International Journal of Modern Physics B

Relative stability of different phases for MgSiO₃ and (Mg_0.75, Fe_0.25)SiO₃ within 0–110 GPa are investigated using first-principles method. For MgSiO₃, the computed equation of state for orthorhombic phase of Pbnm space group agrees well with experimental results. The relative stability reduces from observed Pbnm orthorhombic phase to intermediated tetragonal P4mbm phase, and then to hypothetical cubic phase. For (Mg_0.75, Fe_0.25)SiO₃, the same sequence of relative phase stability is observed. Thus, the low-symmetric orthorhombic MgSiO₃ should be favored in the lower mantle condition, while adding Fe into MgSiO₃ will make it less stable at the same depth.

N-type polycrystalline higher manganese silicide (MnSi_1.7) films are prepared on thermally oxidized silicon substrates by magnetron sputtering. MnSi_1.85, Si, and carbon targets are used in the experiments. By co-sputtering of the MnSi_1.85 and Si targets, n-type MnSi_1.7 films are directly obtained. By increasing the Si content to the deposited films, both the Seebeck coefficient and electrical resistivity increase to high values. A Si intermediate layer between the MnSi_1.7 film and substrate plays an important role on the electrical properties of the films. Without the interlayer, the Seebeck coefficient is not stable and the electrical resistivity is higher. For preparation of MnSi_1.7 films by solid phase reaction, a sandwich structure Si/MnSi_x/Si (x < 1.7) and thermal annealing are used. A carbon cap layer is used as a doping source. With the carbon doping, the electrical resistivity of the MnSi_1.7 film decreases, while the Seebeck coefficient increases slightly. For reactive deposition, the MnSi_x (x < 1.7) film is directly deposited on the heated substrate with a Si intermediate layer. By using a Si cap layer, a MnSi_1.7 film with a Seebeck coefficient of -292 μV/K and electrical resistivity of 23 × 10^-3 Ω-cm at room temperature is obtained. The power factor reaches 1636 μW/mK² at 483 K. With such a high power factor, the n-type MnSi_1.7 material may be superior to p-type MnSi_1.7 material for the development of thermoelectric generators. Several smaller (0.036 - 0.099 eV) and intermediate (0.10 - 0.28 eV) activation energies are observed from the curves of logarithm of the resistivity versus reciprocal temperature. The larger activation energies (0.35 - 1.1 eV) are consistent with the reported energy band gaps for higher manganese silicides.

Fractional calculus of variations (FCV) has recently attracted considerable attention as it is deeply related to the fractional quantization procedure. In this work, the FCV from extended Erdélyi-Kober fractional integral is constructed. Our main goal is to exhibit a general treatment for dissipative systems, in particular the harmonic oscillator (HO) that has time-dependent mass and time-dependent frequency. The general linear equation of damped Erdélyi-Kober harmonic oscillator is constructed from which a time-dependent mass generalized law was derived exhibiting different types of behavior. This relatively new time-dependent mass law permits us to point out several possible cases simultaneously in contrast to many models discussed in the literature and without making use of any types of fractional derivatives. Some results on Hamiltonian part, namely Hamilton equations for the damped HO were obtained and discussed in detail.

We study quantum phases of an anisotropic (XYZ) Heisenberg spin-1/2 chain under a mixed magnetic field (uniform and staggered), along with its variant. Our model consists of two competing relevant operators, anisotropy a, and staggered magnetic field h_s. Depending on the sign of a this model describes either the physics of solitonic confinement, or the Ising transition between gapped phases. In our case excitations are the integer spin particle in contrast to the half-integer spin particle of frustrated dimerized spin chain. We do all analysis in weak coupling limit.

Polycrystalline solid solution of phosphate and vanadate lead calcium apatites Pb_{10 -x}Ca_x(VO₄)₁(PO₄)₅(OH)₂ apatites [x = 0.0, 2.0, 4.0, 6.0, 8.0, and 10.0] was reduced with hydrogen gas at the temperature of 300 and 400°C for 1 h. All these compounds were investigated by XRD and ESR spectroscopy. Only one reduced oxidation state V⁺⁴ was detected. Two types of species like isolated vanadium (+4) and clusters of V⁴⁺ had been observed. The stability of the apatites increases and the particle size of the crystal decreases with calcium doping concentrations. As the concentrations of Ca increase the intensity of ESR lines deviates due to the spin–lattice relaxation interactions. The deviation in the intensity of ESR spectra can be attributed as the coexistence of paramagnetic and antiferromagnetic phases in the apatites.

