Density functional study of α-graphyne derivatives: Energetic stability,atomic and electronic structure

The energetic stability, atomic and electronic structures of α-graphyne and its derivatives (α-GYs) with extended carbon chains were investigated by density functional (DF) calculations in this work. The studied α-GYs consist of hexagon carbon rings sharing their edges with carbon atoms N=1–10. The structure and energy analyses show that α-GYs with even-numbered carbon chains have alternating single and triple C–C bonds (polyyne), energetically more stable than those with odd-numbered carbon chains possessing continuous double C–C bonds (polycumulene). The calculated electronic structures indicate that α-GYs can be either metallic (odd N) or semiconductive (even N) depending on the parity of number of atoms on hexagon edges despite the edge length. The semiconducting α-graphyne derivatives are found to possess Dirac cones (DC) with small direct band gaps 2–40 meV and large electron velocities 0.554×10⁶–0.671×10⁶ m/s, 70–80% of that of graphene. Our DF studies suggest that introducing sp carbon atoms into the hexagon edges of graphene opens up an avenue to switch between metallic and DC electronic structures via tuning the parity of the number of hexagon edge atoms.     

High‐Quality Three‐Dimensional Nanoporous Graphene

We report three‐dimensional (3D) nanoporous graphene with preserved 2D electronic properties, tunable pore sizes, and high electron mobility for electronic applications. The complex 3D network comprised of interconnected graphene retains a 2D coherent electron system of massless Dirac fermions. The transport properties of the nanoporous graphene show a semiconducting behavior and strong pore‐size dependence, together with unique angular independence. The free‐standing, large‐scale nanoporous graphene with 2D electronic properties and high electron mobility holds great promise for practical applications in 3D electronic devices.     

The effect of two ferromagnetic metal stripes on valley polarization of electrons in a graphene

As the key factor for designing the valleytronic devices is to well understand the valley-dependent transport mechanism in graphene, we investigate, in this work, the effect of two ferromagnetic (FM) metal stripes on the valley polarization in a graphene nanostructure with a strain. The nearly 100% valley polarization is observed at certain energy windows and it can be easily controlled through changing the width and the position of the FM stripe as well as the strength of the magnetic field induced by the FM stripe. Our interesting findings reveal the valley-dependent transport mechanism of electrons and promote the realization of the new types of valleytronic devices modulated by the FM stripe and the strain.     

Extended Dirac oscillator in (2 + 1)-dimensional space-time

We consider a soluble covariant extension of the two-dimensional Dirac oscillator (2D DO), which breaks the infinite degeneracy of the energy spectrum. The energy eigenvalues and the corresponding eigenstates of the system are obtained algebraically using chiral creation and annihilation operators. The effect of the coupling to an external constant transverse magnetic field is investigated. The connection with Jaynes-Cummings (JC) and Anti-Jaynes-Cummings (AJC) models of quantum optics, and other features of the system are also discussed.     

Spin Dynamics and Dirac Nodes in a Kagome Lattice

Herein, the spin dynamics for various magnetic configurations arranged on a Kagome lattice is investigated. Using a Holstein–Primakoff expansion of the isotropic Heisenberg Hamiltonian with multiple exchange parameters, the development and evolution of magnetic Dirac nodes with both anisotropy and magnetic field are examined. From the classical energies, the phase diagrams for the ferromagnetic (FM), antiferrimagnetic (AfM), and the 120°  phases are shown as functions of J₁, J₂, J₃, and anisotropy. Furthermore, the production of bosonic Dirac and Weyl nodes in the spin-wave spectra is shown. Through frustration of the magnetic geometry, a connection to the asymmetric properties of the Kagome lattice and the various antiferromagnetic configurations is discerned. Most interesting is the 120°  phase, which does not have Dirac nodes when considering only J₁ due to the formation of an analogous antiferromagnetic honeycomb lattice, but gains Dirac symmetry with next-nearest neighbor interactions. Additionally, the presence of flat modes that are characteristic of cluster excitations is shown. Further study of external frustrations from a magnetic field and anisotropy reveals a tunability of the exchange interactions and nodal points.     

人行荷载下梁式结构振动分析的DQ-IQ混合法

为了评估人行荷载作用下梁式结构的振动舒适度,利用微分求积-积分求积,即DQ-IQ混合法求解移动荷载作用下梁的振动响应。人行荷载作用下梁式结构的振动控制方程是含Dirac函数的偏微分方程,首先利用IQ法离散与时间相关的Dirac函数,再利用DQ法把控制方程转化为二阶常系数微分方程,最后利用Newmark算法求解微分方程。以某钢结构连廊为例,利用DQ法计算结构自振频率并与解析解进行对比,结果验证了节点选取和边界条件施加的合理性,再利用DQ-IQ混合法和振型叠加法分别计算了不同行走步频下连廊的响应,计算结果表明,DQ-IQ混合法具有较高的可靠性和精确性。DQ-IQ混合法也可以推广到诸如车辆荷载作用下路面或桥梁的动力响应等其他移动荷载下结构的振动分析。     

Microscopic calculation of the magnetic field of neutron stars

Based on the Dirac equation describing an electron moving in a uniform and cylindrically symmetric magnetic field which may be the result of the self-consistent mean field of the electrons themselves in a neutron star, we have obtained the eigen solutions and the orbital magnetic moments of electrons in which each eigen orbital can be calculated. From the eigen energy spectrum we find that the lowest energy level is the highly degenerate orbitals with the quantum numbers pZ=0, n=0, and m≥0. At the ground state, the electrons fill the lowest eigen states to form many Landau magnetic cells and each cell is a circular disk with the radius λfree and the thickness λe, where λfree is the electron mean free path determined by Coulomb cross section and electron density and λe is the electron Compton wavelength. The magnetic moment of each cell and the number of cells in the neutron star are calculated, from which the total magnetic moment and magnetic field of the neutron star can be calculated. The results are compared with the observational data and the agreement is reasonable.     

Model-independent constraints on spin observables

We discuss model-independent constraints on spin observables in exclusive and inclusive reactions, with special attention to the case of photoproduction.