International Journal of Modern Physics B

In this paper, by utilizing the variable conductivity with photo-injection in gallium arsenide (GaAs), we have designed an asymmetrical planar terahertz (THz) metamaterial, which is connected with two single-gap split ring resonator (SRR) by GaAs strip and demonstrated the resonant conversion of SRR within the THz range under appropriate optical pumping. As central trailing arm of the structure, GaAs is skillfully inserted between the two cross arms of the THz metamaterial and plays a key role in resonant conversion. Through modulation of its conductivity (σ_GaAs), the variable conductivity of GaAs can make one dual-gap SRR into two connective single-gap SRRs in physical structure, at the same time, the state conversion of two different resonances in the THz metamaterial has been achieved. The simulation results show that the resonant states of THz metamaterial can be switched from one LC and one dipole (state 1) to two LC and one new dipole (state 2) through the intermediate state with the increasing σ_GaAs. This structural design provides a new example to apply variable conductivity to achieve state conversion of resonance and can be extended to the additional application in THz devices.

This work reports on the influences of temperature changes on plasmons of metallic particles that are so small that electric carriers in the conduction band are forced to be at discrete sub-bands due to quantum confinement. In the framework of the electron-in-a-box model and with an every-electron-count computational scheme, the spatial electric distribution inside the particle is calculated. In the calculations, the intra-subband fluctuations are taken into account. The numerical results have shown that the small-particle plasmon frequency shifts with the temperature. The findings suggest that it would be possible to control the plasmons of quantum particles externally.

A parallel three-dimensional lattice Boltzmann scheme for multicomponent immiscible fluids is proposed to simulate bubble rising and coalescence process in viscous flows. The lattice Boltzmann scheme is based on the free-energy model and is parallelized in the share-memory model by using the OpenMP. Bubble interface is described by a diffusion interface method solving the Cahn–Hilliard equation and both the surface tension force and the buoyancy are introduced in a form of discrete body force. To avoid the numerical instability caused by the interface deformation, the 18 point finite difference scheme is utilized to calculate the first- and second-order space derivative. The correction of the parallel scheme handling three-dimensional interfaces is verified by the Laplace law and the dynamic characteristics of an isolated bubble in stationary flows. Subsequently, effects of the initially relative position, accompanied by the size ratio on bubble–bubble interaction are studied. The results show that the present scheme can effectively describe the bubble interface dynamics, even if rupture and restructure occurs. In addition to the repulsion and coalescence phenomenon due to the relative position, the size ratio also plays an insignificant role in bubble deformation and trajectory.

We use a two-compartment model, which includes soma and dendrite, to explore how extracellular subthreshold sinusoidal electric fields (EFs) influence the spike coding of an active neuron. By changing the intensity and the frequency of subthreshold EFs, we find that subthreshold EFs indeed affect neuronal coding remarkably within several stimulus frequency windows where the field effects on spike timing are stronger than that on spiking rate. The field effects are maximized at several harmonics of the intrinsic spiking frequency of an active neuron. Our findings implicate the potential resonance mechanism underlying subthreshold field effects. We also discuss how neuronal morphologic properties constrain subthreshold EF effects on spike timing. The morphologic properties are represented by two parameters, g_c and p, where g_c is the internal conductance between soma and dendrite and geometric factor p characterizes the proportion of area occupied by soma. We find that the contribution to field effects from the variation of p is stronger than that from g_c, which suggests that neuronal geometric features play a crucial role in subthreshold field effects. Theoretically, these insights into how subthreshold sinusoidal EFs modulate ongoing neuron behaviors could contribute to uncovering the relevant mechanism of subthreshold sinusoidal EFs effects on neuronal coding. Furthermore, they are useful in rationally designing noninvasive brain stimulation strategies and developing electromagnetic stimulus techniques.

In this paper, the effect of built-in-polarization field on lattice thermal conductivity of AlN/GaN/AlN quantum well (QW) has been theoretically investigated. The built-in-polarization field at the hetero-interface of GaN/AlN modifies elastic constant, phonon velocity and Debye temperature of GaN QW. The relaxation time of acoustic phonons (AP) in various scattering processes in GaN with and without built-in-polarization field has been computed at room temperature. The result shows that combined relaxation time of AP is enhanced by built-in-polarization field and implies a longer mean free path. The revised intrinsic and extrinsic thermal conductivities of GaN have been estimated. The theoretical analysis shows that up to a certain temperature the polarization field acts as negative effect and reduces the thermal conductivities. However, after this temperature both thermal conductivities are significantly contributed by polarization field. This gives the idea of temperature dependence of polarization effect which signifies the pyro-electric character of GaN. The intrinsic thermal conductivity at room temperature for with and without polarization mechanism is found to be 491 Wm^-1K^-1 and 409 Wm^-1K^-1, respectively i.e., 20% enhancement. However, the extrinsic thermal conductivity at room temperature for with and without polarization mechanism is found to be 280 Wm^-1K^-1 and 245 Wm^-1K^-1, respectively i.e., 13% enhancement. The method we have developed may be taken into account during the simulation of heat transport in optoelectronic nitride devices to minimize the self-heating processes and in polarization engineering strategies to optimize the thermoelectric performance of GaN alloys.

