International Journal of Modern Physics B

The present report is concerned with the dynamical behavior of $\pi$-electronic valley states, under the interaction with transverse zone-boundary optical phonons, in graphene. It is assumed that the phonons are thermal and obey the Bose–Einstein distribution, while the $\pi$-electrons are initially prepared in an experimentally realizable particular valley state. In our study, we take the view that such a mixture is completely described by a time-dependent density operator which is then determined, to the second-order of perturbation, from the governing Schrödinger equation. Employing the density operator so calculated, an analytical expression for the valley polarization, as a function of time, phonon frequency and temperature, is obtained. The results, accompanying with illustrative figures, reveal that the $\pi$-electrons, through the elastic exchange of energy with phonons, change the valley states periodically with characteristics that strongly depend upon the temperature. It is in particular shown that as the temperature is raised, the time-averaged valley polarization approaches zero, as expected. Our calculations also show that the amplitude of valley oscillations is solely determined by the temperature and phonon frequency: an increase in the temperature enlarges the amplitudes in contrast to the phonon frequency which does the reverse. Along these lines, moreover, we demonstrate that the frequency of valley oscillations is determined by the electronic momentum deviation from the valley states, along with the phonon frequency.

A photoconductive fractal antenna significantly improves the performance of photomixing-based continuous wave (CW) terahertz (THz) systems. An analysis has been carried out for the generation of CW-THz radiation by photomixer photoconductive antenna technique. To increase the active area for generation and hence the THz radiation power we used interdigitated electrodes that are coupled with a fractal tree antenna. In this paper, both semiconductor and electromagnetic problems are considered. Here, photomixer devices with Thue-Morse fractal tree antennas in two configurations (narrow and wide) are discussed. This new approach gives better performance, especially in the increasing of THz output power of photomixer devices, when compared with the conventional structures. In addition, applying the interdigitated electrodes improved THz photocurrent, considerably. It produces THz radiation power several times higher than the photomixers with simple gap.

The structural, electronic, optical and vibrational properties of Ni₃C have been studied by density functional theory (DFT) framework with first-principles study. The obtained structural parameters are in good agreement with other works. The electronic study demonstrates metallic behavior of Ni₃C since it has no energy gap at Fermi level. The optical parameters such as real and imaginary dielectric functions, loss function, conductivity, reflection, refraction indexes and absorption coefficients are studied. The phonon investigations confirm that the Ni₃C bulk is dynamically stable and carbon has a major role in optical spectrum of the material at infrared region. Finally, the $T^3$ behavior of $C_v$ at low temperatures is obtained, as expected.

By solving the Schrödinger equation and Poisson’s equation self-consistently, we have calculated the electronic structure for Si-doped GaAs/Al${}_x$Ga${}_{1-x}$As quantum well system at $T=100$ K in the effective mass approximation. We obtain the self-consistent potentials, eigen-envelope functions and the subband energies for different doping concentrations and for different thicknesses of the doping layer. The binding energies of exciton in GaAs/Al${}_x$Ga${}_{1-x}$As quantum wells under different doping conditions are calculated by using a variational method. And the variation of the binding energy with the thickness of the doped layer and the doping concentration is analyzed. It is found that at a given doping concentration, with the increase of thickness of the doping layer, the self-consistent potential becomes wider and more shallow, the binding energy of exciton decreases. At a given thickness of the doping layer, with the increase of the doping concentration, the self-consistent potential becomes narrower and deeper, the binding energy of exciton increases.

Angular-dependent torque measurements have been performed on the electron doped iron-pnictide superconductors BaFe${}_{2-x}$Ni_xAs₂ with series of Ni concentrations. In the superconducting state, an irreversibility, as the evidence for the presence of vortex, is observed between the torque measured with increasing and decreasing angle. Our results in underdoped samples $x=0.065$ show that the irreversible torque signal survives up to a temperature $T_{\mathrm{\text{irr}}}$ well above the superconducting transition temperature $T_c$, suggesting the existence of superconducting vortices above $T_c$, which is likely to originate from superconducting fluctuations.

In this paper we study the interaction between two two-level atoms with a two-mode quantized field in the presence of damping. Dipole–dipole interaction between the two atoms and the correlation between the two modes of field are also taken into account. To solve the model, using appropriate transformations, we reduce the considered model to a well-known Jaynes–Cummings model. After finding the analytical solution for the atom–field system, the effects of damping, field–field correlation and atomic dipole–dipole interaction on the entanglement between atoms and population inversion are investigated, numerically. It is observed that the dynamical behavior of the degree of entanglement for damped systems, in relatively large domains of time, takes a low but constant value adequately far from the beginning of the interaction. In addition, it is found that the value of population inversion after the initial oscillations takes negative values for damped systems and eventually vanishes by increasing time. Also, it is seen that simultaneous presence of both dipole–dipole interaction and field–field correlation provides typical collapse–revival phenomenon in the time-behavior of atomic inversion.

Based on a very simple coarse-grained colloidal model, here we implement an effective hard-sphere theory and numerical simulation to capture the general features of the association equilibria for globular proteins in crowded environment. We measure the activity coefficient, i.e., the deviation from ideal behavior of protein solution, and the crowding factor, i.e., the contribution of crowders to the association equilibria, for proteins in macromolecular crowding. The results show that the association balance in macromolecular crowding depends sensitively on the magnitude of protein–crowder attraction and the relative size of reactant to crowding agent. Since our coarse-grained model is irrelevant to the microscopic details of the molecules, it can be applied to the control of the association equilibria of many globular proteins such as bovine serum albumin, crystallin and lysozyme.

