Pore-scale simulation of gas-water flow in low permeability gas reservoirs

A novel method was developed to establish a realistic three dimensional(3D) network model representing pore space in low permeability sandstone.Digital core of rock sample was established by the combination of micro-CT scanning and image processing,then 3D pore-throat network model was extracted from the digital core through analyzing pore space topology,calculating pore-throat parameters and simplifying the shapes of pores and throats.The good agreements between predicted and measured porosity and absolute permeability verified the validity of this new network model.Gas-water flow mechanism was studied by using pore-scale simulations,and the influence of pore structure parameters,including coordination number,aspect ratio and shape factor,on gas-water flow,was investigated.The present simulation results show that with the increment of coordination number,gas flow ability in network improves and the effect of invading water on blocking gas flow weakens.The smaller the aspect ratio is,the stronger the anisotropy of the network is,resulting in the increase of seepage resistance.It is found that the shape factor mainly affects the end points in relative permeability curves,and for a highly irregular pore or throat with a small shape factor,the irreducible water saturation(Swi) and residual gas saturation(Sgr) are relatively high.

Graphene as Gas Sensors

The triatomic and tetratomic gas molecule adsorption effects on the electrical conductivity of graphene areinvestigated by the tight-binding model,Green’s function method,and coherent potential approximation.We find thatthe electrical conductivity of graphene sheet is sensitive to the adsorption of these gases.

Propagators for Scalar Bound States at Finite Temperature in an NJL Model

We re-examine physical causal propagators for scalar and pseudoscalar bound states at finite temperature in a chiral NJL model, defined by four-point amputated functions subtracted through the gap equation, and prove that they are completely equivalent in the imaginary-time and real-time formalisms by separating carefully the imaginary part of the zero-temperature loop integral. It is shown that the same thermal transformation matrix of the matrix propagators for these bound states in the real-time formalism is precisely the one of the matrix propagator for an elementary scalar particle and this fact shows the similarity of thermodynamic property between a composite and elementary scalar particle. The retarded and advanced propagators for these bound states are also given explicitly from the imaginary-time formalism.

Theoretical Simulations of Irradiation-Induced Sputtering at Tungsten Surface

The irradiation-induced sputtering and the structural damage at tungsten surface are investigated by using molecular dynamics simulations at the level of quantum mechanics. Our simulations indicate that the sputtered atoms appear when the energy of incident primary knock-on atom （PKA） is more than 200 eV and the incident angle of the PKA is larger than 65°. Meanwhile, the irradiation-induced vacancies are less when the incident angle of PKA is in the range of 45°-65°. So, the optimum incident angles of PKA are suggested to reduce the irradiation-induced damage of the W surface. Furthermore, we find that the interstitials contained in the systems accelerate the sputtering whereas the intrinsic vacancies suppress the sputtering when the PKA is near the defects.

上海光源储存环注入段纵向耦合阻抗的计算

对上海光源注入段的纵向耦合阻抗进行了分析计算．注入段耦合阻抗主要来自镀膜陶瓷真空室及过渡段．分别用解析法和Mafia数值计算法对其进行了估算,包括窄带阻抗和宽带阻抗的计算．在对由两个过渡段形成的类腔体进行计算时,没有发现束缚模(Trapped mode);在对宽带阻抗的计算时,发现它和原先人们常用的宽带共振模型有出入,为配合对微波不稳定性的研究,把宽带阻抗转化成了有效宽带阻抗．

基于平面超材料的Fano谐振可调谐研究

基于两段相同金属分裂环谐振器构成的单层结构,从理论及实验方面研究了平面超材料的可调谐Fano谐振.实验测量了平面超材料对TE和TM入射波的电磁响应,利用电磁波的入射角度控制Fano谐振的强度,实现了谐振的开关特性,谐振频率红移可达到21%.基于有限元法给出了平面超材料的场分布,强的正常色散表明平面超材料的电磁响应可类比经典电磁诱导透明现象,仿真与实验结果相符合.对称结构超材料Fano谐振的开/关特性为超材料性能的可调谐控制提供了有效途径.

Neural network representation of electronic structure from ab initio molecular dynamics

Despite their rich information content,electronic structure data amassed at high volumes in ab initio molecular dynamics simulations are generally under-utilized.We introduce a transferable high-fidelity neural network representation of such data in the form of tight-binding Hamiltonians for crystalline materials.This predictive representation of ab initio electronic structure,combined with machinelearning boosted molecular dynamics,enables efficient and accurate electronic evolution and sampling.When it is applied to a one-dimension charge-density wave material,carbyne,we are able to compute the spectral function and optical conductivity in the canonical ensemble.The spectral functions evaluated during soliton-antisoliton pair annihilation process reveal significant renormalization of low-energy edge modes due to retarded electron-lattice coupling beyond the Born-Oppenheimer limit.The availability of an efficient and reusable surrogate model for the electronic structure dynamical system will enable calculating many interesting physical properties,paving the way to previously inaccessible or challenging avenues in materials modeling.

