensional persistent random walk, which we call a zero-one-only process. It makes a step in the same direction as the previous step with probability p, and stops to change the direction with 1 − p. By using the generating-function method, we calculate its characteristic quantities such as the statistical moments and probability of the first return.

The structural and elastic properties of 3d transition metal tetraboride TMB₄ (TM = V, Cr, Mn) have been studied by using density functional theory (DFT) within the local density approximation (LDA) and generalized gradient approximation (GGA) for exchange-correlation function. Our results are in agreement well with available experimental data and previous theoretical studies. The full elastic constants and bulk shear modulus of TMB₄ (TM = V, Cr, Mn) are obtained in the wide pressure range. Results indicated that these phases of TMB₄ (TM = V, Cr, Mn) are strongly pressure dependent. And the CrB₄ in Immm structure is the most ultraincompressible among the considered phases at 0 GPa. By the mechanical stability criteria, it is predicted that these phases of TMB₄ (TM = V, Cr, Mn) compounds are stable up to 100 GPa. In addition, the calculated B/G ratio indicated that all the structures of TMB₄ (TM = V, Cr, Mn) possess brittle nature in the range of pressure from 0 GPa to 100 GPa except the Pnnm structure of MnB₄, which is higher than 1.75, indicating that the Pnnm structure of MnB₄ is prone to ductility when the pressure is above 73 GPa.

We characterize the solute mobility behavior driven by interstitial mechanism in FCC diluted alloys using a classical molecular static technique (CMS). In the same line of ideas as the multi-frequency model, we calculate the tracer self- and solute diffusion coefficients. Specifically, we perform our calculations for the Al–U diluted alloy. We verify that in the Al–U system, mixed dumb-bells are observed to be unstable and U mobility is driven by crowdions. From previous results of diffusion in same alloys containing only vacancies, qualitatively we conclude that, experimental data are in perfect agreement with previous calculations of solute U diffusion driven by a vacancy mechanism. Also we give a possible migration path for solute U atoms through interstitial migration, where we have found that U enhances the Al mobility in the alloy.

In this work we have investigated the effect of external electric field on the electronic structure and refractive index changes of a GaN/AlN constant total effective length multiple quantum well. A decreasing oscillatory behavior for energy levels was seen when the electric field increased and in some energy values the energy levels intersected. By increasing the number of wells, oscillator strength dropped suddenly and moved to higher electric field in a critical electric field. By using the number of wells and the total length of the structure as two tuning tools, we were able to select our previously determined critical electric field.

We present the electronic, magnetic and structural properties of the magnetic transition metal oxides PbMO₃ (M = Fe, Co, Ni) in cubic perovskite structure. The calculations are based on the density functional theory (DFT) within plane-wave pseudopotential method and local spin density approximation (LSDA) of the exchange-correlation functional. On-site Coulomb interaction is also included in calculations (LSDA+U). The systems are considered in ferromagnetic (FM) and G-type antiferromagnetic (G-AFM) order. FM structures are energetically more favored than G-AFM and than non-magnetic states for all the systems studied. The spin-polarized electronic band structures show that all the structures have metallic property in FM order without Hubbard-U interaction (U_eff = 0). However, the inclusion of on-site Coulomb interaction (U_eff = 7 eV) opens a semiconducting gap for majority spin channel of PbFeO₃ and of PbNiO₃ resulting in a half-metallic character. PbCoO₃ system remains as metallic with LSDA+U scheme. Bonding features of all structures are largely determined by the hybridizations between O–p and d-states of transition metal atoms. The partial magnetic moment of Fe atom in PbFeO₃ is enhanced by inclusion of Hubbard-U interaction (2.55 μ_B ⇒ 3.78 μ_B). Total magnetic moments of half-metallic PbFeO₃ and of PbNiO₃ compounds are very close to integer values.

This paper deals with the studies of lasing characteristics along with the gain spectra of compressively strained and step SCH based In_0.71Ga_0.21Al_0.08As/InP lasing nano-heterostructure within TE polarization mode, taking into account the variation in well width of the single quantum well of the nano-heterostructure. In addition, the compressive conduction and valence bands dispersion profiles for quantum well of the material composition In_0.71Ga_0.21Al_0.08As at temperature 300 K and strain ~1.12% have been studied using 4 × 4 Luttinger Hamiltonian. For the proposed nano-heterostructure, the quantum well width dependence of differential gain, refractive index change and relaxation oscillation frequency with current density have been studied. Moreover, the G–J characteristics of the nano-heterostructure at different well widths have also been investigated, that provided significant information about threshold current density, threshold gain and transparency current density. The results obtained in the study of nano-heterostructure suggest that the gain and relaxation oscillation frequency both are decreased with increasing quantum well width but the required lasing wavelength is found to shift towards higher values. On behalf of qualitative analysis of the structure, the well width of 6 nm is found more suitable for lasing action at the wavelength of 1.55 μm due to minimum optical attenuation and minimum dispersion within the waveguide. The results achieved are, therefore, very important in the emerging area of nano-optoelectronics.

