International Journal of Modern Physics B

One central issue under intense debate in the study of the iron-based superconductors is the origin of the structural phase transition that changes the crystal lattice symmetry from tetragonal to orthorhombic. This structural phase transition, occurring universally in almost every family of the iron-based superconductors, breaks the lattice C₄ rotational symmetry and results in an anisotropy in a number of physical properties. Due to the unique topology of the Fermi surface, both orbital- and spin-based scenarios have been proposed as the driving force. In this review, we focus on theories from the orbital-based scenario and discuss several related experiments. It is pointed out that although both scenarios lead to the same macroscopic phases and are not distinguishable in bulk measurements of the thermodynamic properties, the elementary excitations could be fundamentally different, and provide us with the possibility to resolve this long-standing debate between orbital- and spin-based theories.

The precipitation phenomena and the related hardening in an Al–Cu–Mg–Si alloy were studied by calorimetry, X-ray diffraction analysis and microhardness measurements. The main calorimetric peaks were identified to be due to β^′′, θ′ and Q′ phases precipitation. The hardening during aging at room temperature and 160°C, was respectively, explained by atomic clusters and GP zones formation and by GP zones and β^′′/θ′ phases coprecipitation. Although the mechanical properties variation during aging at 200°C is simple, the corresponding microstructural evolution is complex: on the basis of the DSC results, the increasing of microhardness values, is mainly due to the coprecipitation of GP zones and β^′′/θ′ phases, however, the maximum hardening is explained by the coexistence of β^′′/θ′ and θ^′′ phases. Another important conclusion is that during aging at 160°C and 200°C, the θ′ phase is essentially developed from GP zones.

In this paper some structural, magnetic and electronic properties of Zn_1-xV_xO for 0 ≤ x ≤ 0.5, such as optimized lattice constant, cohesive energy, formation enthalpy, density of states, band structure, effective mass and Fermi surface are being investigated. In calculating these properties first principle approach is being used. The calculations performed using DFT theory with full potential linear augmented plane wave (FP-LAPW) and GGA approximation. It is shown that by substituting V instead of Zn, Zn_1-xV_xO compound will gain magnetic properties. The band structure of Zn_1-xV_xO shows that metallic behavior increases with increasing substituted V. This substitution increases extremal area in Fermi surface around Γ point. The results obtained from calculated cohesive energy and formation enthalpy show that substituting V increases the stability of Zn_1-xV_xO. The calculated band gap is in a good agreement with other theoretical results.

Polycrystalline (1-x)YBa₂Cu₃O_7-y + xBaTiO₃–CoFe₂O₄(x = 0.0, 0.2, 0.4, 0.6 wt.%) superconductors were prepared by solid state route. XRD analysis reveals no significant change in "b" parameter and increase in "a" and "c" parameters. SEM micrographs show no change in grain size of the samples. With the increase of BaTiO₃–CoFe₂O₄ (BTO–CFO) addition it has been analyzed that the superconducting transition temperatures (T_c) determined from standard four-probe method was decreased and dropped sharply with higher wt.% addition. Excess conductivity fluctuation analysis using Aslamazov–Larkin model fitting reveals transition of two dominant regions (2D and 3D) above T_c. The decrease in 2D–3D crossover temperature T_LD (Lawerence–Doniach temperature) in the mean field region has been observed as a consequent dominance of 3D region to increase in wt.% in the composite. The increasing value of ρ_wl and ρ₀ and the decreasing trend in the value of zero-resistance critical temperature (T_c0) indicates that the connectivity between grains decreases gradually with the addition of magneto–electric composite BTO–CFO.

The effects of carbon nanotubes (CNTs) on mechanical and tribiological properties of the NiFe/CNT composites prepared by high energy mechanical alloying and hot pressing, were investigated. Bulk samples were prepared by sintering of cold pressed (300 MPa) samples at 1040°C for 1 h. X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and optical microscopy were employed for evaluation of the phase composition, surface morphology and porosities of the samples. The microhardness of as-milled Ni₃Fe and NiFe powders reached to 720 and 650 VHN, respectively. The hardness of NiFe and Ni₃Fe bulk samples reduced to 190 and 270 Vickers because of the grain growth during sintering and remaining porosity. The hardness of NiFe-CNT and Ni₃Fe-CNT bulk samples reached to 360 and 400 Vickers, respectively. The friction and wear properties of the bulk samples were investigated under dry conditions using a pin-on-disk test rig under an applied load of 8 N. The wear rate, mass loss and friction coefficient of the composite samples remarkably reduced in comparison with NiFe and Ni₃Fe matrix alloys which demonstrate effects of the CNTs on mechanical and tribiological behavior of the composites resulting from the excellent mechanical properties and unique topological structure of the CNTs.

Spectral properties of periodic one-dimensional array of nanorings in a magnetic field are investigated. Two types of the superlattice are considered. In the first one, rings are connected by short one-dimensional wires while in the second one rings have immediate contacts between each other. The dependence of the electron energy on the quasimomentum is obtained from the Schrödinger equation for the Bloch wavefunction. We have found an interesting feature of the system, namely, presence of discrete energy levels in the spectrum. The levels can be located in the gaps or in the bands depending on parameters of the system. The levels correspond to bound states and electrons occupying these levels are located on individual rings or couples of neighboring rings and do not contribute to the charge transport. The wavefunction for the bound states corresponding to the discrete levels is obtained. Modification of electron energy spectrum with variation of system parameters is discussed.

An epidemic spreading through an adaptive network, where the susceptible are able to avoid contact with the infective by rewiring their network connections, with random and regular contacts is considered. This paper uses a new mathematical model obtained by pair approximation to explore the effect of rewiring of the epidemic dynamics. We show that chaotic behavior can emerge while considering rewiring. The obtained results show that rewiring plays an very important role on the dynamical behavior of the epidemic model.

This paper presents the general method for the adaptive function Q-S synchronization of different chaotic (hyper-chaotic) systems. Based upon the Lyapunov stability theory, the dynamical evolution can be achieved by the Q-S synchronization with a desired scaling function between the different chaotic (hyper-chaotic) systems. This approach is successfully applied to two examples: Chen hyper-chaotic system drives the Lorenz hyper-chaotic system; Lorenz system drives Lü hyper-chaotic system. Numerical simulations are used to validate and demonstrate the effectiveness of the proposed scheme.

A high precision fast projective synchronization method for chaotic systems with unknown parameters was proposed by introducing optimal matrix. Numerical simulations indicate that the precision be improved about three orders compared with other common methods under the same condition of software and hardware. Moreover, when average error is less than 10^-3, the synchronization speed is 6500 times than common methods, the iteration needs only 4 times. The unknown parameters also were identified rapidly. The theoretical analysis and proof also were given.