International Journal of Modern Physics B

Time-reversal symmetric topological insulator (TI) is a novel state of matter that a bulk-insulating state carries dissipationless spin transport along the surfaces, embedded by the Z₂ topological invariant. In the noninteracting limit, this exotic state has been intensively studied and explored with realistic systems, such as HgTe/(Hg, Cd)Te quantum wells. On the other hand, electronic correlation plays a significant role in many solid-state systems, which further influences topological properties and triggers topological phase transitions. Yet an interacting TI is still an elusive subject and most related analyses rely on the mean-field approximation and numerical simulations. Among the approaches, the mean-field approximation fails to predict the topological phase transition, in particular at intermediate interaction strength without spontaneously breaking symmetry. In this paper, we develop an analytical approach based on a combined perturbative and self-consistent mean-field treatment of interactions that is capable of capturing topological phase transitions beyond either method when used independently. As an illustration of the method, we study the effects of short-ranged interactions on the Z₂ TI phase, also known as the quantum spin Hall (QSH) phase, in three generalized versions of the Kane–Mele (KM) model at half-filling on the honeycomb lattice. The results are in excellent agreement with quantum Monte Carlo (QMC) calculations on the same model and cannot be reproduced by either a perturbative treatment or a self-consistent mean-field treatment of the interactions. Our analytical approach helps to clarify how the symmetries of the one-body terms of the Hamiltonian determine whether interactions tend to stabilize or destabilize a topological phase. Moreover, our method should be applicable to a wide class of models where topological transitions due to interactions are in principle possible, but are not correctly predicted by either perturbative or self-consistent treatments.

We investigated the nonsequential double ionization (NSDI) of Xe by linearly polarized (LP) and elliptically polarized (EP) laser pulses using the classical ensemble model. For the two cases, the ensemble from NSDI was separated into two kinds of subensemble according to the relative momentum along the y-axis , respectively. The results show only for the case of relatively small , the momentum distribution along the x-axis can show a V-like shape obviously, which is according with the experiment [Phys. Rev. Lett.99, 263003 (2007)]. What is important is that we firstly examine the role of the repulsion interaction between two electrons for the two kinds of subensemble, and find that the process of NSDI for the case of relatively small exits a strong repulsion interaction which contributes significantly to the V-like shape. However, for the case of relatively big , the repulsion interaction between two electrons is relatively weak, and both the nuclear Coulomb attraction and electric field force dominate the process of NSDI.

Nickel zinc aluminum nanoferrites with general formula Ni_0.5Zn_0.5Fe_2-xAl_xO₄(x = 0.0, 0.05, 0.1, 0.15, 0.2 and 0.25) were prepared by citrate-gel autocombustion method and heat treated in air at 1100°C for 4 h. The crystallography, surface morphology and magnetic properties were studied by using X-ray diffraction (XRD), scanning electron microscope (SEM) and vibrating sample magnetometer (VSM), respectively. XRD analysis confirms that the system exhibits polycrystalline single phase cubic spinel structure and absence of any secondary phases. The crystallite size estimated from Scherrer formula for the Gaussian peak (311) has been found to be around 30 nm. The results obtained show that with Al³⁺ doping, the lattice parameter decreases due to smaller ionic radii of Al³⁺ ions replaces larger Fe³⁺ ions. The distribution of Al³⁺ ions over tetrahedral (A) and octahedral (B) sites is estimated from X-ray intensity calculations. The arrangement of magnetic ions among the tetrahedral and octahedral sites due to the substitution of Al³⁺ ions modifies the saturation magnetization (M_s) and coercivity (H_c). The room temperature magnetization values increases up to x = 0.1 and then decreases for the increase in aluminum concentration above x > 0.1. Surface topography of the powder samples exhibits nearly spherical shape microstructure and the average grain size has been found to be 180 nm (x = 0.05). The observed variations in the structural and magnetic properties are discussed in the light of existing understanding.

This paper is focused on the excitation rates of free electrons of an aluminum (Al) bulk irradiated by an elliptically polarized laser in simulation, using time-dependent density functional theory (TDDFT). The polarized 400 nm, 10 fs laser pulse consisted of two elementary sinusoidal beams, and is adjusted by changing the phase difference φ and the intersection angle θ of the polarization directions between the two beams. The simulation includes cases of φ = π/2 with θ = 30°, θ = 45°, θ = 60°, θ = 90°, θ = 120°, θ = 135°, θ = 150°, and cases of θ = 90° with φ = π/4, φ = π/3, φ = π/2, φ = 2π/3, φ = 3π/4. The absorbed energy, the excitation rates and the density distributions of free electrons after laser termination are investigated. At the given power intensity (1×10¹⁴Wcm^-2), pulse width (10 fs) and wavelength (400 nm) of each elementary laser beam, computational results indicate that the excitation rate of free electrons is impacted by three major factors: the long axis direction of the laser projected profile, the amplitude difference of the first main oscillation (1st AD), and the total amplitude difference of main oscillations (TAD) of the external electric field. Among the aforementioned three factors for the excitation rate of free electrons, the direction of long axis plays the most significant role. The screen effect is crucial to compare the importance of the remaining two factors. The analysis approach to investigate the electron dynamics under an elliptically polarized laser is both pioneering and effective.