We study the two-component Ginzburg–Landau (GL) theory, in the presence of a self-consistent vortex line, to obtain the penetration depth (λ) and the effective healing length (ξ), in the asymptotic limit r → ∞. All these parameters versus T/T_c are analyzed for the materials MgB₂, V₃Si and LiFeAs in the interval 0.88 ≤ T/T_c ≤ 1.0, where the GL theory is assumed to be valid. We find that κ ≡ λ/ξ, which is another parameter not related to the GL parameter, is T-independent for V₃Si and LiFeAs, while is T-dependent for the compound MgB₂. This result suggests that even though all these three materials display two-gap superconductivity overall, near T_c superconductivity in V₃Si and LiFeAs seems to be different from the one in MgB₂. The use of this parameter, κ, as a new way to "study" the superconducting materials, under the presence of a single vortex, is valid for 0.88T_c≲ T ≤ T_c, namely, in the GL formalism.

This paper addresses the effects of applied magnetic field and partial slip effects in peristalsis of water-based nanofluids in an asymmetric flow configuration. Analysis is carried out using silver and copper nanoparticles. Viscous dissipation, mixed convection, Ohmic heating and heat generation/absorption are considered. Mathematical modeling is done employing lubrication approximations. Resulting coupled system is solved numerically. Physical quantities like axial velocity, pressure gradient, temperature and heat transfer rate are graphically analyzed. Comparison between the silver–water and copper–water nanofluids is presented and analyzed. Results show that the maximum velocity, temperature and heat transfer rate at the wall in silver–water nanofluid are comparatively greater than that of copper–water nanofluid. It is also observed that addition of nanoparticles results in a decrease in the velocity and temperature of fluid. However, the heat transfer rate at the wall is enhanced through addition of nanoparticles.

Nitinol as a superelastic shape memory alloy (SMA) has been the focus of physical-chemical studies in recent decades in respect to functionality of biocompatibility in the body. Superelastic properties of nitinol are the direct results of the electronic structure of this material while dealing with the ab initio behavior of microstructure. In the present work, the elastic properties and electronic structure of B2-phase binary TiNi_(1-x)Cu_x (x = 0, 0.25 and 0.75) shape memory alloys are discussed aiming at understanding of the physical properties underlying superelastic behavior. The calculations have been performed with the program package WIEN2K, in the framework of first-principle, all-electron density functional theory (DFT) within the scheme of the generalized gradient approximation (GGA). The optimized lattice parameters and independent elastic constants are obtained for use in the calculation of the bulk and shear moduli, Young modulus, Poisson ratio and Zener anisotropy parameter. For different alloying fractions x, the tetragonal (C′) and trigonal (C₄₄) shear constants are calculated and brittle/ductile behavior of these compounds is discussed. Finally, a qualitative discussion of dependence of elastic behavior of these compounds upon the electronic density of states (DOS) is presented.

The sol–gel method was developed to synthesize A₅B₅O₁₇-type Na_0.4K_0.6Ca₄Nb₅O₁₇ layered perovskite ceramics, using NaNO₃, KNO₃, CaNO₃⋅4H₂O and NbCl₅ precursors. Samples were calcined at 950°C and sintered at 1200–1350°C. The phase and microstructural analyses of samples were carried out using X-ray diffractometer (XRD) and scanning electron microscope. X-ray diffraction analysis revealed single phase monoclinic symmetry, within the detection limit of in-house XRD facility. Microstructural analysis shows ~ 10 μm elongated rod-like grains. The microwave dielectric properties of the sintered composition at 1300°C were: relative permittivity (ε_r) = 42, quality factor (Q × f₀) = 8270 GHz and temperature coefficient of resonant frequency (τ_f) = -10.1 ppm/°C.

Ab initio density functional calculations are performed to investigate the dielectric properties of LnBSiO₅ (Ln = Ce, Pr, Nd) with the stillwellite structure. The calculated structural parameters are found to agree well with existing experimental results. The three compounds possess insulating electronic structure with nearly isotropic high frequency dielectric permittivity tensors. On the other hand, the static dielectric permittivity tensors are found to be less isotropic. The anisotropy of static dielectric tensors are found to increase as the atomic number of the lanthanide increases.