With a first-principles method based on density functional theory, the effect of the alloying element titanium (Ti) on the thermodynamic stability and electronic structure of hydrogen (H) in pure vanadium (V) is investigated. The interactions between H and the vacancy and the defect solution energies in a dilute V–Ti binary alloy are calculated. The results show that: (i) a single H atom prefers to reside in a tetrahedral interstitial site in dilute V–Ti binary alloy systems; (ii) H atoms tend to bond at the vacancy sites; a mono-vacancy is shown to be capable of trapping three H atoms; and (iii) the presence of Ti in pure V can increase the H trapping energy and reduce the H trapping capability of the vacancy defects. This indicates that doping with Ti to form dilute V–Ti binary alloys can inhibit the solution for H, and thus suppress the retention of H. These results provide useful insight into V-based alloys as a candidate structural material in fusion reactors.

Almost all existing approaches for community detection only make use of the network topology information, which completely ignore the background information of the network. However, in many real world applications, we may know some prior information that could be useful in detecting the community structures. Specifically, the true community assignments of certain nodes are known in advance. In this paper, a novel semi-supervised community detection approach is proposed based on label propagation, which can utilize prior information to guide the discovery process of community structure. Our algorithm can propagate the labels from the labeled nodes to the whole network nodes. The algorithm is evaluated on several artificial and real-world networks and shows that it is highly effective in recovering communities.

The dynamics of charge carriers (electrons) hopping through a nonhomogeneous medium in semiconductor layer is investigated by changing a thermal noise of strength D and an external harmonic potential V(x). The nonhomogeneous medium exhibits denser trap distribution around the center, which biases the electrons to therein concentrate. Applying also a monostable potential at the center further enhances the accumulation of electrons. However, by applying a nonhomogeneous hot temperature in the vicinity of the potential minimum forced the electrons to diffuse away from the center and redistribute around two points. Thermally activated rate of hopping and diffusion of electrons in a nonhomogeneous medium, as a function of model parameters, is also considered in the high barrier limit. Using two states approximation, I have also studied the stochastic resonance (SR) of the electrons dynamics in the presence of a time-varying signal. I found a strong spectral amplification η and lower temperature occurrence of its peak as compared to previous works [M. Asfaw, B. Aragie and M. Bekele, Eur. Phys. J. B 79, 371 (2011); B. Aragie, Y. B. Tateka and M. Bekele, Eur. Phys. J. B 87, 101 (2014)].

The authors draw from half a century of meteorological research to expose flaws in a recent proposal to build 300-m-tall tornado-prevention walls across the U.S. Great Plains. The idea behind the walls is that they would prevent cold and warm air masses from clashing and would therefore suppress tornadoes. The problem with this proposal, however, is that atmospheric fronts ("airmass clashes") are neither a necessary nor a sufficient condition for tornadoes and that the proposed walls would not prevent the formation of fronts in the first place. Additional misconceptions about supercells thunderstorms and tornado formation also are identified.

Dahl and Markowski are wrong and misleading to claim that the major tornadoes in USA Tornado Alley are not related to the collisions between northbound warm air flow and southbound cold air flow. In addition, they use incompressible and inviscid fluid model for atmosphere in their simulations about the interaction between air wind and the wall. Such approach ignores the basic physics and thus cannot reach any meaningful results. As air is compressible, the collision between the wind and wall will compress air, eventually lead the air density to decrease fast with the height and make the air flow stratified. The viscosity will produce wind shear, turbulent eddies and greatly reduce the wind's forwarding speed. Laboratory experiments and the Nature have all shown that hills with height about 300 m will not block winds completely to change the climate, but can effectively reduce the wind speed, weaken the air mass collisions and eliminate the major tornadoes. All these strongly support the theory that building east–west ranged walls of 300 m high and 50 m wide will eliminate major tornado threat in Tornado Alley.