We investigate the topological phases of a three-dimensional (3D) topological insulator (TI) without the top–bottom inversion symmetry. We calculate the momentum depended spin Chern number to extract the phase diagram. Various phases are found and we address the dependence of phase boundaries on the strength of inversion asymmetry. Opposite to the quasi-two-dimensional thin film TI, in our 3D system the TI state is stabilized by the structure inversion asymmetry (SIA). With a strong SIA the 3D TI phase can exist even under a large Zeeman field. In a tight-binding form, the surface modes are discussed to confirm with the phase diagram. Particularly we find that the SIA cannot destroy the surface states but open a gap on its spectrum.

Fe nanoparticles have been investigated by means of molecular dynamics simulation. The nucleation and crystal growth is analyzed through the potential energy and number of different types of atoms. The simulation shows that when the amorphous sample is annealed at 900 K, it is crystallized into bcc phase. We found that as the crystal cluster has a size larger than some critical value, the mean potential energy of different types of atoms decreases in following orders: amorphous-atom → surface-crystal atom → crystal-atom. As a result, the crystal cluster is stable and tends to have a nearly spherical shape. Further, it was shown that small nuclei form frequently in the core and rarely in the surface area. After a long annealing time a cluster expands and reaches the critical radius. Then this cluster grows exponentially with times. The fully crystallized sample consists of the core with crystalline structure and surface shell with amorphous porous structure. The Fe nanoparticle has a number of polymorphs which are stable upon annealing at 300 K. We have analyzed the pair radial distribution function (PRDF) for obtained polymorphs. We found that as the fraction of crystal-atoms is less than 0.18, the PRDF is like those of amorphous metal. However, the left sub-peak is higher than right sub-peak when the fraction of crystal-atoms is less than 0.05.

We consider quantum wave propagation in one-dimensional quasiperiodic lattices. We propose an iterative construction of quasiperiodic potentials from sequences of potentials with increasing spatial period. At each finite iteration step, the eigenstates reflect the properties of the limiting quasiperiodic potential properties up to a controlled maximum system size. We then observe approximate Metal–Insulator Transitions (MIT) at the finite iteration steps. We also report evidence on mobility edges, which are at variance to the celebrated Aubry–André model. The dynamics near the MIT shows a critical slowing down of the ballistic group velocity in the metallic phase, similar to the divergence of the localization length in the insulating phase.

Using the massless Dirac–Weyl model for monolayer graphene sheet, we study the low-lying spectra of a single Dirac electron system bound to an on-center positively charged Coulomb impurity, under both electrostatic potential and magnetic field. Numerical results obtained from diagonalization show that, the increase of the electrostatic potential causes the low-lying states to evolve from one Landau-type plateau to the higher ones, and the whole spectra exhibit similar features with slight shifts in eigenenergies when the on-center impurity is considered. Electrostatic-potential dependent optical spectra with their corresponding absorption coefficients as functions of incident photon energies for transitions between low-lying states are presented.

In this paper, we propose a railway faults spreading model which improved the SIR model and made it suitable for analyzing the dynamic process of faults spreading. To apply our model into a real network, the accident causation network of "7.23" China Yongwen high-speed railway accident is employed. This network is improved into a directed network, which more clearly reflects the causation relationships among the accident factors and provides help for our studies. Simulation results quantitatively show that the influence of failures can be diminished via choosing the appropriate initial recovery factors, reducing the time of the failure detected, decreasing the transmission rate of faults and increasing the propagating rate of corrected information. The model is useful to simulate the railway faults spreading and quantitatively analyze the influence of failures.

This study combines the use of the full potential linear-augmented plane wave method (FP-LAPW) within the framework of the density functional theory (DFT) and the optical matrix approach for modeling the multilayer assembly. A new class of heterostructures with sufficient number of alternating layers of rutile-TiO₂ (as a high index material) and α-Al₂O₃ (as a low index material) are proposed and their transmittance spectra are investigated. This study shows that the number of alternating layers, and the thickness and arrangement of them should be considered in making a heterostructured filter. The relation between heterostructure parameters and narrow-band-pass peaks of transmittance spectra is investigated. The proposed model seems to be successful in predicting the optical behavior of heterostructures and simulations agree well with the experimental observations. In addition, our model is very flexible and the effect of other parameters such as incident angle and light polarization can be easily investigated.