Nowadays, the studies of the effects on the optical bandgap, absorption spectrum, and electrical properties of Ag–N co-doped ZnO have been extensively investigated. However, Ag and N atoms in doped systems are randomly doped, and the asymmetric structure of ZnO is yet to be explored. In this paper, the geometric structure, stability, density of states, absorption spectra and conductivity of pure and Ag–N co-doped Zn${}_{1-x}$Ag${}_x$O${}_{1-x}$N${}_x$($x$=0.03125, 0.0417 and 0.0625) in different orientations are calculated by using plane-wave ultrasoft pseudopotential on the basis of density functional theory with GGA$+$U method. Results show that the volume, equivalent total energy and formation energy of the doped system increase as the concentration of Ag–N co-doped Zn${}_{1-x}$Ag${}_x$O${}_{1-x}$N${}_x$ increases at the same doping mode. The doped systems also become unstable, and difficulty in doping. At the same concentration of Ag–N co-doped Zn${}_{1-x}$Ag${}_x$O${}_{1-x}$N${}_x$, the systems with Ag–N along the $c$-axis orientation is unstable, and doping is difficult. The optical bandgap of Ag–N co-doped systems is narrower than that of the pure ZnO. At the same doping mode, the optical bandgap of the systems with Ag–N perpendicular to the $c$-axis orientation becomes narrow as the concentration of Ag–N co-doped Zn${}_{1-x}$Ag${}_x$O${}_{1-x}$N${}_x$ increases. The absorption spectra of the doped systems exhibit a red shift, and this red shift becomes increasingly significant as the concentration of Ag–N co-doped Zn${}_{1-x}$Ag${}_x$O${}_{1-x}$N${}_x$ increases. Under the same condition, the relative hole concentrations of the doped systems increases, the hole effective mass in valence band maximum decreases, the hole mobility decreases, the ionization energy decreases, Bohr radius increases, the conductance increases and the conductivity become better. Our results may be used as a basis for the designing and preparation of new optical and electrical materials for Ag–N co-doped ZnO applied in low temperature end of temperature difference battery.

The influence of position-dependent mass (PDM) on nonlinear optical absorption (OA) coefficients and refractive index changes (RICs) is theoretically studied, within the framework of the iterative method and the compact-density-matrix approach. Through solving Schrödinger equation, the eigenfunctions and energy eigenvalues are obtained. The analytic expressions of the optical absorption coefficients and the refraction index changes are derived theoretically. The results reveal that the OA coefficients and the RICs are strongly affected by PDM, and both the peaks of them increase when considering PDM.

A number of factors influences railway safety. It is an important work to identify important influencing factors and to build the relationship between railway accident and its influencing factors. The maximal information coefficient (MIC) is a good measure of dependence for two-variable relationships which can capture a wide range of associations. Employing MIC, a graph model is proposed for preventing railway accidents which avoids complex mathematical computation. In the graph, nodes denote influencing factors of railway accidents and edges represent dependence of the two linked factors. With the increasing of dependence level, the graph changes from a globally coupled graph to isolated points. Moreover, the important influencing factors are identified from many factors which are the monitor key. Then the relationship between railway accident and important influencing factors is obtained by employing the artificial neural networks. With the relationship, a warning mechanism is built by giving the dangerous zone. If the related factors fall into the dangerous zone in railway operations, the warning level should be raised. The built warning mechanism can prevent railway accidents and can promote railway safety.

This paper proposes a new radial membrane acoustic metamaterial (RMAM) structure, wherein a layer membrane substrate is covered with a rigid ring (polymethyl methacrylate frame and aluminum lump). The dispersion relationships, transmission spectra and displacement fields of the eigenmodes of this radial membrane acoustic metamaterial are studied with FEM. In contrast to the traditional radial phononic crystals (RPCs), the proposed structures can open bandgaps (BGs) in lower frequency range (0–300 Hz). Simulation results show that the physical mechanism behind the bandgaps is the coupling effects between the rotational vibration of aluminum lump and the transverse vibration of membrane. Geometrical parameters which can adjust the bandgaps’ widths or positions are analyzed. Finally, we investigate the axial sound transmission loss of this acoustic metamaterial structure, and discuss the effects of factor loss, membrane thickness and the number of layers of unit cell on the axial sound transmission loss. Dynamic effective density proves the accuracy of the FEM results. This kind of structure has potential application in pipe or circular ring structure for damping/noise reduction.

Structural, electronic, elastic and thermal properties of Al₄Si₂C₅ under constant pressure were investigated by using first-principles theory. The total volume of the cell decreased by almost 15.7% under 40 GPa which is smaller than that of Al₄SiC₄ (16%), while the linear compressibility along $a$- or $b$-axis direction showed better anti-deformation behavior than that of along $c$-axis direction. The peak heights of total density of state (TDOS) and partial density of state (PDOS) curves of Al₄Si₂C₅ are slightly lowered with forced high pressure. Meanwhile, the mechanical properties of Al₄Si₂C₅-like elastic constants and elastic moduli accelerate with the pressure increasing from 0 GPa to 40 GPa; the thermal expansion coefficient $\alpha$ increases rapidly at lower temperature and this tendency gradually approaches a linear increase when the temperature is above 1000 K. At particular temperature, $\alpha$ decreases continuously with the pressure accelerating. Heat capacity at constant volume ($C_V)$ with pressure was also evaluated, the results displayed that $C_V$ is sensitive with the temperature rather than the pressure. The elastic anisotropy and Debye temperature with pressure were successfully obtained and discussed.