International Journal of Modern Physics B

We present a lossy nonlinear transmission RLC line and show how the coupled Ginzburg–Landau equations can be derived in the small amplitude and long wavelength limit using a standard reductive perturbation method and complex expansion. Soliton-like solution of the simplified equation was searched and the instability of a class of phase-winding solutions was explored.

Extending the Frohlich polaron problem to an anharmonic lattice, the author studies a polaronic state with a large radius of the wave function. The appropriate anharmonic part of the electron–phonon interaction Hamiltonian is derived, based on the methods of quantum field theory. Then, with the help of the perturbation theory, the anharmonic correction to the electron–phonon coupling constant is straightforwardly calculated. The estimate of this correction shows that the anharmonicity can considerably increase the e–ph coupling constant if the longitudinal phonon frequency is lower than ~10 meV. Several materials, molecular crystals α-sexithiophene (α-6T), α, ω dihexylsexithiophene (H-6T), and perovskite-type oxides, in which the anharmonic polaronic effects should be pronounced, are discussed. A comparison of the results with other anharmonic models of a polaronic state is drawn.

In this paper, nonlinear dispersive equations and seventh-order Sawada–Kotera equation are solved using homotopy perturbation method (HPM) and variational iteration method (VIM). The results obtained by the proposed methods are then compared with that of Adomian decomposition method (ADM). The comparisons demonstrate that the two obtained solutions are in excellent agreement. The numerical results calculated show that the methods can be accurately implemented to these types of nonlinear equations. The results of HPM and VIM confirm the correctness of those obtained by Adomian decomposition method.

This paper consists of two parts: (1) mechanism of formation of hole-pairs and quantum stripes in cuprate superconductors, and (2) resistivity versus temperature variation in YBa₂ Cu₃O_7-δ superconductor with different doping levels in the range 400–T_c. (1) On deoxygenation, CuO₂ plane of high-temperature cuprate superconductors are broken into small magnetically isolated fragments, predominantly Cu-tetramer (CuO)₄. Its electron paramagnetic resonance spectra show fine and hyperfine splittings. It was concluded that these splittings are due to FM coupling of the electronic spins of the four holes on the four Cu-ions in (CuO)₄ as well as the FM coupling of the nuclear spins of the four Cu-ions. A magnetic field is generated perpendicular to CuO₂ plane due to the spin magnetic moment of four holes and their orbiting around the (CuO)₄ frame. In the nondeoxygenated superconductors, this magnetic field leads to the formation of quantum stripes of free charge carriers and in special cases to the formation of preformed hole-pairs. This is the first model of a hole-pair, though its existence has been conjectured from various experimental results. (2) The electrical conduction in superconductors along c-axis seems to arise from the movement of free holes and along a-axis from the parallel combination of resistivities arising from the movement of free holes and paired holes, but no single formula connecting ρ and T is applicable to explain resistivities either along c- or a-axis in the entire temperature range. Nature of charge carriers seems to change continuously as the temperature is lowered. In the range 200–T_c, both along the c- and a-axes, the experimental resistivities depart significantly from those calculated by the formula applicable in the range 400–200 K. Plausible explanation has been suggested for the departure of experimental resistivities from the calculated ones in the low temperature region.

The length of the optical fiber which contains the sensor must be able to withstand the tensile loads that will be placed on it during the duration of its service lifetime. It is important to assess the effect of the UV radiation and removal of coating on the strength of the fiber within the region where the grating has been written. In this study, we have identified the various mechanical and laser fabrication parameters which constrain the strength of fiber gratings. These factors include the laser pulse rate, laser energy per pulse, UV exposure time, methods of coating removal and environmental degradation (moisture). The variation in strength with respect to these factors is discussed in detail. However, it is observed in all the investigations that the average strength of all the fiber grating samples decreased. Finally, the enhancement of mechanical strength of fiber gratings can be achieved by adjusting the laser settings and improving the coating removal methods.

Ca_3-xBi_xCo₄O₉(x = 0.0, 0.3) samples have been prepared at 1223 K by conventional solid-state reaction method. XRD and SEM investigations reveal that c-axis-oriented structure could be formed in Ca_2.7Bi_0.3 Co₄O₉ samples, whereas grains in Ca₃Co₄ O₉ samples distribute randomly. Moreover, Bi doping increases the grain size and relative density of Ca_2.7 Bi_0.3Co₄O₉. The electrical conductivity along the ab plane for Ca_2.7Bi_0.3Co₄O₉ is about four times as large as that along the c-axis, but the Seebeck coefficient is almost isotropic, which leads to a remarkable rise of the power factor in ab plane for Ca_2.7Bi_0.3 Co₄O₉ compared with untextured Ca₃Co₄ O₉. The textured structure in Ca_2.7Bi_0.3Co₄O₉ sample should be attributed to the effect of Bi doping.

The electronic band structures of some A⁵B⁶C⁷-type ternary compounds, BiSeI, BiSI, BiSCl, BiSBr, BiSeBr and SbSeBr, are investigated using the density functional theory and pseudopotential theory under the generalized gradient approximation (GGA). The electronic band structures obtained show that these crystals, except for BiSeI, have an indirect band gap.

The Pb(Zr_0.52Ti_0.48)O₃ ceramics with 0.75, 1.0, 1.25 and 1.50%wt of Nb₂O₅ addition were prepared by a conventional mixed oxide technique. It was found that the average grain size of the ceramics decreased from 6 to 1 μm with increasing doping concentration to 1.5%wt. The room temperature dielectric constant and d₃₃ reached maximum values of 1050 and 285 pC/N, respectively, at 1%wt Nb-doping. The electrical coercivity (E_c) of Nb-doped PZT decreased, while the polarization values increased, with increasing doping concentration. Most importantly, this study shows that Nb₂O₅-doped PZT ceramics exhibits soft piezoelectric and enhanced ferroelectric behaviors, and the optimum properties can be obtained using suitable doping concentration.

We have investigated some of the thermodynamic properties of spin-polarized liquid ³He at finite temperature using the lowest order constrained variational method. For this system, the free energy, entropy and pressure are calculated for different values of the density, temperature and polarization. We have also presented the dependence of specific heat, saturation density and incompressibility on temperature and polarization.

In the present study, Mg_xMn_1–xFe₂O₄ series of ferrites with x = 0.1, 0.3, 0.5, 0.7 and 0.9 were prepared by the citrate precursor technique. One set of samples was subjected to normal and the other to hot-pressed technique. Microstructural properties were studied with the help of X-ray diffraction (XRD) and scanning electron microscopy (SEM) for both sets of samples. Hot pressing of Mg–Mn ferrites results in an improvement of their magnetic and micro-structural properties, as it simultaneously controls grain growth and porosity. Hot pressing of Mg–Mn ferrites, however, results in deterioration of their dc resistivity. The cation distribution has been studied by X-ray analysis and magnetization. The variation of saturation magnetization and Curie temperature with increasing concentration of Mg²⁺ ions can be explained on the basis of cation distribution and Neel two sublattice models. Possible mechanisms, models and theories contributing to these results have been discussed.