Tight-binding models for ultracold atoms in optical lattices：general formulation and applications

Tight-binding models for ultracold atoms in optical lattices can be properly defined by using the concept of maximally localized Wannier functions for composite bands. The basic principles of this approach are reviewed here, along with different applications to lattice potentials with two minima per unit cell, in one and two spatial dimensions. Two independent methods for computing the tight-binding coefficients—one ab initio, based on the maximally localized Wannier functions, the other through analytic expressions in terms of the energy spectrum—are considered. In the one dimensional case, where the tight-binding coefficients can be obtained by designing a specific gauge transformation, we consider both the case of quasi resonance between the two lowest bands, and that between s and p orbitals. In the latter case, the role of the Wannier functions in the derivation of an effective Dirac equation is also reviewed. Then, we consider the case of a two dimensional honeycomb potential, with particular emphasis on the Haldane model, its phase diagram, and the breakdown of the Peierls substitution. Tunable honeycomb lattices, characterized by movable Dirac points, are also considered. Finally, general considerations for dealing with the interaction terms are presented.

A brief review on Majorana bound states in topological superconductors

Topological superconductivity has drawn much attention recently, and most interests are focused on the Majorana bound states existing at the edges of one-dimensional topological superconductors. These Majorana bound states are ideal platform for studying non-Abelian statistics. Meanwhile, they are proposed to be useful in quantum computation. In this review, we introduce the basic concepts and models in this area. We begin from the Kitaev model, which is the most concise model for one-dimensional topological superconductivity. Then, we discuss how to realize this model with spin-orbit coupling in realistic materials. Finally, we show some simple methods to detect the Majorana bound states and study their novel properties with the help of adjacent quantum dots.

Conductance Spectra in Graphene-Superconductor Junctions

The conductance spectra of a graphene ribbon and graphene-superconductor （G-S） junctions are investi- gated, using the tight-binding model and non-equilibrium Green＇s function formalism. It is found that the quantized conductance related to graphene＇s edge-states is robust against perturbations in the model parameters for a graphene monolayer ribbon with the zigzag boundary. With appropriate a new bound state with odd-frequency symmetry is found in conductance amplitude is followed model parameter of the spin-orbit interaction strength, the G-S junction. An enhancement in the zero-energy

Monolayer NbF:a 4d-analogue of cuprates

The electronic structure and possible electronic orders in monolayer NbF4 are investigated by density functional theory and functional renormalization group.Because of the niobium-centered octahedra,the energy band near the Fermi level is found to derive from the 4 dxyorbital,well separated from the other bands.Local Coulomb interaction drives the undoped system into an antiferromagnetic insulator.Upon suitable electron/hole doping,the system is found to develop dx2à-y2 wave superconductivity with sizable transition temperature.Therefore,the monolayer NbF4 may be an exciting 4d1 analogue of cuprates,providing a new two-dimensional platform for high-Tc superconductivity.

Topological Insulators on the Ruby Lattice with Rashba Spin-Orbit Coupling

We investigate a tight-binding model of the ruby lattice with Rashba spin-orbit coupling. We calculate the band structure of the lattice and evaluate the Z2 topological indices. According to the Z2 topological indices and the band structure, we present the phase diagrams of the lattice with different filling fractions. We findthat topological insulators occur in some range of parameters at 1/6, 1/3, 1/2, 2/3 and 5/6 filling fractions. We analyze and discuss the characteristics of these topological insulators and their edge states.

SU(2) symmetry in a Hubbard model with spin-orbit coupling

We study the underlying symmetry in a spin-orbit coupled tight-binding model with Hubbard interaction.It is shown that,in the absence of the on-site interaction,the system possesses the SU(2)symmetry arising from the time reversal symmetry.The influence of the on-site interaction on the symmetry depends on the topology of the networks:The SU(2)symmetry is shown to be the spin rotation symmetry of a simply-connected lattice even in the presence of the Hubbard interaction.On the contrary,the on-site interaction breaks the SU(2)symmetry of a multi-connected lattice.This fact indicates that a discrete spin-orbit coupled system has exclusive features from its counterpart in a continuous system.The obtained rigorous result is illustrated by a simple ring system.

Is the existence of a J/ψJ/ψbound state plausible?

In a recent measurement LHCb reported pronounced structures in the J/ψJ/ψspectrum.One of the various possible explanations of those is that they emerge from non-perturbative interactions of vector charmonia.It is thus important to understand whether it is possible to form a bound state of two charmonia interacting through the exchange of gluons,which hadronise into two pions at the longest distance.In this paper,we demonstrate that,given our current understanding of hadron-hadron interactions,the exchange of correlated light mesons(pions and kaons)is able to provide sizeable attraction to the di-J/ψsystem,and it is possible for two J/ψmesons to form a bound state.As a side result we find from an analysis of the data for theψ/(2 S)J/ψππtransition including bothππand KK final state interactions an improved value for theψ(2 S)→J/ψtransition chromo-electric polarisability:|αψ(2 S)J/ψ|=(1.8±0.1)GeV-3,where the uncertainty also includes the one induced by the final state interactions.