The nanocrystallization process in amorphous Fe₈₁B_13.5Si_3.5C₂ ribbons caused by isothermal annealing below the crystallization temperature is studied. X-ray diffraction and Mossbauer spectroscopy measurements are used to identify the formation of new Fe containing compounds such as the ribbons annealed at various temperatures. The ferromagnetic resonance measurements for an as-cast ribbon and the 495°, 525°C and 600°C annealed ribbons exhibit a resonance line at 63.64 mT for φ = 0°. The sample annealed at 425°C shows two resonance peaks at 95.45 mT and 295.46 mT. These are due to the nanocrystalline α–Fe(Si) phase. The resonance-line widths are seen to broaden after the ribbons are annealed at 495°C and 525°C, at which time, the amorphous matrix crystallizes into nano-grains of α–Fe(Si), t–Fe₂B and t–Fe₃B phases. The kinetics of the crystallization is discussed in terms of the relative change in the line width of the samples annealed at 495°C for different annealing times. These results yielded an Avrami exponent, n of 0.84 which is consistent with diffusion-controlled growth with a nucleation rate close to zero.

The ground-state self-trapping energy and effective mass of surface polarons in a freestanding wurtzite GaN nanowire (NW) are studied using the second-order perturbation approach. Based on the dielectric continuum and Loudon's uniaxial crystal models, the polar optical phonon modes in the one-dimensional (1D) systems are analyzed, and the vibrating spectra of surface optical (SO) modes and electron–SO phonon coupling functions are discussed and analyzed. The calculations of the ground-state polaron self-trapping energy and the correction of effective mass due to the SO phonon modes in the 1D GaN NWs reveal that the polaron self-trapping energy and the correction of effective mass is far larger than those in 1D GaAs NW systems. The reasons for this obvious difference in the two 1D structures can be attributed to the different electron–phonon coupling constants and electron effective masses of bulk material constituting the two types of 1D confined systems. Finally, the polaronic properties of the wurtzite 1D GaN NWs are compared with those of wurtzite GaN-based two-dimensional quantum wells. The physical origins leading to these characteristics and their distinction in the different-dimensionality systems is carefully analyzed.

We examine the doping dependence of SC gap (Δ) and chemical potential (μ) of the hole-doped cuprates by using the Fulde model. It is assumed that SC arises due to BCS pairing mechanism in lattices of Cu–O planes. The expression for SC, AFM, and doping concentration are calculated analytically by using the Green function technique of D. N. Zubarev. The value of SC gap (Δ) and chemical potential (μ) are solved self-consistently for different model parameters. The variation of critical temperature and chemical potential with doping are also studied.

The elastic analysis of a Griffith crack in an icosahedral quasicrystal is performed using the general solution and systematic Fourier transform-dual integral equations method, the analytic expressions for the displacement and stress fields are obtained. The asymptotic behavior of the stress field around the crack tip indicates that the stress near the crack tip exhibits the square root singularity. The singularity in the phason stress field arising from those of the phonon and phonon–phason coupling is also discussed. The most important physical quantities of fracture theory, crack stress intensity factor and energy release rate, are determined.

We have investigated the electrical transport properties of Dirac electrons in a monolayer graphene sheet in the presence of both electric and magnetic modulations. The effects of the modulations on quantum transport when they are in phase and out of phase are considered. We present the energy spectrum and the bandwidth of the Dirac electrons in the presence of both the modulations. We determine the σ_yy component of the magnetoconductivity tensor for this system which is shown to exhibit Weiss oscillations. Asymptotic expressions for σ_yy are also calculated to better illustrate the effects of in-phase and out-of-phase modulations. We find that the position of the oscillations in magnetoconductivity depends on the relative strength of the two modulations. When the two modulations are out of phase there is a complete suppression of Weiss oscillations for a particular relative strength of the modulations.